Initial Evaluation of the Windows Subsystem for Linux

Recently I had my first experiences with the Windows Subsystem for Linux (WSL), evaluating its potential as an environment for getting work done. This subsystem, introduced to Windows 10 in August 2016, allows Windows to natively run x86 and x86-64 Linux binaries. It’s essentially the counterpart to Wine, which allows Linux to natively run Windows binaries.

WSL interfaces with Linux programs only at the kernel level, servicing system calls the same way the Linux kernel would. The subsystem’s main job is translating Linux system calls into NT requests. There’s a series of articles about its internals if you’re interested in learning more.

I was honestly impressed by how well this all works, especially since Microsoft has long had an affinity for producing flimsy imitations (Windows console, PowerShell, Arial, etc.). WSL’s design allows Microsoft to dump an Ubuntu system wholesale inside Windows — and, more recently, other Linux distributions — bypassing a bunch of annoying issues, particularly in regards to glibc.

WSL processes can exec(2) Windows binaries, which then run in under their appropriate subsystem, similar to binfmt on Linux. In theory this nice interop should allow for some automation Linux-style even for Windows’ services and programs. More on that later.

There are some notable issues, though.

Lack of device emulation

No soundcard devices are exposed to the subsystem, so Linux programs can’t play sound. There’s a hack to talk PulseAudio with a Windows’ process that can access, but that’s about it. Generally there’s not much reason to be playing media or games under WSL, but this can be an annoyance if you’re, say, writing software that synthesizes audio.

Really, there’s almost no device emulation at all and /proc is pretty empty. You won’t see hard drives or removable media under /dev, nor will you see USB devices like webcams and joysticks. A lot of the useful things you might do on a Linux system aren’t available under WSL.

No Filesystem in Userspace (FUSE)

Microsoft hasn’t implemented any of the system calls for FUSE, so don’t expect to use your favorite userspace filesystems. The biggest loss for me is sshfs, which I use frequently.

If FUSE was supported, it would be interesting to see how the rest of Windows interacts with these mounted filesystems, if at all.

Fragile services

Services running under WSL are flaky. The big issue is that when the initial WSL shell process exits, all WSL processes are killed and the entire subsystem is torn down. This includes any services that are running. That’s certainly surprising to anyone with experience running services on any kind of unix system. This is probably the worst part of WSL.

While systemd is the standard for Linux these days and may even be “installed” in the WSL virtual filesystem, it’s not actually running and you can’t use systemctl to interact with services. Services can only be controlled the old fashioned way, and, per above, that initial WSL console window has to remain open while services are running.

That’s a bit of a damper if you’re intending to spend a lot of time remotely SSHing into your Windows 10 system. So yes, it’s trivial to run an OpenSSH server under WSL, but it won’t feel like a proper system service.

Limited graphics support

WSL doesn’t come with an X server, so you have to supply one separately (Xming, etc.) that runs outside WSL, as a normal Windows process. WSL processes can connect to that server (DISPLAY) allowing you to run most Linux graphical software.

However, this means there’s no hardware acceleration. There will be no GLX extensions available. If your goal is to run the Emacs or Vim GUIs, that’s not a big deal, but it might matter if you were interested in running a browser under WSL. It also means it’s not a suitable environment for developing software using OpenGL.

Filesystem woes

The filesystem manages to be both one of the smallest issues as well as one of the biggest.

Filename translation

On the small issue side is filename translation. Under most Linux filesystems — and even more broadly for unix — a filename is just a bytestring. They’re not necessarily UTF-8 or any other particular encoding, and that’s partly why filenames are case-sensitive — the meaning of case depends on the encoding.

However, Windows uses a pseudo-UTF-16 scheme for filenames, incompatible with bytestrings. Since WSL lives within a Windows’ filesystem, there must be some bijection between bytestring filenames and pseudo-UTF-16 filenames. It will also have to reject filenames that can’t be mapped. WSL does both.

I couldn’t find any formal documentation about how filename translation works, but most of it can be reverse engineered through experimentation. In practice, Linux filenames are UTF-8 encoded strings, and WSL’s translation takes advantage of this. Filenames are decoded as UTF-8 and re-encoded as UTF-16 for Windows. Any byte that doesn’t decode as valid UTF-8 is silently converted to REPLACEMENT CHARACTER (U+FFFD), and decoding continues from the next byte.

I wonder if there are security consequences for different filenames silently mapping to the same underlying file.

Exercise for the reader: How is an unmatched surrogate half from Windows translated to WSL, where it doesn’t have a UTF-8 equivalent? I haven’t tried this yet.

Even for valid UTF-8, there are many bytes that most Linux filesystems allow in filenames that Windows does not. This ranges from simple things like ASCII backslash and colon — special components of Windows’ paths — to unusual characters like newlines, escape, and other ASCII control characters. There are two different ways these are handled:

  1. The C drive is available under /mnt/c, and WSL processes can access regular Windows files under this “mountpoint.” Attempting to access filenames with invalid characters under this mountpoint always results in ENOENT: “No such file or directory.”

  2. Outside of /mnt/c is WSL territory, and Windows processes aren’t supposed to touch these files. This allows for more freedom when translating filenames. REPLACEMENT CHARACTER is still used for invalid UTF-8 sequences, but the forbidden characters, including backslashes, are all permitted. They’re translated to #XXXX where X is hexadecimal for the normally invalid character. For example, a:b becomes a#003Ab.

While WSL doesn’t let you get away with all the crazy, ill-advised filenames that Linux allows, it’s still quite reasonable. Since Windows and Linux filenames aren’t entirely compatible, there’s going to be some trade-off no matter how this translation is done.

Filesystem performance

On the other hand, filesystem performance is abysmal, and I doubt the subsystem is to blame. This isn’t a surprise to anyone who’s used moderately-sized Git repositories on Windows, where the large numbers of loose files brings things to a crawl. This has been a Windows issue for years, and that’s even before you start plugging in the typically “security” services — virus scanners, whitelists, etc. — that are typically present on a Windows system and make this even worse.

To test out WSL, I went around my normal business compiling tools and making myself at home, just as I would on Linux. Doing nearly anything in WSL was noticably slower than doing the same on Linux on the exact same hardware. I didn’t run any benchmarks, but I’d expect to see around an order of magnitude difference on average for filesystem operations. Building LLVM and Clang took a couple hours rather than the typical 20 minutes.

I don’t expect this issue to get fixed anytime soon, and it’s probably always going to be a notable limitation of WSL.

So is WSL useful?

One of my hopes for WSL appears to be unfeasible. I thought it might be a way to avoid porting software from POSIX to Win32. I could just supply Windows users with the same Linux binary and they’d be fine. However, WSL requires switching Windows into a special “developer mode,” putting it well out of reach of the vast majority of users, especially considering the typical corporate computing environment that will lock this down. In practice, WSL is only useful to developers. I’m sure this is no accident. (Developer mode is no longer required as of October 2017.)

Mostly I see WSL as a Cygwin killer. Unix is my IDE and, on Windows, Cygwin has been my preferred go to for getting a solid unix environment for software development. Unlike WSL, Cygwin processes can make direct Win32 calls, which is occasionally useful. But, in exchange, WSL will overall be better equipped. It has native Linux tools, including a better suite of debugging tools — even better than you get in Windows itself — Valgrind, strace, and properly-working GDB (always been flaky in Cygwin). WSL is not nearly as good as actual Linux, but it’s better than Cygwin if you can get access to it.

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Chris Wellons (PGP)
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