Mapping Multiple Memory Views in User Space

Modern operating systems run processes within virtual memory using a piece of hardware called a memory management unit (MMU). The MMU contains a page table that defines how virtual memory maps onto physical memory. The operating system is responsible for maintaining this page table, mapping and unmapping virtual memory to physical memory as needed by the processes it’s running. If a process accesses a page that is not currently mapped, it will trigger a page fault and the execution of the offending thread will be paused until the operating system maps that page.

This functionality allows for a neat hack: A physical memory address can be mapped to multiple virtual memory addresses at the same time. A process running with such a mapping will see these regions of memory as aliased — views of the same physical memory. A store to one of these addresses will simultaneously appear across all of them.

Some useful applications of this feature include:

Both POSIX and Win32 allow user space applications to create these aliased mappings. The original purpose for these APIs is for shared memory between processes, where the same physical memory is mapped into two different processes’ virtual memory. But the OS doesn’t stop us from mapping the shared memory to a different address within the same process.

POSIX Memory Mapping

On POSIX systems (Linux, *BSD, OS X, etc.), the three key functions are shm_open(3), ftruncate(2), and mmap(2).

First, create a file descriptor to shared memory using shm_open. It has very similar semantics to open(2).

int shm_open(const char *name, int oflag, mode_t mode);

The name works much like a filesystem path, but is actually a different namespace (though on Linux it is a tmpfs mounted at /dev/shm). Resources created here (O_CREAT) will persist until explicitly deleted (shm_unlink(3)) or until the system reboots. It’s an oversight in POSIX that a name is required even if we never intend to access it by name. File descriptors can be shared with other processes via fork(2) or through UNIX domain sockets, so a name isn’t strictly required.

OpenBSD introduced shm_mkstemp(3) to solve this problem, but it’s not widely available. On Linux, as of this writing, the O_TMPFILE flag may or may not provide a fix (it’s undocumented).

The portable workaround is to attempt to choose a unique name, open the file with O_CREAT | O_EXCL (either atomically create the file or fail), shm_unlink the shared memory object as soon as possible, then cross our fingers. The shared memory object will still exist (the file descriptor keeps it alive) but will not longer be accessible by name.

int fd = shm_open("/example", O_RDWR | O_CREAT | O_EXCL, 0600);
if (fd == -1)
    handle_error(); // non-local exit
shm_unlink("/example");

The shared memory object is brand new (O_EXCL) and is therefore of zero size. ftruncate sets it to the desired size. This does not need to be a multiple of the page size. Failing to allocate memory will result in a bus error on access.

size_t size = sizeof(uint32_t);
ftruncate(fd, size);

Finally mmap the shared memory into place just as if it were a file. We can choose an address (aligned to a page) or let the operating system choose one for use (NULL). If we don’t plan on making any more mappings, we can also close the file descriptor. The shared memory object will be freed as soon as it completely unmapped (munmap(2)).

int prot = PROT_READ | PROT_WRITE;
uint32_t *a = mmap(NULL, size, prot, MAP_SHARED, fd, 0);
uint32_t *b = mmap(NULL, size, prot, MAP_SHARED, fd, 0);
close(fd);

At this point both a and b have different addresses but point (via the page table) to the same physical memory. Changes to one are reflected in the other. So this:

*a = 0xdeafbeef;
printf("%p %p 0x%x\n", a, b, *b);

Will print out something like:

0x6ffffff0000 0x6fffffe0000 0xdeafbeef

It’s also possible to do all this only with open(2) and mmap(2) by mapping the same file twice, but you’d need to worry about where to put the file, where it’s going to be backed, and the operating system will have certain obligations about syncing it to storage somewhere. Using POSIX shared memory is simpler and faster.

Windows Memory Mapping

Windows is very similar, but directly supports anonymous shared memory. The key functions are CreateFileMapping, and MapViewOfFileEx.

First create a file mapping object from an invalid handle value. Like POSIX, the word “file” is used without actually involving files.

size_t size = sizeof(uint32_t);
HANDLE h = CreateFileMapping(INVALID_HANDLE_VALUE,
                             NULL,
                             PAGE_READWRITE,
                             0, size,
                             NULL);

There’s no truncate step because the space is allocated at creation time via the two-part size argument.

Then, just like mmap:

uint32_t *a = MapViewOfFile(h, FILE_MAP_ALL_ACCESS, 0, 0, size);
uint32_t *b = MapViewOfFile(h, FILE_MAP_ALL_ACCESS, 0, 0, size);
CloseHandle(h);

If I wanted to choose the target address myself, I’d call MapViewOfFileEx instead, which takes the address as additional argument.

From here on it’s the same as above.

Generalizing the API

Having some fun with this, I came up with a general API to allocate an aliased mapping at an arbitrary number of addresses.

int  memory_alias_map(size_t size, size_t naddr, void **addrs);
void memory_alias_unmap(size_t size, size_t naddr, void **addrs);

Values in the address array must either be page-aligned or NULL to allow the operating system to choose, in which case the map address is written to the array.

It returns 0 on success. It may fail if the size is too small (0), too large, too many file descriptors, etc.

Pass the same pointers back to memory_alias_unmap to free the mappings. When called correctly it cannot fail, so there’s no return value.

The full source is here: memalias.c

POSIX

Starting with the simpler of the two functions, the POSIX implementation looks like so:

void
memory_alias_unmap(size_t size, size_t naddr, void **addrs)
{
    for (size_t i = 0; i < naddr; i++)
        munmap(addrs[i], size);
}

The complex part is creating the mapping:

int
memory_alias_map(size_t size, size_t naddr, void **addrs)
{
    char path[128];
    snprintf(path, sizeof(path), "/%s(%lu,%p)",
             __FUNCTION__, (long)getpid(), addrs);
    int fd = shm_open(path, O_RDWR | O_CREAT | O_EXCL, 0600);
    if (fd == -1)
        return -1;
    shm_unlink(path);
    ftruncate(fd, size);
    for (size_t i = 0; i < naddr; i++) {
        addrs[i] = mmap(addrs[i], size,
                        PROT_READ | PROT_WRITE, MAP_SHARED,
                        fd, 0);
        if (addrs[i] == MAP_FAILED) {
            memory_alias_unmap(size, i, addrs);
            close(fd);
            return -1;
        }
    }
    close(fd);
    return 0;
}

The shared object name includes the process ID and pointer array address, so there really shouldn’t be any non-malicious name collisions, even if called from multiple threads in the same process.

Otherwise it just walks the array setting up the mappings.

Windows

The Windows version is very similar.

void
memory_alias_unmap(size_t size, size_t naddr, void **addrs)
{
    (void)size;
    for (size_t i = 0; i < naddr; i++)
        UnmapViewOfFile(addrs[i]);
}

Since Windows tracks the size internally, it’s unneeded and ignored.

int
memory_alias_map(size_t size, size_t naddr, void **addrs)
{
    HANDLE m = CreateFileMapping(INVALID_HANDLE_VALUE,
                                 NULL,
                                 PAGE_READWRITE,
                                 0, size,
                                 NULL);
    if (m == NULL)
        return -1;
    DWORD access = FILE_MAP_ALL_ACCESS;
    for (size_t i = 0; i < naddr; i++) {
        addrs[i] = MapViewOfFileEx(m, access, 0, 0, size, addrs[i]);
        if (addrs[i] == NULL) {
            memory_alias_unmap(size, i, addrs);
            CloseHandle(m);
            return -1;
        }
    }
    CloseHandle(m);
    return 0;
}

In the future I’d like to find some unique applications of these multiple memory views.

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Chris Wellons