[ 046/171 ] mqueue: sys_mq_open: do not call mnt_drop_write() if read-only

From: Steven Rostedt
Date: Thu Apr 11 2013 - 16:29:31 EST stable review patch.
If anyone has any objections, please let me know.


From: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>

[ Upstream commit 38d78e587d4960d0db94add518d27ee74bad2301 ]

mnt_drop_write() must be called only if mnt_want_write() succeeded,
otherwise the mnt_writers counter will diverge.

mnt_writers counters are used to check if remounting FS as read-only is
OK, so after an extra mnt_drop_write() call, it would be impossible to
remount mqueue FS as read-only. Besides, on umount a warning would be
printed like this one:

[ BUG: bad unlock balance detected! ]
3.9.0-rc3 #5 Not tainted
a.out/12486 is trying to release lock (sb_writers) at:
but there are no more locks to release!

Signed-off-by: Vladimir Davydov <vdavydov@xxxxxxxxxxxxx>
Cc: Doug Ledford <dledford@xxxxxxxxxx>
Cc: KOSAKI Motohiro <kosaki.motohiro@xxxxxxxxxxxxxx>
Cc: "Eric W. Biederman" <ebiederm@xxxxxxxxxxxx>
Cc: Al Viro <viro@xxxxxxxxxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
Signed-off-by: Steven Rostedt <rostedt@xxxxxxxxxxx>
ipc/mqueue.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/ipc/mqueue.c b/ipc/mqueue.c
index 9a08acc..9e4cf7f 100644
--- a/ipc/mqueue.c
+++ b/ipc/mqueue.c
@@ -841,7 +841,8 @@ out_putfd:
fd = error;
- mnt_drop_write(mnt);
+ if (!ro)
+ mnt_drop_write(mnt);
return fd;

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