do_last(): don't let a bogus return value from ->open() et.al. to confuse us

do_last(): don't let a bogus return value from ->open() et.al. to confuse us

Orabug: stable_rc4

[ Upstream commit c80567c82ae4814a41287618e315a60ecf513be6 ]

... into returning a positive to path_openat(), which would interpret that
as "symlink had been encountered" and proceed to corrupt memory, etc.
It can only happen due to a bug in some ->open() instance or in some LSM
hook, etc., so we report any such event *and* make sure it doesn't trick
us into further unpleasantness.

Cc: stable@vger.kernel.org # v3.6+, at least
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Sasha Levin <sasha.levin@oracle.com>
(cherry picked from commit 3960cde3e356057bd60adce1b625a7d178b9581c)

Signed-off-by: Dan Duval <dan.duval@oracle.com>