Re: [PATCH] mm: kmem: properly initialize local objcg variable in current_obj_cgroup()

[Shakeel Butt]

On Wed, Nov 15, 2023 at 11:04 PM Vlastimil Babka <vbabka@xxxxxxx> wrote:
>
> On 11/16/23 03:51, Roman Gushchin wrote:
> > Actually the problem is caused by uninitialized local variable in
> > current_obj_cgroup(). If the root memory cgroup is set as an active
> > memory cgroup for a charging scope (as in the trace, where systemd
> > tries to create the first non-root cgroup, so the parent cgroup is
> > the root cgroup), the "for" loop is skipped and uninitialized objcg is
> > returned, causing a panic down the accounting stack.
> >
> > The fix is trivial: initialize the objcg variable to NULL
> > unconditionally before the "for" loop.
> >
> > Fixes: e86828e5446d ("mm: kmem: scoped objcg protection")
> > Reported-by: Erhard Furtner <erhard_f@xxxxxxxxxxx>
> > Closes: https://github.com/ClangBuiltLinux/linux/issues/1959
> > Signed-off-by: Roman Gushchin (Cruise) <roman.gushchin@xxxxxxxxx>
> > Cc: Shakeel Butt <shakeelb@xxxxxxxxxx>
> > Cc: Vlastimil Babka <vbabka@xxxxxxx>
> > Cc: David Rientjes <rientjes@xxxxxxxxxx>
> > Cc: Dennis Zhou <dennis@xxxxxxxxxx>
> > Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
> > Cc: Michal Hocko <mhocko@xxxxxxxxxx>
> > Cc: Muchun Song <muchun.song@xxxxxxxxx>
> > Cc: stable@xxxxxxxxxxxxxxx
>
> Acked-by: Vlastimil Babka <vbabka@xxxxxxx>
>
> We could also do this to make it less confusing?
>
> diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> index 774bd6e21e27..a08bcec661b6 100644
> --- a/mm/memcontrol.c
> +++ b/mm/memcontrol.c
> @@ -3175,7 +3175,6 @@ __always_inline struct obj_cgroup *current_obj_cgroup(void)
>                 objcg = rcu_dereference_check(memcg->objcg, 1);
>                 if (likely(objcg))
>                         break;
> -               objcg = NULL;
>         }
>
>         return objcg;
>
>
> > ---
> >  mm/memcontrol.c | 1 +
> >  1 file changed, 1 insertion(+)
> >
> > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > index 774bd6e21e27..b138501e6489 100644
> > --- a/mm/memcontrol.c
> > +++ b/mm/memcontrol.c
> > @@ -3165,6 +3165,7 @@ __always_inline struct obj_cgroup *current_obj_cgroup(void)
> >       return NULL;
> >
> >  from_memcg:
> > +     objcg = NULL;
> >       for (; !mem_cgroup_is_root(memcg); memcg = parent_mem_cgroup(memcg)) {
> >               /*
> >                * Memcg pointer is protected by scope (see set_active_memcg())
>

For Roman's change and Vlastimil's cleanup:

Acked-by: Shakeel Butt <shakeelb@xxxxxxxxxx>