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Date: Tue, 16 Feb 2021 12:53:37 -0500
From: Rich Felker <dalias@...c.org>
To: musl@...ts.openwall.com
Subject: Re: [PATCH] handle AT_SYMLINK_NOFOLLOW
On Tue, Feb 16, 2021 at 09:30:22AM -0800, Khem Raj wrote:
> From: Richard Purdie <richard.purdie@...uxfoundation.org>
>
> For faccessat(), AT_SYMLINK_NOFOLLOW is a supported flag by the
> Linux kernel and musl should really handle it correctly rather
> than return EINVAL. Noticed from code in systemd.
>
> Signed-off-by: Richard Purdie <richard.purdie@...uxfoundation.org>
> Signed-off-by: Khem Raj <raj.khem@...il.com>
This patch does not work. It just makes the error go away by silently
doing the wrong thing instead of reporting it.
> ---
> src/unistd/faccessat.c | 11 ++++++-----
> 1 file changed, 6 insertions(+), 5 deletions(-)
>
> diff --git a/src/unistd/faccessat.c b/src/unistd/faccessat.c
> index 8e8689c1..22c30bc6 100644
> --- a/src/unistd/faccessat.c
> +++ b/src/unistd/faccessat.c
> @@ -9,6 +9,7 @@ struct ctx {
> const char *filename;
> int amode;
> int p;
> + int flag;
> };
>
> static int checker(void *p)
> @@ -18,7 +19,7 @@ static int checker(void *p)
> if (__syscall(SYS_setregid, __syscall(SYS_getegid), -1)
> || __syscall(SYS_setreuid, __syscall(SYS_geteuid), -1))
> __syscall(SYS_exit, 1);
> - ret = __syscall(SYS_faccessat, c->fd, c->filename, c->amode, 0);
> + ret = __syscall(SYS_faccessat, c->fd, c->filename, c->amode, c->flag & AT_SYMLINK_NOFOLLOW);
The SYS_faccessat syscall does not take a flags argument. That's the
whole reason for having this emulation mechanism. The 0 being left
there is a historical error and it should be removed; the kernel does
not inspect it and is not intended to sincw the old syscall has only 3
arguments.
> __syscall(SYS_write, c->p, &ret, sizeof ret);
> return 0;
> }
> @@ -30,11 +31,11 @@ int faccessat(int fd, const char *filename, int amode, int flag)
> if (ret != -ENOSYS) return __syscall_ret(ret);
> }
>
> - if (flag & ~AT_EACCESS)
> + if (flag & ~(AT_EACCESS | AT_SYMLINK_NOFOLLOW))
> return __syscall_ret(-EINVAL);
EINVAL is the normal error code Linux returns for flags not
recognized/supported by the running kernel. It's also the
POSIX-documented "may fail" code for this. The code *before* this
test, using the new SYS_faccessat2 syscall, handles the
AT_SYMLINK_NOFOLLOW flag if you have a kernel that can support it.
I suppose it might be possible to emulate AT_SYMLINK_NOFOLLOW on
old kernels using procfs magic symlinks, but I haven't checked the
details to be sure, and IMO it does not make sense to make the
fallback code here more complex when it's for a nonstandard feature
that's not expected to be present on old kernels, rather than a
POSIX-mandated feature like AT_EACCESS.
> - if (!flag || (getuid()==geteuid() && getgid()==getegid()))
> - return syscall(SYS_faccessat, fd, filename, amode);
> + if (!(flag & AT_EACCESS) || (getuid()==geteuid() && getgid()==getegid()))
> + return syscall(SYS_faccessat, fd, filename, amode, flag);
Same issue here -- there is no flag argument.
Rich
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