linux-kernel - [PATCH for-3.4-fixes] cgroup: cgroup_attach_task() could return -errno after success

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Message-ID: <20120329162620.GD28941@google.com>
Date:	Thu, 29 Mar 2012 09:26:20 -0700
From:	Tejun Heo <tj@...nel.org>
To:	Jiri Kosina <jkosina@...e.cz>
Cc:	Mandeep Singh Baines <msb@...omium.org>, lizefan@...wei.com,
	linux-kernel@...r.kernel.org, cgroups@...r.kernel.org,
	Paul Menage <paul@...lmenage.org>,
	Kay Sievers <kay.sievers@...y.org>
Subject: [PATCH for-3.4-fixes] cgroup: cgroup_attach_task() could return
 -errno after success

61d1d219c4 "cgroup: remove extra calls to find_existing_css_set" made
cgroup_task_migrate() return void.  An unfortunate side effect was
that cgroup_attach_task() was depending on that function's return
value to clear its @retval on the success path.  On cgroup mounts
without any subsystem with ->can_attach() callback,
cgroup_attach_task() ended up returning @retval without initializing
it on success.

For some reason, gcc failed to warn about it and it didn't cause
cgroup_attach_task() to return non-zero value in many cases, probably
due to difference in register allocation.  When the problem
materializes, systemd fails to populate /systemd cgroup mount and
fails to boot.

Fix it by initializing @retval to zero on declaration.

Signed-off-by: Tejun Heo <tj@...nel.org>
Reported-by: Jiri Kosina <jkosina@...e.cz>
LKML-Reference: <alpine.LNX.2.00.1203282354440.25526@...ox.suse.cz>
---
Jiri, can you please confirm the fix?

Thanks.

 kernel/cgroup.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/kernel/cgroup.c b/kernel/cgroup.c
index f4ea4b6..ed64cca 100644
--- a/kernel/cgroup.c
+++ b/kernel/cgroup.c
@@ -1883,7 +1883,7 @@ static void cgroup_task_migrate(struct cgroup *cgrp, struct cgroup *oldcgrp,
  */
 int cgroup_attach_task(struct cgroup *cgrp, struct task_struct *tsk)
 {
-	int retval;
+	int retval = 0;
 	struct cgroup_subsys *ss, *failed_ss = NULL;
 	struct cgroup *oldcgrp;
 	struct cgroupfs_root *root = cgrp->root;
--
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