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

From: Alexey Gladkov <[email-masked]>
Date: Tue, 11 Jul 2023 15:38:03 +0200

On Tue, Jul 11, 2023 at 02:10:58PM +0200, Florian Weimer wrote:
> * 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?

To me this looks like a bug. I'm fine if the operation ends with
EOPNOTSUPP, but in that case the mode bits shouldn't change.

> 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.

tmpfs
syscall_0x1c3(0xffffff9c, 0x7ffd58758574, 0, 0x100, 0x7f6cf18adc70, 0x7ffd58756ad8) = 0
+++ exited with 0 +++
l--------- 1 root root 1 Jul 11 16:36 /tmp/dir/link -> f
=== dropping caches ===
l--------- 1 root root 1 Jul 11 16:36 /tmp/dir/link -> f

ext4
syscall_0x1c3(0xffffff9c, 0x7ffedfdb4574, 0, 0x100, 0x7f7f40b45c70, 0x7ffedfdb3ae8) = -1 EOPNOTSUPP (Operation not supported)
+++ exited with 1 +++
l--------- 1 root root 1 Jul 11 16:36 /tmp/dir/link -> f
=== dropping caches ===
l--------- 1 root root 1 Jul 11 16:36 /tmp/dir/link -> f

xfs
syscall_0x1c3(0xffffff9c, 0x7ffcd03ce574, 0, 0x100, 0x7ff2f2980c70, 0x7ffcd03cdd38) = -1 EOPNOTSUPP (Operation not supported)
+++ exited with 1 +++
l--------- 1 root root 1 Jul 11 16:36 /tmp/dir/link -> f
=== dropping caches ===
l--------- 1 root root 1 Jul 11 16:36 /tmp/dir/link -> f

btrfs
syscall_0x1c3(0xffffff9c, 0x7fff13d2e574, 0, 0x100, 0x7f9b67f59c70, 0x7fff13d2ca88) = -1 EOPNOTSUPP (Operation not supported)
+++ exited with 1 +++
l--------- 1 root root 1 Jul 11 16:36 /tmp/dir/link -> f
=== dropping caches ===
l--------- 1 root root 1 Jul 11 16:36 /tmp/dir/link -> f

reiserfs
syscall_0x1c3(0xffffff9c, 0x7ffdf75af574, 0, 0x100, 0x7f7ad0634c70, 0x7ffdf75ae478) = 0
+++ exited with 0 +++
l--------- 1 root root 1 Jul 11 16:43 /tmp/dir/link -> f
=== dropping caches ===
l--------- 1 root root 1 Jul 11 16:43 /tmp/dir/link -> f

-- 
Rgrds, legion