linux-kernel - Re: [PATCH] mm: Fix int overflow in callers of do_shrink

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Message-ID: <ec297f1e-529e-799b-b98c-51472cd64b15@virtuozzo.com>
Date:   Sat, 29 Sep 2018 02:58:54 +0300
From:   Kirill Tkhai <ktkhai@...tuozzo.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     gorcunov@...nvz.org, mhocko@...e.com, aryabinin@...tuozzo.com,
        hannes@...xchg.org, penguin-kernel@...ove.SAKURA.ne.jp,
        shakeelb@...gle.com, jbacik@...com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm: Fix int overflow in callers of do_shrink_slab()

On 29.09.2018 00:15, Andrew Morton wrote:
> On Fri, 28 Sep 2018 14:28:32 +0300 Kirill Tkhai <ktkhai@...tuozzo.com> wrote:
> 
>> do_shrink_slab() returns unsigned long value, and
>> the placing into int variable cuts high bytes off.
>> Then we compare ret and 0xfffffffe (since SHRINK_EMPTY
>> is converted to ret type).
>>
>> Thus, big number of objects returned by do_shrink_slab()
>> may be interpreted as SHRINK_EMPTY, if low bytes of
>> their value are equal to 0xfffffffe. Fix that
>> by declaration ret as unsigned long in these functions.
> 
> Sigh.  How many times has this happened.
> 
>> Reported-by: Cyrill Gorcunov <gorcunov@...nvz.org>
> 
> What did he report?  Was it code inspection?  Did the kernel explode? 
> etcetera.  I'm thinking that the fix should be backported but to
> determine that, we need to understand the end-user runtime effects, as
> always.  Please.

Yeah, it was just code inspection. It's need to be a really unlucky person
to meet this in real life -- the probability is very small.

The runtime effect would be the following. Such the unlucky person would
have a single shrinker, which is never called for a single memory cgroup,
despite there are objects charged.

Thanks,
Kirill