* 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 <legion@kernel.org>

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