Subject: Re: [PATCH v3 5/5] selftests: add fchmodat4(2) selftest

From: Florian Weimer <[email-masked]>
Date: Tue, 11 Jul 2023 14:10:58 +0200

* Alexey Gladkov:

> The test marks as skipped if a syscall with the AT_SYMLINK_NOFOLLOW flag
> fails. This is because not all filesystems support changing the mode
> bits of symlinks properly. These filesystems return an error but change
> the mode bits:
>
> newfstatat(4, "regfile", {st_mode=S_IFREG|0640, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
> newfstatat(4, "symlink", {st_mode=S_IFLNK|0777, st_size=7, ...}, AT_SYMLINK_NOFOLLOW) = 0
> syscall_0x1c3(0x4, 0x55fa1f244396, 0x180, 0x100, 0x55fa1f24438e, 0x34) = -1 EOPNOTSUPP (Operation not supported)
> newfstatat(4, "regfile", {st_mode=S_IFREG|0640, st_size=0, ...}, AT_SYMLINK_NOFOLLOW) = 0
>
> This happens with btrfs and xfs:
>
>  $ /kernel/tools/testing/selftests/fchmodat4/fchmodat4_test
>  TAP version 13
>  1..1
>  ok 1 # SKIP fchmodat4(symlink)
>  # Totals: pass:0 fail:0 xfail:0 xpass:0 skip:1 error:0
>
>  $ stat /tmp/ksft-fchmodat4.*/symlink
>    File: /tmp/ksft-fchmodat4.3NCqlE/symlink -> regfile
>    Size: 7               Blocks: 0          IO Block: 4096   symbolic link
>  Device: 7,0     Inode: 133         Links: 1
>  Access: (0600/lrw-------)  Uid: (    0/    root)   Gid: (    0/    root)
>
> Signed-off-by: Alexey Gladkov <[email-masked]>

This looks like a bug in those file systems?

As an extra test, “echo 3 > /proc/sys/vm/drop_caches” sometimes has
strange effects in such cases because the bits are not actually stored
on disk, only in the dentry cache.

Thanks,
Florian