Blowpipe is a toy crypto tool that creates a
Blowfish-encrypted pipe. It doesn’t open any files and instead
encrypts and decrypts from standard input to standard output. This
pipe can encrypt individual files or even encrypt a network
connection (à la netcat).
Most importantly, since Blowpipe is intended to be used as a pipe
(duh), it will never output decrypted plaintext that hasn’t been
authenticated. That is, it will detect tampering of the encrypted
stream and truncate its output, reporting an error, without producing
the manipulated data. Some very similar tools that aren’t considered
toys lack this important feature, such as aespipe.
Purpose
Blowpipe came about because I wanted to study Blowfish, a 64-bit block
cipher designed by Bruce Schneier in 1993. It’s played an important
role in the history of cryptography and has withstood cryptanalysis
for 24 years. Its major weakness is its small block size, leaving it
vulnerable to birthday attacks regardless of any other property of the
cipher. Even in 1993 the 64-bit block size was a bit on the small
side, but Blowfish was intended as a drop-in replacement for the Data
Encryption Standard (DES) and the International Data Encryption
Algorithm (IDEA), other 64-bit block ciphers.
The main reason I’m calling this program a toy is that, outside of
legacy interfaces, it’s simply not appropriate to deploy a 64-bit
block cipher in 2017. Blowpipe shouldn’t be used to encrypt more than
a few tens of GBs of data at a time. Otherwise I’m fairly confident
in both my message construction and my implementation. One detail is a
little uncertain, and I’ll discuss it later when describing message
format.
A tool that I am confident about is Enchive, though since
it’s intended for file encryption, it’s not appropriate for use
as a pipe. It doesn’t authenticate until after it has produced most of
its output. Enchive does try its best to delete files containing
unauthenticated output when authentication fails, but this doesn’t
prevent you from consuming this output before it can be deleted,
particularly if you pipe the output into another program.
Usage
As you might expect, there are two modes of operation: encryption (-E)
and decryption (-D). The simplest usage is encrypting and decrypting a
file:
In both cases you will be prompted for a passphrase which can be up to
72 bytes in length. The only verification for the key is the first
Message Authentication Code (MAC) in the datastream, so Blowpipe
cannot tell the difference between damaged ciphertext and an incorrect
key.
In a script it would be smart to check Blowpipe’s exit code after
decrypting. The output will be truncated should authentication fail
somewhere in the middle. Since Blowpipe isn’t aware of files, it can’t
clean up for you.
Another use case is securely transmitting files over a network with
netcat. In this example I’ll use a pre-shared key file, keyfile.
Rather than prompt for a key, Blowpipe will use the raw bytes of a given
file. Here’s how I would create a key file:
$ head -c 32 /dev/urandom > keyfile
First the receiver listens on a socket (bind(2)):
$ nc -lp 2000 | blowpipe -D -k keyfile > data.zip
Then the sender connects (connect(2)) and pipes Blowpipe through:
If all went well, Blowpipe will exit with 0 on the receiver side.
Blowpipe doesn’t buffer its output (but see -w). It performs one
read(2), encrypts whatever it got, prepends a MAC, and calls
write(2) on the result. This means it can comfortably transmit live
sensitive data across the network:
Kernel messages will appear on the other end as they’re produced by
dmesg. Though keep in mind that the size of each line will be known to
eavesdroppers. Blowpipe doesn’t pad it with noise or otherwise try to
disguise the length. Those lengths may leak useful information.
Blowfish
This whole project started when I wanted to play with Blowfish as a
small drop-in library. I wasn’t satisfied with the selection,
so I figured it would be a good exercise to write my own. Besides, the
specification is both an enjoyable and easy read (and
recommended). It justifies the need for a new cipher and explains the
various design decisions.
I coded from the specification, including writing a script
to generate the subkey initialization tables. Subkeys are initialized
to the binary representation of pi (the first ~10,000 decimal digits).
After a couple hours of work I hooked up the official test vectors to
see how I did, and all the tests passed on the first run. This wasn’t
reasonable, so I spent awhile longer figuring out how I screwed up my
tests. Turns out I absolutely nailed it on my first shot. It’s a
really great sign for Blowfish that it’s so easy to implement
correctly.
Blowfish’s key schedule produces five subkeys requiring 4,168 bytes of
storage. The key schedule is unusually complex: Subkeys are repeatedly
encrypted with themselves as they are being computed. This complexity
inspired the bcrypt password hashing scheme, which
essentially works by iterating the key schedule many times in a loop,
then encrypting a constant 24-byte string. My bcrypt implementation
wasn’t nearly as successful on my first attempt, and it took hours of
debugging in order to match OpenBSD’s outputs.
The encryption and decryption algorithms are nearly identical, as is
typical for, and a feature of, Feistel ciphers. There are no branches
(preventing some side-channel attacks), and the only operations are
32-bit XOR and 32-bit addition. This makes it ideal for implementation
on 32-bit computers.
One tricky point is that encryption and decryption operate on a pair
of 32-bit integers (another giveaway that it’s a Feistel cipher). To
put the cipher to practical use, these integers have to be serialized
into a byte stream. The specification doesn’t choose a byte
order, even for mixing the key into the subkeys. The official test
vectors are also 32-bit integers, not byte arrays. An implementer
could choose little endian, big endian, or even something else.
However, there’s one place in which this decision is formally made:
the official test vectors mix the key into the first subkey in big
endian byte order. By luck I happened to choose big endian as well,
which is why my tests passed on the first try. OpenBSD’s version of
bcrypt also uses big endian for all integer encoding steps, further
cementing big endian as the standard way to encode Blowfish integers.
Blowfish library
The Blowpipe repository contains a ready-to-use, public domain Blowfish
library written in strictly conforming C99. The interface is just three
functions:
Technically the key can be up to 72 bytes long, but the last 16 bytes
have an incomplete effect on the subkeys, so only the first 56 bytes
should matter. Since bcrypt runs the key schedule multiple times, all
72 bytes have full effect.
The library also includes a bcrypt implementation, though it will only
produce the raw password hash, not the base-64 encoded form. The main
reason for including bcrypt is to support Blowpipe.
Message format
The main goal of Blowpipe was to build a robust, authenticated
encryption tool using only Blowfish as a cryptographic primitive.
It uses bcrypt with a moderately-high cost as a key derivation
function (KDF). Not terrible, but this is not a memory hard KDF,
which is important for protecting against cheap hardware brute force
attacks.
Encryption is Blowfish in “counter” CTR mode. A 64-bit
counter is incremented and encrypted, producing a keystream. The
plaintext is XORed with this keystream like a stream cipher. This
allows the last block to be truncated when output and eliminates
some padding issues. Since CRT mode is trivially malleable, the MAC
becomes even more important. In CTR mode, blowfish_decrypt() is
never called. In fact, Blowpipe never uses it.
The authentication scheme is Blowfish-CBC-MAC with a unique key and
encrypt-then-authenticate (something I harmlessly got wrong
with Enchive). It essentially encrypts the ciphertext again with a
different key, but in Cipher Block Chaining mode (CBC), but
it only saves the final block. The final block is prepended to the
ciphertext as the MAC. On decryption the same block is computed
again to ensure that it matches. Only someone who knows the MAC key
can compute it.
Of all three Blowfish uses, I’m least confident about authentication.
CBC-MAC is tricky to get right, though I am following the
rules: fixed length messages using a different key than encryption.
Wait a minute. Blowpipe is pipe-oriented and can output data without
buffering the entire pipe. How can there be fixed-length messages?
The pipe datastream is broken into 64kB chunks. Each chunk is
authenticated with its own MAC. Both the MAC and chunk length are
written in the chunk header, and the length is authenticated by the
MAC. Furthermore, just like the keystream, the MAC is continued from
previous chunk, preventing chunks from being reordered. Blowpipe can
output the content of a chunk and discard it once it’s been
authenticated. If any chunk fails to authenticate, it aborts.
This also leads to another useful trick: The pipe is terminated with a
zero length chunk, preventing an attacker from appending to the
datastream. Everything after the zero-length chunk is discarded. Since
the length is authenticated by the MAC, the attacker also cannot
truncate the pipe since that would require knowledge of the MAC key.
The pipe itself has a 17 byte header: a 16 byte random bcrypt salt and 1
byte for the bcrypt cost. The salt is like an initialization vector (IV)
that allows keys to be safely reused in different Blowpipe instances.
The cost byte is the only distinguishing byte in the stream. Since even
the chunk lengths are encrypted, everything else in the datastream
should be indistinguishable from random data.
Portability
Blowpipe runs on POSIX systems and Windows (Mingw-w64 and MSVC). I
initially wrote it for POSIX (on Linux) of course, but I took an unusual
approach when it came time to port it to Windows. Normally I’d invent a
generic OS interface that makes the appropriate host system
calls. This time I kept the POSIX interface (read(2), write(2),
open(2), etc.) and implemented the tiny subset of POSIX that I needed
in terms of Win32. That implementation can be found under w32-compat/.
I even dropped in a copy of my own getopt().
One really cool feature of this technique is that, on Windows, Blowpipe
will still “open” /dev/urandom. It’s intercepted by my own open(2),
which in response to that filename actually calls
CryptAcquireContext() and pretends like it’s a file. It’s all
hidden behind the file descriptor. That’s the unix way.
I’m considering giving Enchive the same treatment since it would simply
and reduce much of the interface code. In fact, this project has taught
me a number of ways that Enchive could be improved. That’s the value of
writing “toys” such as Blowpipe.
Gap buffers are a common data structure for representing a text buffer
in a text editor. Emacs famously uses gap buffers — long-standing proof
that gap buffers are a perfectly sufficient way to represent a text
buffer.
Gap buffers are very easy to implement. A bare minimum
implementation is about 60 lines of C.
Gap buffers are especially efficient for the majority of typical
editing commands, which tend to be clustered in a small area.
Except for the gap, the content of the buffer is contiguous, making
the search and display implementations simpler and more efficient.
There’s also the potential for most of the gap buffer to be
memory-mapped to the original file, though typical encoding and
decoding operations prevent this from being realized.
Due to having contiguous content, saving a gap buffer is basically
just two write(2) system calls. (Plus fsync(2), etc.)
A gap buffer is really a pair of buffers where one buffer holds all of
the content before the cursor (or point for Emacs), and the other
buffer holds the content after the cursor. When the cursor is moved
through the buffer, characters are copied from one buffer to the
other. Inserts and deletes close to the gap are very efficient.
Typically it’s implemented as a single large buffer, with the
pre-cursor content at the beginning, the post-cursor content at the
end, and the gap spanning the middle. Here’s an illustration:
The top of the animation is the display of the text content and cursor
as the user would see it. The bottom is the gap buffer state, where
each character is represented as a gray block, and a literal gap for
the cursor.
Ignoring for a moment more complicated concerns such as undo and
Unicode, a gap buffer could be represented by something as simple as
the following:
structgapbuf{char*buf;size_ttotal;/* total size of buf */size_tfront;/* size of content before cursor */size_tgap;/* size of the gap */};
This is close to how Emacs represents it. In the structure
above, the size of the content after the cursor isn’t tracked directly,
but can be computed on the fly from the other three quantities. That is
to say, this data structure is normalized.
As an optimization, the cursor could be tracked separately from the
gap such that non-destructive cursor movement is essentially free. The
difference between cursor and gap would only need to be reconciled for
a destructive change — an insert or delete.
Multiple cursors is fairly recent text editor invention that
has gained a lot of popularity recent years. It seems every major
editor either has the feature built in or a readily-available
extension. I myself used Magnar Sveen’s well-polished package
for several years. Though obviously the concept didn’t originate in
Emacs or else it would have been called multiple points, which
doesn’t quite roll off the tongue quite the same way.
The concept is simple: If the same operation needs to done in many
different places in a buffer, you place a cursor at each position, then
drive them all in parallel using the same commands. It’s super flashy
and great for impressing all your friends.
However, as a result of improving my typing skills, I’ve
come to the conclusion that multiple cursors is all hat and no
cattle. It doesn’t compose well with other editing commands, it
doesn’t scale up to large operations, and it’s got all sorts of flaky
edge cases (off-screen cursors). Nearly anything you can do with
multiple cursors, you can do better with old, well-established editing
paradigms.
Somewhere around 99% of my multiple cursors usage was adding a common
prefix to a contiguous serious of lines. As similar brute force
options, Emacs already has rectangular editing, and Vim already has
visual block mode.
The most sophisticated, flexible, and robust alternative is a good old
macro. You can play it back anywhere it’s needed. You can zip it across
a huge buffer. The only downside is that it’s less flashy and so you’ll
get invited to a slightly smaller number of parties.
But if you don’t buy my arguments about multiple cursors being
tasteless, there’s still a good technical argument: Gap buffers are
not designed to work well in the face of multiple cursors!
For example, suppose we have a series of function calls and we’d like to
add the same set of arguments to each. It’s a classic situation for a
macro or for multiple cursors. Here’s the original code:
foo();
bar();
baz();
The example is tiny so that it will fit in the animations to come.
Here’s the desired code:
foo(x, y);
bar(x, y);
baz(x, y);
With multiple cursors you would place a cursor inside each set of
parenthesis, then type x, y. Visually it looks something like this:
Text is magically inserted in parallel in multiple places at a time.
However, if this is a text editor that uses a gap buffer, the
situation underneath isn’t quite so magical. The entire edit doesn’t
happen at once. First the x is inserted in each location, then the
comma, and so on. The edits are not clustered so nicely.
From the gap buffer’s point of view, here’s what it looks like:
For every individual character insertion the buffer has to visit each
cursor in turn, performing lots of copying back and forth. The more
cursors there are, the worse it gets. For an edit of length n with
m cursors, that’s O(n * m) calls to memmove(3). Multiple cursors
scales badly.
Compare that to the old school hacker who can’t be bothered with
something as tacky and modern (eww!) as multiple cursors, instead
choosing to record a macro, then play it back:
The entire edit is done locally before moving on to the next location.
It’s perfectly in tune with the gap buffer’s expectations, only needing
O(m) calls to memmove(3). Most of the work flows neatly into the
gap.
So, don’t waste your time with multiple cursors, especially if you’re
using a gap buffer text editor. Instead get more comfortable with your
editor’s macro feature. If your editor doesn’t have a good macro
feature, get a new editor.
If you want to make your own gap buffer animations, here’s the source
code. It includes a tiny gap buffer implementation:
As of today, I’ve been blogging for 10 years. In this time I’ve written
302,000 words across 343 articles — a rate of one article every
week and a half. These articles form a record of my professional
progress, touching on both “hard” technical skills and “soft”
communication skills. My older articles are a personal reminder of how
far I’ve come. They are proof that I’m not as stagnant as I sometimes
feel, and it helps me to sympathize with others who are currently in
those earlier stages of their own career.
That index where you can find these 343 articles is sorted
newest-first, because it correlates with best-first. It’s a trend I
hope to continue.
History
Before blogging, I had a simple Penn State student page showcasing a
few small — and, in retrospect, trivial — side projects (1,
2, 3), none of which went anywhere. Around the beginning
of my final semester of college, I was inspired by Mark Dominus’ The
Universe of Discourse and Andy Owen’s Friendly Robot
Overlord (gone since 2011) to start my own blosxom
blog. It would be an outlet to actively discuss my projects. Some time
later GitHub was founded, and I switched to a static blog
hosted by GitHub Pages, which is where it lives to this day.
It’s been more challenging to manage all this content than I ever
anticipated. It’s like maintaining a large piece of software, except
it’s naturally more fragile. Any time I make a non-trivial change to
the CSS, I have to inspect the archives to check if I broke older
articles. If I did, sometimes it’s a matter of further adjusting the
CSS. Other times I’ll mass edit a couple hundred articles in order to
normalize some particular aspect, such as heading consistency or image
usage. (Running a macro over an Emacs’ Dired buffer is great
for this.)
I decided in those early days to Capitalize Every Word of the
Title, and I’ve stuck with this convention purely out of
consistency even though it’s looked weird to me for years. I don’t
want to edit the old titles, and any hard changeover date would be
even weirder (in the index listing).
With more foresight and experience, I could have laid down better
conventions for myself from the beginning. Besides the universal
impossibility of already having experience before it’s needed, there’s
also the issue that the internet’s conventions have changed, too. This
blog is currently built with HTML5, but this wasn’t always the case —
especially considering that predates HTML5. When I switched to
HTML5, I also adjusted some of my own conventions to match,
since, at the time, I was still writing articles in raw HTML.
The mobile revolution also arrived since starting this blog. Today,
about one third of visitors read the blog from a mobile device. I’ve
also adjusted the CSS to work well on these devices. To that third of
you: I hope you’re enjoying the experience!
Just in case you haven’t tried it, the blog also works really well with
terminal-based browsers, such as Lynx and ELinks. Go ahead and give it a
shot. The header that normally appears at the top of the page is
actually at the bottom of the HTML document structure. It’s out of the
way for browsers that ignore CSS.
If that’s not enough, last year I also spent effort making the printed
style of my articles look nice. Take a look at the printed version of
this article (i.e. print preview, print to PDF), and make sure to turn
off the little headers added by the browser. A media selector provides
a separate print stylesheet. Chrome / Chromium has consistently had
the best results, followed by Firefox. Someday I’d like for browsers
to be useful as typesetting engines for static documents — as an
alternative to LaTeX and Groff — but they’ve still got a ways to go
with paged media. (Trust me, I’ve tried.)
With more foresight, I could have done something better with my
permanent URLs. Notice how they’re just dates and don’t include the
title. URLs work better when they include human-meaningful context.
Ideally I should be able to look at any one of my URLs and know what
it’s about. Again, this decision goes all the way back to those early
days when I first configured blosxom, not knowing any better.
URLs are forever, and I don’t want to break all my old links.
Consistency is better than any sort of correction. I’m also practically
limited to one article per day, though this has never been a real
problem.
Motivation
For me, an important motivation for writing is to say something unique
about a topic. For example, I’m not interested in writing a tutorial
unless either no such tutorial already exists, or there’s some vital
aspect all the existing tutorials miss. Each article should add new
information to the internet, either raw information or through
assembling existing information in a new way or with a unique
perspective.
I also almost constantly feel like I’m behind the curve, like I don’t
know enough or I don’t have enough skill. As many of you know, the
internet is really effective at causing these feelings. Every day lots
of talented people are sharing interesting and complicated things
across all sorts of topics. For topics that overlap my own desired
expertise, those projects have me thinking, “Man, I have no idea how
to even start doing that. There’s so much I don’t know, and so much
more I need to learn.” Writing articles as I learn is a great way to
keep on top of new subjects.
This is tied to another problem: I have a tendency to assume the
things I’ve known for awhile are common knowledge. This shows up in a
few ways. First, if everyone knows what I know, plus they know a bunch
of things that I don’t know, then I’ve got a lot of catching up to do.
That’s another source of feeling behind the curve.
Second, when writing an article on a topic where I’ve got years of
experience, I leave out way too many important details, assuming the
reader already knows them. When an article I regard as valuable gets a
weak response, it’s probably related to this issue.
Third, after three years of teaching, it seems like it’s
becoming more difficult to put myself in the student’s shoes. I’m
increasingly further away from my own days learning those early topics,
and it’s harder to remember the limitations of my knowledge at that
time. Having this blog really helps, and I’ve re-read some of my older
articles to recall my mindset at the time.
Another way the blog helps is that it’s like having my own textbook.
When teaching a topic to someone — and not necessarily a formal mentee
— or even when just having a discussion, I will reference my articles
when appropriate. Since they’re designed to say something unique, my
article may be the only place to find certain information in a
conveniently packaged form.
Finally, the last important motivating factor is that I want to
spread my preferred memes. Obviously the way I do things is
the Right Way, and the people who do things differently (the Wrong
Way) are stupid and wrong. By writing about the sorts of topics and
technologies I enjoy — C, low-level design, Emacs, Vim, programming on
unix-like systems, my personal programming style — I’m encouraging
others to follow my lead. Surely I’m responsible for at least one
Emacs convert out there!
“Formal” professional writing
Despite having three or four novels worth of (mostly) technical writing
here, my formal workplace writing leaves much to be desired. I’m no
standout in this area. In the same period of time I’ve written just a
handful of formal memos, each about the same length as a long blog post.
Why so few? These memos are painful to write. In order to be
officially recognized, the formal memo process must be imposed upon
me. What this means is that, compared to a blog post, these memos take
at least an order of magnitude longer to write.
The process involves more people, dragged out over a long period of
time. The writing is a lot less personal, which, in my opinion, makes it
drier and less enjoyable. After the initial draft is complete, I have to
switch to vastly inferior tools: emailing the same Microsoft Office
documents back and forth between various people, without any proper
source control. The official, mandated memo template was created by
someone who didn’t know how to effectively operate a word processor, who
had a poor sense of taste (ALL CAPS HEADINGS), and who obviously had no
training in typesetting or style.
At the end of this long process, it’s filed into a system with no
practical search capability, and where it will be quietly purged after
five years, never to be seen again. Outside of the reviewers who were
directly involved in the memo process, somewhere between zero and two
people will have actually read it. Literally.
Arguably the memo might more polished than a blog post. I’m skeptical of
this, but suppose that’s true. I’d still much rather have written ten
less-polished blog posts than one more-polished memo. That’s also ten
shots to produce, by chance, a more valuable article than the single,
precious memo.
Let’s broaden the scope to academic papers. Thanks to some great
co-workers — all three of whom are smarter and handsomer than me — a
year ago I finally got a published academic paper under my belt (and
more to come): ROP Gadget Prevalence and Survival under
Compiler-based Binary Diversification Schemes (and I said
memos were dry!). A ton of work went into this paper, and it’s far
more substantial than any memo or single blog post. The process was a
lot more pleasant (LaTeX instead of Word), and the results are
definitely much more polished than a typical blog post. It reads well
and has interesting information to present.
This all sounds great until you consider the impact. According to
ACM’s statistics, the paper has been accessed 130 times as of this
writing. (Yes, providing an unofficial link to the paper like I just
did above doesn’t help, but I ran out of those crappy “free” links.
Sue me.) Sure, the PDF might have been passed around in untrackable
ways, but I also bet a lot of those accesses were just downloads that
were never read. So let’s split the difference and estimate around
130 people read it.
What kind of impact does a blog post have? Talking about these numbers
feels a bit taboo, like discussing salaries, but it’s important for the
point I’m about to make.
#1 most popular, not even counting the Japanese publication
Note that all but the last has been published for less than time than
our paper. The average time on these pages is between 5 and 6 minutes,
so these are actual readers, not just visitors that take one glance and
leave. Thanks to the information age, a technical blog article on
established blog can reach an audience 100 times larger than a journal
for a fraction of the effort and cost. There are other benefits, too:
I get immediate feedback in the form of comments and email (open
peer review).
The content is available for free (open access). It’s trivial to
link and share blog articles.
Even more, this entire blog is in the public domain. If you don’t
believe me, check out the public domain dedication in the footer of
this page. It’s been there for years, and you can verify that
yourself. Every single change to this blog in the past 6 years
has been publicly documented (transparency).
When I write about a topic, I make it a goal to provide the code and
data to try it for yourself (open data and materials). This code
and data is also either in the public domain, or as close to it as
possible.
Link aggregators and social media are great at finding the best stuff
and making it more visible (censorship resistance). When I have a
big hit, it’s often Reddit or Hacker News driving people to my
article. Sometimes it’s other blogs.
In 2017, a blog is by far the most effective way to publish and share
written information, and have more scientific quality than
journals. More people should be publishing through blog
articles than traditional forms of publications, which are far less
effective.
Since this has proven to be such an effective medium, I’m going to
make a promise right here, right now. And as I explained above with
transparency, there are no take-backs on this one. If you’re reading
this, it’s on the public record forever. I promise to deliver
another 10 years of free, interesting, useful content. This stuff is
going to be even better, too. On this blog’s 20th anniversary,
September 1st, 2027, we’ll see how I did.
Vim and Emacs have different internals models for the current working
directory, and these models influence the overall workflow for each
editor. They decide how files are opened, how shell commands are
executed, and how the build system is operated. These effects even reach
outside the editor to influence the overall structure of the project
being edited.
In the traditional unix model, which was eventually adopted
everywhere else, each process has a particular working directory
tracked by the operating system. When a process makes a request to the
operating system using a relative path — a path that doesn’t begin
with a slash — the operating system uses the process’ working
directory to convert the path into an absolute path. When a process
forks, its child starts in the same directory. A process can change
its working directory at any time using chdir(2), though
most programs never need to do it. The most obvious way this system
call is exposed to regular users is through the shell’s built-in cd
command.
Vim’s spiritual heritage is obviously rooted in vi, one of the classic
unix text editors, and the most elaborate text editor standardized by
POSIX. Like vi, Vim closely follows the unix model for working
directories. At any given time Vim has exactly one working directory.
Shell commands that are run within Vim will start in Vim’s working
directory. Like a shell, the cd ex command changes and queries Vim’s
working directory.
Emacs eschews this model and instead each buffer has its own working
directory tracked using a buffer-local variable, default-directory.
Emacs internally simulates working directories for its buffers like an
operating system, resolving absolute paths itself, giving credence to
the idea that Emacs is an operating system (“lacking only a decent
editor”). Perhaps this model comes from ye olde lisp machines?
In contrast, Emacs’ M-x cd command manipulates the local variable
and has no effect on the Emacs process’ working directory. In fact,
Emacs completely hides its operating system working directory from
Emacs Lisp. This can cause some trouble if that hidden working
directory happens to be sitting on filesystem you’d like to unmount.
Vim can be configured to simulate Emacs’ model with its autochdir
option. When set, Vim will literally chdir(2) each time the user
changes buffers, switches windows, etc. To the user, this feels just
like Emacs’ model, but this is just a convenience, and the core
working directory model is still the same.
Single instance editors
For most of my Emacs career, I’ve stuck to running a single,
long-lived Emacs instance no matter how many different tasks I’m
touching simultaneously. I start the Emacs daemon shortly after
logging in, and it continues running until I log out — typically only
when the machine is shut down. It’s common to have multiple Emacs
windows (frames) for different tasks, but they’re all bound to the
same daemon process.
While with care it’s possible to have a complex, rich Emacs
configuration that doesn’t significantly impact Emacs’ startup time, the
general consensus is that Emacs is slow to start. But since it has a
really solid daemon, this doesn’t matter: hardcore Emacs users only ever
start Emacs occasionally. The rest of the time they’re launching
emacsclient and connecting to the daemon. Outside of system
administration, it’s the most natural way to use Emacs.
The case isn’t so clear for Vim. Vim is so fast that many users fire
it up on demand and exit when they’ve finished the immediate task. At
the other end of the spectrum, others advocate using a single
instance of Vim like running a single Emacs daemon. In my
initial dive into Vim, I tried the single-instance, Emacs way of
doing things. I set autochdir out of necessity and pretended each
buffer had its own working directory.
At least for me, this isn’t the right way to use Vim, and it all comes
down to working directories. I want Vim to be anchored at the
project root with one Vim instance per project. Everything is
smoother when it happens in the context of the project’s root
directory, from opening files, to running shell commands (ctags in
particular), to invoking the build system. With autochdir, these
actions are difficult to do correctly, particularly the last two.
Invoking the build
I suspect the Emacs’ model of per-buffer working directories has, in a
Sapir-Whorf sort of way, been responsible for leading developers
towards poorly-designed, recursive Makefiles. Without a global
concept of working directory, it’s inconvenient to invoke the build
system (M-x compile) in some particular grandparent directory that
is the root of the project. If each directory has its own Makefile, it
usually makes sense to invoke make in the same directory as the file
being edited.
Over the years I’ve been reinventing the same solution to this
problem, and it wasn’t until I spent time with Vim and its alternate
working directory model that I truly understood the problem. Emacs
itself has long had a solution lurking deep in its bowels, unseen by
daylight: dominating files. The function I’m talking about is
locate-dominating-file:
(locate-dominating-file FILE NAME)
Look up the directory hierarchy from FILE for a directory containing
NAME. Stop at the first parent directory containing a file NAME, and
return the directory. Return nil if not found. Instead of a string,
NAME can also be a predicate taking one argument (a directory) and
returning a non-nil value if that directory is the one for which we’re
looking.
The trouble of invoking the build system at the project root is that
Emacs doesn’t really have a concept of a project root. It doesn’t know
where it is or how to find it. The vi model inherited by Vim is to
leave the working directory at the project root. While Vim can
simulate Emacs’ working directory model, Emacs cannot (currently)
simulate Vim’s model.
Instead, by identifying a file name unique to the project’s root (i.e.
a “dominating” file) such as Makefile or build.xml, then
locate-dominating-file can discover the project root. All that’s
left is wrapping M-x compile so that default-directory is
temporarily adjusted to the project’s root.
That looks very roughly like this (and needs more work):
It’s a pattern I’ve used again and again and
again, working against the same old friction. By running one
Vim instance per project at the project’s root, I get the correct
behavior for free.
In my first decade writing Makefiles, I developed the bad habit of
liberally using GNU Make’s extensions. I didn’t know the line between
GNU Make and the portable features guaranteed by POSIX. Usually it
didn’t matter much, but it would become an annoyance when building on
non-Linux systems, such as on the various BSDs. I’d have to specifically
install GNU Make, then remember to invoke it (i.e. as gmake) instead
of the system’s make.
I’ve since become familiar and comfortable with make’s official
specification, and I’ve spend the last year writing strictly
portable Makefiles. Not only has are my builds now portable across all
unix-like systems, my Makefiles are cleaner and more robust. Many of the
common make extensions — conditionals in particular — lead to fragile,
complicated Makefiles and are best avoided anyway. It’s important to be
able to trust your build system to do its job correctly.
This tutorial should be suitable for make beginners who have never
written their own Makefiles before, as well as experienced developers
who want to learn how to write portable Makefiles. Regardless, in
order to understand the examples you must be familiar with the usual
steps for building programs on the command line (compiler, linker,
object files, etc.). I’m not going to suggest any fancy tricks nor
provide any sort of standard starting template. Makefiles should be dead
simple when the project is small, and grow in a predictable, clean
fashion alongside the project.
I’m not going to cover every feature. You’ll need to read the
specification for yourself to learn it all. This tutorial will go over
the important features as well as the common conventions. It’s important
to follow established conventions so that people using your Makefiles
will know what to expect and how to accomplish the basic tasks.
If you’re running Debian, or a Debian derivative such as Ubuntu, the
bmake and freebsd-buildutils packages will provide the bmake and
fmake programs respectively. These alternative make implementations
are very useful for testing your Makefiles’ portability, should you
accidentally make use of a GNU Make feature. It’s not perfect since each
implements some of the same extensions as GNU Make, but it will catch
some common mistakes.
What’s in a Makefile?
I am free, no matter what rules surround me. If I find them tolerable,
I tolerate them; if I find them too obnoxious, I break them. I am free
because I know that I alone am morally responsible for everything I
do. ―Robert A. Heinlein
At make’s core are one or more dependency trees, constructed from
rules. Each vertex in the tree is called a target. The final
products of the build (executable, document, etc.) are the tree roots. A
Makefile specifies the dependency trees and supplies the shell commands
to produce a target from its prerequisites.
In this illustration, the “.c” files are source files that are written
by hand, not generated by commands, so they have no prerequisites. The
syntax for specifying one or more edges in this dependency tree is
simple:
target [target...]: [prerequisite...]
While technically multiple targets can be specified in a single rule,
this is unusual. Typically each target is specified in its own rule. To
specify the tree in the illustration above:
The order of these rules doesn’t matter. The entire Makefile is parsed
before any actions are taken, so the tree’s vertices and edges can be
specified in any order. There’s one exception: the first non-special
target in a Makefile is the default target. This target is selected
implicitly when make is invoked without choosing a target. It should be
something sensible, so that a user can blindly run make and get a useful
result.
A target can be specified more than once. Any new prerequisites are
appended to the previously-given prerequisites. For example, this
Makefile is identical to the previous, though it’s typically not written
this way:
There are six special targets that are used to change the behavior
of make itself. All have uppercase names and start with a period.
Names fitting this pattern are reserved for use by make. According to
the standard, in order to get reliable POSIX behavior, the first
non-comment line of the Makefile must be .POSIX. Since this is a
special target, it’s not a candidate for the default target, so game
will remain the default target:
In practice, even a simple program will have header files, and sources
that include a header file should also have an edge on the dependency
tree for it. If the header file changes, targets that include it should
also be rebuilt.
We’ve constructed a dependency tree, but we still haven’t told make how
to actually build any targets from its prerequisites. The rules also
need to specify the shell commands that produce a target from its
prerequisites.
If you were to create the source files in the example and invoke make,
you will find that it actually does know how to build the object
files. This is because make is initially configured with certain
inference rules, a topic which will be covered later. For now, we’ll
add the .SUFFIXES special target to the top, erasing all the built-in
inference rules.
Commands immediately follow the target/prerequisite line in a rule. Each
command line must start with a tab character. This can be awkward if
your text editor isn’t configured for it, and it will be awkward if you
try to copy the examples from this page.
Each line is run in its own shell, so be mindful of using commands like
cd, which won’t affect later lines.
The simplest thing to do is literally specify the same commands you’d
type at the shell:
I tried to walk into Target, but I missed. ―Mitch Hedberg
When invoking make, it accepts zero or more targets from the dependency
tree, and it will build these targets — e.g. run the commands in the
target’s rule — if the target is out-of-date. A target is out-of-date
if it is older than any of its prerequisites.
# build the "game" binary (default target)
$ make
# build just the object files
$ make graphics.o physics.o input.o
This effect cascades up the dependency tree and causes further targets
to be rebuilt until all of the requested targets are up-to-date. There’s
a lot of room for parallelism since different branches of the tree can
be updated independently. It’s common for make implementations to
support parallel builds with the -j option. This is non-standard, but
it’s a fantastic feature that doesn’t require anything special in the
Makefile to work correctly.
Similar to parallel builds is make’s -k (“keep going”) option, which
is standard. This tells make not to stop on the first error, and to
continue updating targets that are unaffected by the error. This is nice
for fully populating Vim’s quickfix list or Emacs’ compilation
buffer.
It’s common to have multiple targets that should be built by default. If
the first rule selects the default target, how do we solve the problem
of needing multiple default targets? The convention is to use phony
targets. These are called “phony” because there is no corresponding
file, and so phony targets are never up-to-date. It’s convention for a
phony “all” target to be the default target.
I’ll make game a prerequisite of a new “all” target. More real targets
could be added as necessary to turn them into defaults. Users of this
Makefile will also expect make all to build the entire project.
Another common phony target is “clean” which removes all of the built
files. Users will expect make clean to delete all generated files.
So far the Makefile hardcodes cc as the compiler, and doesn’t use any
compiler flags (warnings, optimization, hardening, etc.). The user
should be able to easily control all these things, but right now they’d
have to edit the entire Makefile to do so. Perhaps the user has both
gcc and clang installed, and wants to choose one or the other
without changing which is installed as cc.
To solve this, make has macros that expand into strings when
referenced. The convention is to use the macro named CC when talking
about the C compiler, CFLAGS when talking about flags passed to the C
compiler, LDFLAGS for flags passed to the C compiler when linking, and
LDLIBS for flags about libraries when linking. The Makefile should
supply defaults as needed.
A macro is expanded with $(...). It’s valid (and normal) to reference
a macro that hasn’t been defined, which will be an empty string. This
will be the case with LDFLAGS below.
Macro values can contain other macros, which will be expanded
recursively each time the macro is expanded. Some make implementations
allow the name of the macro being expanded to itself be a macro, which
is turing complete, but this behavior is non-standard.
Macros are overridden by macro definitions given as command line
arguments in the form name=value. This allows the user to select their
own build configuration. This is one of make’s most powerful and
under-appreciated features.
$ make CC=clang CFLAGS='-O3 -march=native'
If the user doesn’t want to specify these macros on every invocation,
they can (cautiously) use make’s -e flag to set overriding macros
definitions from the environment.
$ export CC=clang
$ export CFLAGS=-O3
$ make -e all
Some make implementations have other special kinds of macro assignment
operators beyond simple assignment (=). These are unnecessary, so
don’t worry about them.
Inference rules so that you can stop repeating yourself
The road itself tells us far more than signs do. ―Tom Vanderbilt,
Traffic: Why We Drive the Way We Do
There’s repetition across the three different object files. Wouldn’t it
be nice if there was a way to communicate this pattern? Fortunately
there is, in the form of inference rules. It says that a target with
a certain extension, with a prerequisite with another certain extension,
is built a certain way. This will make more sense with an example.
In an inference rule, the target indicates the extensions. The $<
macro expands to the prerequisite, which is essential to making
inference rules work generically. Unfortunately this macro is not
available in target rules, as much as that would be useful.
For example, here’s an inference rule that teaches make how to build an
object file from a C source file. This particular rule is one that
is pre-defined by make, so you’ll never need to write this one yourself.
I’ll include it for completeness.
.c.o:$(CC)$(CFLAGS)-c$<
These extensions must be added to .SUFFIXES before they will work.
With that, the commands for the rules about object files can be omitted.
The first empty .SUFFIXES clears the suffix list. The second one adds
.c and .o to the now-empty suffix list.
Other target conventions
Conventions are, indeed, all that shield us from the shivering void,
though often they do so but poorly and desperately. ―Robert Aickman
Users usually expect an “install” target that installs the built
program, libraries, man pages, etc. By convention this target should use
the PREFIX and DESTDIR macros.
The PREFIX macro should default to /usr/local, and since it’s a
macro the user can override it to install elsewhere, such as in their
home directory. The user should override it for both building and
installing, since the prefix may need to be built into the binary (e.g.
-DPREFIX=$(PREFIX)).
The DESTDIR is macro is used for staged builds, so that it gets
installed under a fake root directory for the sake of packaging. Unlike
PREFIX, it will not actually be run from this directory.
You may also want to provide an “uninstall” phony target that does the
opposite.
make PREFIX=$HOME/.local install
Other common targets are “mostlyclean” (like “clean” but don’t delete
some slow-to-build targets), “distclean” (delete even more than
“clean”), “test” or “check” (run the test suite), and “dist” (create a
package).
Complexity and growing pains
One of make’s big weak points is scaling up as a project grows in size.
Recursive Makefiles
As your growing project is broken into subdirectories, you may be
tempted to put a Makefile in each subdirectory and invoke them
recursively.
Don’t use recursive Makefiles. It breaks the dependency
tree across separate instances of make and typically results in a
fragile build. There’s nothing good about it. Have one Makefile at the
root of your project and invoke make there. You may have to teach your
text editor how to do this.
When talking about files in subdirectories, just include the
subdirectory in the name. Everything will work the same as far as make
is concerned, including inference rules.
Keeping your object files separate from your source files is a nice
idea. When it comes to make, there’s good news and bad news.
The good news is that make can do this. You can pick whatever file names
you like for targets and prerequisites.
obj/input.o:src/input.c
The bad news is that inference rules are not compatible with
out-of-source builds. You’ll need to repeat the same commands for each
rule as if inference rules didn’t exist. This is tedious for large
projects, so you may want to have some sort of “configure” script, even
if hand-written, to generate all this for you. This is essentially what
CMake is all about. That, plus dependency management.
Dependency management
Another problem with scaling up is tracking the project’s ever-changing
dependencies across all the source files. Missing a dependency means the
build may not be correct unless you make clean first.
If you go the route of using a script to generate the tedious parts of
the Makefile, both GCC and Clang have a nice feature for generating all
the Makefile dependencies for you (-MM, -MT), at least for C and
C++. There are lots of tutorials for doing this dependency generation on
the fly as part of the build, but it’s fragile and slow. Much better to
do it all up front and “bake” the dependencies into the Makefile so that
make can do its job properly. If the dependencies change, rebuild your
Makefile.
For example, here’s what it looks like invoking gcc’s dependency
generator against the imaginary input.c for an out-of-source build:
Unfortunately this feature strips the leading paths from the target, so,
in practice, using it is always more complicated than it should be (e.g.
it requires the use of -MT).
Microsoft’s Nmake
Microsoft has an implementation of make called Nmake, which comes with
Visual Studio. It’s nearly a POSIX-compatible make, but
necessarily breaks from the standard in some places. Their cl.exe
compiler uses .obj as the object file extension and .exe for
binaries, both of which differ from the unix world, so it has different
built-in inference rules. Windows also lacks a Bourne shell and the
standard unix tools, so all of the commands will necessarily be
different.
There’s no equivalent of rm -f on Windows, so good luck writing a
proper “clean” target. No, del /f isn’t the same.
So while it’s close to POSIX make, it’s not practical to write a
Makefile that will simultaneously work properly with both POSIX make
and Nmake. These need to be separate Makefiles.
May your Makefiles be portable
It’s nice to have reliable, portable Makefiles that just work anywhere.
Code to the standards and you don’t need feature tests or
other sorts of special treatment.
I recently developed Pokerware, an offline passphrase
generator that operates in the same spirit as Diceware.
The primary difference is that it uses a shuffled deck of playing
cards as its entropy source rather than dice. Draw some cards and use
them to select a uniformly random word from a list. Unless you’re some
sort of tabletop gaming nerd, a deck of cards is more readily
available than five 6-sided dice, which would typically need to be
borrowed from the Monopoly board collecting dust on the shelf, then
rolled two at a time.
There are various flavors of two different word lists here:
Hardware random number generators are difficult to verify
and may not actually be as random as they promise, either
intentionally or unintentionally. For the particularly paranoid,
Diceware and Pokerware are an easily verifiable alternative for
generating secure passphrases for cryptographic purposes.
At any time, a deck of 52 playing cards is in one of 52! possible
arrangements. That’s more than 225 bits of entropy. If you give your
deck a thorough shuffle, it will be in an arrangement that
has never been seen before and will never be seen again. Pokerware
draws on some of these bits to generate passphrases.
The Pokerware list has 5,304 words (12.4 bits per word), compared to
Diceware’s 7,776 words (12.9 bits per word). My goal was to invent a
card-drawing scheme that would uniformly select from a list in the same
sized ballpark as Diceware. Much smaller and you’d have to memorize more
words for the same passphrase strength. Much larger and the words on the
list would be more difficult to memorize, since the list would contain
longer and less frequently used words. Diceware strikes a nice balance
at five dice.
One important difference for me is that I like my Pokerware word
lists a lot more than the two official Diceware lists. My lists only
have simple, easy-to-remember words (for American English speakers, at
least), without any numbers or other short non-words. Pokerware has
two official lists, “formal” and “slang,” since my early testers
couldn’t agree on which was better. Rather than make a difficult
decision, I took the usual route of making no decision at all.
The “formal” list is derived in part from Google’s Ngram
Viewer, with my own additional filters and tweaking. It’s called
“formal” because the ngrams come from formal publications and represent
more formal kinds of speech.
The “slang” list is derived from every reddit comment between
December 2005 and May 2017, tamed by the same additional filters. I
have this data on hand, so I may as well put it to use. I
figured more casually-used words would be easier to remember. Due to
my extra filtering, there’s actually a lot of overlap between these
lists, so the differences aren’t too significant.
If you have your own word list, perhaps in a different language, you
can use the Makefile in the repository to build your own Pokerware
lookup table, both plain text and PDF. The PDF is generated using
Groff macros.
Passphrase generation instructions
Thoroughly shuffle the deck.
Draw two cards. Sort them by value, then suit. Suits are in
alphabetical order: Clubs, Diamonds, Hearts, Spades.
Draw additional cards until you get a card that doesn’t match the
face value of either of your initial two cards. Observe its suit.
Using your two cards and observed suit, look up a word in the table.
Place all cards back in the deck, shuffle, and repeat from step 2
until you have the desired number of words. Each word is worth 12.4
bits of entropy.
A word of warning about step 4: If you use software to do the word list
lookup, beware that it might save your search/command history — and
therefore your passphrase — to a file. For example, the less pager
will store search history in ~/.lesshst. It’s easy to prevent that
one:
$ LESSHISTFILE=- less pokerware-slang.txt
Example word generation
Suppose in step 2 you draw King of Hearts (KH/K♥) and Queen of Clubs
(QC/Q♣).
In step 3 you first draw King of Diamonds (KD/K♦), discarding it because
it matches the face value of one of your cards from step 2.
Next you draw Four of Spades (4S/4♠), taking spades as your extra suit.
In order, this gives you Queen of Clubs, King of Hearts, and Spades:
QCKHS or Q♣K♥♠. This corresponds to “wizard” in the formal word list and
would be the first word in your passphrase.
A deck of cards as an office tool
I now have an excuse to keep a deck of cards out on my desk at work.
I’ve been using Diceware — or something approximating it since I’m not
so paranoid about hardware RNGs — for passwords for over 8 years
now. From now I’ll deal new passwords from an in-reach deck of
cards. Though typically I need to tweak the results to meet outdated
character-composition requirements.
Last week I was discussing CVE-2017-7529 with my intern.
Specially crafted input to Nginx causes an integer overflow which has the
potential to leak sensitive information. But how could an integer overflow
be abused to trick a program into leaking information? To answer this
question, I put together the simplest practical example I could imagine.
This small C program converts a vector image from a custom format
(described below) into a Netpbm image, a conveniently simple
format. The program defensively and carefully parses its input, but
still makes a subtle, fatal mistake. This mistake not only leads to
sensitive information disclosure, but, with a more sophisticated attack,
could be used to execute arbitrary code.
After getting the hang of the interface for the program, I encourage you
to take some time to work out an exploit yourself. Regardless, I’ll reveal
a functioning exploit and explain how it works.
A new vector format
The input format is line-oriented and very similar to Netpbm itself. The
first line is the header, starting with the magic number V2 (ASCII)
followed by the image dimensions. The target output format is Netpbm’s
“P2” (text gray scale) format, so the “V2” parallels it. The file must end
with a newline.
V2 <width> <height>
What follows is drawing commands, one per line. For example, the s
command sets the value of a particular pixel.
s <x> <y> <00–ff>
Since it’s not important for the demonstration, this is the only command I
implemented. It’s easy to imagine additional commands to draw lines,
circles, Bezier curves, etc.
Here’s an example (example.txt) that draws a single white point in the
middle of the image:
V2 256 256
s 127 127 ff
The rendering tool reads standard input to standard output:
$ render < example.txt > example.pgm
Here’s what it looks like rendered:
However, you will notice that when you run the rendering tool, it prompts
you for username and password. This is silly, of course, but it’s an
excuse to get “sensitive” information into memory. It will accept any
username/password combination where the username and password don’t match
each other. The key is this: It’s possible to craft a valid image that
leaks the the entered password.
Tour of the implementation
Without spoiling anything yet, let’s look at how this program works. The
first thing to notice is that I’m using a custom “obstack”
allocator instead of malloc() and free(). Real-world allocators have
some defenses against this particular vulnerability. Plus a specific
exploit would have to target a specific libc. By using my own allocator,
the exploit will mostly be portable, making for a better and easier
demonstration.
The allocator interface should be pretty self-explanatory, except for two
details. This is an obstack allocator, so freeing an object also frees
every object allocated after it. Also, it doesn’t call malloc() in the
background. At initialization you give it a buffer from which to allocate
all memory.
There are no vulnerabilities in these functions (I hope!). It’s just
here for predictability.
Next here’s the “authentication” function. It reads a username and
password combination from /dev/tty. It’s only an excuse to get a flag in
memory for this capture-the-flag game. The username and password must be
less than 32 characters each.
Next up is a paranoid wrapper for strtoul() that defensively checks its
inputs. If it’s out of range of an unsigned long, it bails out. If
there’s trailing garbage, it bails out. If there’s no number at all, it
bails out. If you make prolonged eye contact, it bails out.
unsignedlongsafe_strtoul(char*nptr,char**endptr,intbase){errno=0;unsignedlongn=strtoul(nptr,endptr,base);if(errno){perror(nptr);exit(EXIT_FAILURE);}elseif(nptr==*endptr){fprintf(stderr,"Expected an integer\n");exit(EXIT_FAILURE);}elseif(!isspace(**endptr)){fprintf(stderr,"Invalid character '%c'\n",**endptr);exit(EXIT_FAILURE);}returnn;}
The main() function parses the header using this wrapper and allocates
some zeroed memory:
Then there’s a command processing loop, also using safe_strtoul(). It
carefully checks bounds against width and height. Finally it writes
out a Netpbm, P2 (.pgm) format.
Where did those junk numbers come from in the image data? Plug them into
an ASCII table and you’ll get “mysecret”. Despite allocating the image
with naive_calloc(), the password has found its way into the image! How
could this be?
What happened is that width * height overflows an unsigned long.
(Well, technically speaking, unsigned integers are defined not to
overflow in C, wrapping around instead, but it’s really the same thing.)
In naive_calloc(), the overflow results in a value of 16, so it only
allocates and clears 16 bytes. The requested allocation “succeeds” despite
far exceeding the available memory. The caller has been given a lot less
memory than expected, and the memory believed to have been allocated
contains a password.
The final part that writes the output doesn’t multiply the integers and
doesn’t need to test for overflow. It uses a nested loop instead,
continuing along with the original, impossible image size.
How do we fix this? Add an overflow check at the beginning of the
naive_calloc() function (making it no longer naive). This is what the
real calloc() does.
if(nmemb&&size>-1UL/nmemb)return0;
The frightening takeaway is that this check is very easy to forget. It’s
a subtle bug with potentially disastrous consequences.
In practice, this sort of program wouldn’t have sensitive data resident in
memory. Instead an attacker would target the program’s stack with those
s commands — specifically the return pointers — and perform a ROP
attack against the application. With the exploit header above and a
platform where long the same size as a size_t, the program will behave
as if all available memory has been allocated to the image, so the s
command could be used to poke custom values anywhere in memory. This is
a much more complicated exploit, and it has to contend with ASLR and
random stack gap, but it’s feasible.
The most recent Smarter Every Day (#172) explains a phenomenon
that results from rolling shutter. You’ve likely seen this effect in
some of your own digital photographs. When a CMOS digital camera
captures a picture, it reads one row of the sensor at a time. If the
subject of the picture is a fast-moving object (relative to the
camera), then the subject will change significantly while the image is
being captured, giving strange, unreal results:
In the Smarter Every Day video, Destin illustrates the effect by
simulating rolling shutter using a short video clip. In each frame of
the video, a few additional rows are locked in place, showing the
effect in slow motion, making it easier to understand.
At the end of the video he thanks a friend for figuring out how to get
After Effects to simulate rolling shutter. After thinking about this
for a moment, I figured I could easily accomplish this myself with
just a bit of C, without any libraries. The video above this paragraph
is the result.
I previously described a technique to edit and manipulate
video without any formal video editing tools. A unix pipeline is
sufficient for doing minor video editing, especially without sound.
The program at the front of the pipe decodes the video into a raw,
uncompressed format, such as YUV4MPEG or PPM. The tools in
the middle losslessly manipulate this data to achieve the desired
effect (watermark, scaling, etc.). Finally, the tool at the end
encodes the video into a standard format.
For the “decode” program I’ll be using ffmpeg now that it’s back in
the Debian repositories. You can throw a video in virtually any
format at it and it will write PPM frames to standard output. For the
encoder I’ll be using the x264 command line program, though ffmpeg
could handle this part as well. Without any filters in the middle,
this example will just re-encode a video:
The filter tools in the middle only need to read and write in the raw
image format. They’re a little bit like shaders, and they’re easy to
write. In this case, I’ll write C program that simulates rolling
shutter. The filter could be written in any language that can read and
write binary data from standard input to standard output.
Update: It appears that input PPM streams are a rather recent
feature of libavformat (a.k.a lavf, used by x264). Support for PPM
input first appeared in libavformat 3.1 (released June 26th, 2016). If
you’re using an older version of libavformat, you’ll need to stick
ppmtoy4m in front of x264 in the processing pipeline.
In the past, my go to for raw video data has been loose PPM frames and
YUV4MPEG streams (via ppmtoy4m). Fortunately, over the years a lot
of tools have gained the ability to manipulate streams of PPM images,
which is a much more convenient format. Despite being raw video data,
YUV4MPEG is still a fairly complex format with lots of options and
annoying colorspace concerns. PPM is simple RGB without
complications. The header is just text:
The maximum depth is virtually always 255. A smaller value reduces the
image’s dynamic range without reducing the size. A larger value involves
byte-order issues (endian). For video frame data, the file will
typically look like:
P6
1920 1080
255
<frame RGB>
Unfortunately the format is actually a little more flexible than this.
Except for the new line (LF, 0x0A) after the maximum depth, the
whitespace is arbitrary and comments starting with # are permitted.
Since the tools I’m using won’t produce comments, I’m going to ignore
that detail. I’ll also assume the maximum depth is always 255.
Here’s the structure I used to represent a PPM image, just one frame
of video. I’m using a flexible array member to pack the data at the
end of the structure.
Finally, a function to read a frame, reusing an existing buffer if
possible. The most complex part of the whole program is just parsing
the PPM header. The %*c in the scanf() specifically consumes the
line feed immediately following the maximum depth.
Since this program will only be part of a pipeline, I’m not worried
about checking the results of fwrite() and fread(). The process
will be killed by the shell if something goes wrong with the pipes.
However, if we’re out of video data and get an EOF, scanf() will
fail, indicating the EOF, which is normal and can be handled cleanly.
An identity filter
That’s all the infrastructure we need to built an identity filter that
passes frames through unchanged:
Processing a frame is just matter of adding some stuff to the body of
the while loop.
A rolling shutter filter
For the rolling shutter filter, in addition to the input frame we need
an image to hold the result of the rolling shutter. Each input frame
will be copied into the rolling shutter frame, but a little less will be
copied from each frame, locking a little bit more of the image in place.
The shutter_step controls how many rows are capture per frame of
video. Generally capturing one row per frame is too slow for the
simulation. For a 1080p video, that’s 1,080 frames for the entire
simulation: 18 seconds at 60 FPS or 36 seconds at 30 FPS. If this
program were to accept command line arguments, controlling the shutter
rate would be one of the options.
Here are some of the results for different shutter rates: 1, 3, 5, 8,
10, and 15 rows per frame. Feel free to right-click and “View Video”
to see the full resolution video.
Source and original input
This post contains the full source in parts, but here it is all together:
