[LTP] ftruncate04 broken on kernels without mandatory locking

[Jiri Jaburek <jjaburek@redhat.com>]

Upstream kernel commit

  9e8925b67a809bb27ce4b7d352d67f25cf1d7fc5
  locks: Allow disabling mandatory locking at compile time

added a config option to remove support for mandatory locking
(mount -o mand, MS_MANDLOCK), which went into v4.5.

Some distributions (like Fedora) already disable it, causing
ftruncate04 to fail:

ftruncate04    0  TINFO  :  TMPDIR does not support mandatory locks
ftruncate04    0  TINFO  :  Found free device '/dev/loop0'
ftruncate04    0  TINFO  :  Formatting /dev/loop0 with ext2 opts=''
extra opts=''
mke2fs 1.42.13 (17-May-2015)
ftruncate04    1  TBROK  :  safe_macros.c:728: ftruncate04.c:247:
mount(/dev/loop0, dir/, ext2, 64, (nil)) failed: errno=EPERM(1):
Operation not permitted
ftruncate04    2  TBROK  :  safe_macros.c:728: Remaining cases broken

and indeed

$ mount /dev/loop0 /mnt/
$ umount /mnt
$ mount /dev/loop0 /mnt/ -o mand
mount: permission denied

The question is how to best fix the testcase - do the mount unsafely
and assume EPERM should be TCONF? Or somehow check kernel config?
Maybe do the mount first without MS_MANDLOCK (to rule out other perm
issues) and then remount with MS_MANDLOCK, checking EPERM?

Any other ideas?

Thanks,
Jiri