Subject: Re: [PATCH 3/9] Makefile: turn on NO_MMAP when building with ASan

From: Collin Funk <[email-masked]>
Date: Wed, 12 Nov 2025 00:17:24 -0800

Jeff King <[email-masked]> writes:

> Git often uses mmap() to access on-disk files. This leaves a blind spot
> in our SANITIZE=address builds, since ASan does not seem to handle mmap
> at all. Nor does the OS notice most out-of-bounds access, since it tends
> to round up to the nearest page size (so depending on how big the map
> is, you might have to overrun it by up to 4095 bytes to trigger a
> segfault).
>
> The previous commit demonstrates a memory bug that we missed. We could
> have made a new test where the out-of-bounds access was much larger, or
> where the mapped file ended closer to a page boundary. But the point of
> running the test suite with sanitizers is to catch these problems
> without having to construct specific tests.
>
> Let's enable NO_MMAP for our ASan builds by default, which should give
> us better coverage. This does increase the memory usage of Git, since
> we're copying from the filesystem into heap. But the repositories in the
> test suite tend to be small, so the overhead isn't really noticeable
> (and ASan already has quite a performance penalty).
>
> There are a few other known bugs that this patch will help flush out.
> However, they aren't directly triggered in the test suite (yet). So
> it's safe to turn this on now without breaking the test suite, which
> will help us add new tests to demonstrate those other bugs as we fix
> them.
>
> Signed-off-by: Jeff King <[email-masked]>

I see that an interceptor was added in 2023 [1]. Maybe your compiler is
older than that?

On my system:

    $ cat main.c 
    #include <stdlib.h>
    #include <unistd.h>
    #include <sys/mman.h>
    int
    main (void)
    {
      char *ptr = mmap (NULL, getpagesize (), PROT_READ | PROT_WRITE,
    		    MAP_ANONYMOUS, -1, 0);
      if (ptr == NULL)
        abort ();
      ptr[getpagesize () + 1] = 'a';
      return 0;
    }
    $ gcc --version | head -n 1
    gcc (GCC) 15.2.1 20251022 (Red Hat 15.2.1-3)
    $ clang --version | head -n 1
    clang version 21.1.4 (Fedora 21.1.4-1.fc43)
    $ gcc -fsanitize=address main.c && ./a.out 2>&1 | grep ^SUMMARY:
    SUMMARY: AddressSanitizer: SEGV (/home/collin/a.out+0x400554) (BuildId: 1b7a82189bfffb3f73d420e138b9859add25901a) in main
    $ clang -fsanitize=address main.c && ./a.out 2>&1 | grep ^SUMMARY:
    SUMMARY: AddressSanitizer: SEGV (/home/collin/a.out+0x4e9ee6) (BuildId: aca1d168eacebaa239082d8a45ab74c8470f4b31) in main

Collin

[1] https://github.com/llvm/llvm-project/commit/a34e702aa16fde4cc76e9360d985a64e008e0b23