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

[Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>]

Tejun Heo (tj-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org) wrote:
> 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-DgEjT+Ai2ygdnm+yROfE0A@public.gmane.org>

Reviewed-by: Mandeep Singh Baines <msb-F7+t8E8rja9g9hUCZPvPmw@public.gmane.org>

> Reported-by: Jiri Kosina <jkosina-AlSwsSmVLrQ@public.gmane.org>
> LKML-Reference: <alpine.LNX.2.00.1203282354440.25526-ztGlSCb7Y1iN3ZZ/Hiejyg@public.gmane.org>
> ---
> 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;