linux-kernel - Re: [PATCH] mm: kmem: properly initialize local objcg variable in current_obj

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Date:   Thu, 16 Nov 2023 16:13:45 +0100
From:   Vlastimil Babka <vbabka@...e.cz>
To:     Erhard Furtner <erhard_f@...lbox.org>
Cc:     Roman Gushchin <roman.gushchin@...ux.dev>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, Shakeel Butt <shakeelb@...gle.com>,
        David Rientjes <rientjes@...gle.com>,
        Dennis Zhou <dennis@...nel.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Michal Hocko <mhocko@...nel.org>,
        Muchun Song <muchun.song@...ux.dev>, stable@...r.kernel.org
Subject: Re: [PATCH] mm: kmem: properly initialize local objcg variable in
 current_obj_cgroup()

On 11/16/23 15:56, Erhard Furtner wrote:
> On Thu, 16 Nov 2023 08:04:18 +0100
> Vlastimil Babka <vbabka@...e.cz> 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@...lbox.org>
>> > Closes: https://github.com/ClangBuiltLinux/linux/issues/1959
>> > Signed-off-by: Roman Gushchin (Cruise) <roman.gushchin@...ux.dev>
>> > Cc: Shakeel Butt <shakeelb@...gle.com>
>> > Cc: Vlastimil Babka <vbabka@...e.cz>
>> > Cc: David Rientjes <rientjes@...gle.com>
>> > Cc: Dennis Zhou <dennis@...nel.org>
>> > Cc: Johannes Weiner <hannes@...xchg.org>
>> > Cc: Michal Hocko <mhocko@...nel.org>
>> > Cc: Muchun Song <muchun.song@...ux.dev>
>> > Cc: stable@...r.kernel.org  
>> 
>> Acked-by: Vlastimil Babka <vbabka@...e.cz>
>> 
>> 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;
>> 
>> 
> 
> I can confirm the 1st patch from Roman fixes the issue on my amd64 and on my i686 box.

Good.

> The 2nd patch from Vlastimil unfortunately does not (only tried on amd64).

Ah no, I meant mine as an additional related cleanup that's related enough
that it can be part of Roman's fix. But it's not a fix on its own.
> Regards,
> Erhard