Re: [musl] faccessat behavior on old kernels (<5.8)

[Rich Felker <dalias@libc.org>]

On Mon, Jun 19, 2023 at 11:49:44PM +0200, Gabriel Ravier wrote:
> On 6/19/23 20:14, Mike Gilbert wrote:
> >I am not subscribed, so please CC me on replies.
> >
> >I received a bug report on Gentoo Linux.
> >
> >https://bugs.gentoo.org/908765
> >
> >There appears to be a difference in behavior between musl and glibc
> >when running on Linux kernels that lack support for the faccessat2
> >system call.
> >
> >On glibc, the following call returns 0. On musl, it returns -1 and
> >sets errno to EINVAL.
> >
> >faccessat(AT_FDCWD, "/dev/null", F_OK, AT_SYMLINK_NOFOLLOW);
> >
> >On older kernels, the underlying faccessat2 syscall returns -1 / ENOSYS.
> >glibc follows that up with an fstatat64 with equivalent arguments.
> >musl immediately fails with -1 / EINVAL.
> >
> >Relevant code:
> >
> >https://sourceware.org/git/?p=glibc.git;a=blob;f=sysdeps/unix/sysv/linux/faccessat.c;h=0ccbd778b5f4d61f9121b6aeb59782c21ae647a0;hb=a704fd9a133bfb10510e18702f48a6a9c88dbbd5#l36
> >
> >https://git.musl-libc.org/cgit/musl/tree/src/unistd/faccessat.c?h=v1.2.4#n34
> 
> To be more precise, the difference is that musl refuses to use its
> fallback when `AT_SYMLINK_NOFOLLOW` is set, whereas glibc does so -
> I don't know if musl's workaround would work in this case, though,
> given how different it is from anything glibc does.

Yes. Being that AT_SYMLINK_NOFOLLOW is nonstandard functionality for
faccessat, it wasn't even originally implemented. It's available as a
Linux extension if you have a version of Linux that provides a native
syscall to do it, but that's all.

If there were a compelling reason to emulate it, that could probably
be done, but so far there doesn't seem to have been one. The access
family of functions have inherent TOCTOU races and the generally bad
problem of using the real ids rather than effective ids to compute
access permission. It's almost always better to just attempt the
operation you want rather than using one of the access family.

Rich