Re: [PATCH] mm: memcg: initialize *locked in memcg1_oom_prepare() stub

[Shakeel Butt <shakeel.butt@linux.dev>]

On Fri, Jun 26, 2026 at 05:43:02AM -0700, Breno Leitao wrote:
> mem_cgroup_oom() passes an uninitialized "locked" to memcg1_oom_prepare()
> and reads it back in memcg1_oom_finish():
> 
> 	bool locked, ret;
> 	...
> 	if (!memcg1_oom_prepare(memcg, &locked))
> 		return false;
> 	ret = mem_cgroup_out_of_memory(memcg, mask, order);
> 	memcg1_oom_finish(memcg, locked);
> 
> This relies on memcg1_oom_prepare() setting *locked whenever it returns
> true.  The CONFIG_MEMCG_V1=y version does, but the stub used when
> CONFIG_MEMCG_V1=n returns true without touching *locked, so
> memcg1_oom_finish() consumes an uninitialized value.

On CONFIG_MEMCG_V1=n, memcg1_oom_finish() is an empty function and I assume
compiler will just remove it completely. Maybe on CONFIG_UBSAN=y kernel,
compiler is not removing memcg1_oom_finish90.

> On a memcg OOM this
> is reported by UBSAN:
> 
>   UBSAN: invalid-load in mm/memcontrol.c:1932:27
>   load of value 0 is not a valid value for type 'bool' (aka '_Bool')
> 
> Initialize *locked to false in the stub; with cgroup v1 compiled out
> there is no OOM lock to take.
> 
> Fixes: e93d4166b40a ("mm: memcg: put cgroup v1-specific code under a config option")
> Cc: stable@vger.kernel.org
> Signed-off-by: Breno Leitao <leitao@debian.org>

Anyways, this is not a performance critical code path, so this is fine.

Acked-by: Shakeel Butt <shakeel.butt@linux.dev>