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Date: Mon, 19 Jun 2023 23:49:44 +0200
From: Gabriel Ravier <gabravier@...il.com>
To: musl@...ts.openwall.com, Mike Gilbert <floppym@...too.org>
Subject: Re: faccessat behavior on old kernels (<5.8)
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.
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