linux-kernel - Re: [PATCH v2] Fixing directly deferencing a _

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Message-ID: <b0654dab-eb43-444e-a820-db169812e1ba@efficios.com>
Date:   Thu, 26 Oct 2023 10:51:36 -0400
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Mateusz Guzik <mjguzik@...il.com>,
        "Michael S. Tsirkin" <mst@...hat.com>
Cc:     Abhinav Singh <singhabhinav9051571833@...il.com>,
        akpm@...ux-foundation.org, brauner@...nel.org, surenb@...gle.com,
        michael.christie@...cle.com, npiggin@...il.com,
        shakeelb@...gle.com, peterz@...radead.org,
        linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH v2] Fixing directly deferencing a __rcu pointer warning

On 2023-10-26 10:06, Mateusz Guzik wrote:
> On 10/26/23, Michael S. Tsirkin <mst@...hat.com> wrote:
>> On Thu, Oct 26, 2023 at 05:46:21PM +0530, Abhinav Singh wrote:
>>> This patch fixes the warning about directly dereferencing a pointer
>>> tagged with __rcu annotation.
>>>
>>> Dereferencing the pointers tagged with __rcu directly should
>>> always be avoided according to the docs. There is a rcu helper
>>> functions rcu_dereference(...) to use when dereferencing a __rcu
>>> pointer. This functions returns the non __rcu tagged pointer which
>>> can be dereferenced just like a normal pointers.
>>>
>>> Signed-off-by: Abhinav Singh <singhabhinav9051571833@...il.com>
>>
>> Well yes but these need to be called under rcu_read_lock.
>> Who does it here?
>> If no one then maybe you found an actual bug and we need to
>> fix it not paper over it.
>>
> 
> There is no bug here.
> 
> p is the newly created thread, ->real_cred was initialized just prior
> to this code and there is nobody to whack the creds from under it.
> 
> Second bit in the patch changes one real_parent deref, but leaves 2
> others just above it. Once more no bug since the entire thing happens
> under tasklist_lock, but the patch should either sort all these cases
> or none.

Drive-by comment: perhaps use rcu_dereference_protected() ?

Thanks,

Mathieu

> 
> I think it would help if the submitter had shown warnings they see.
> 

-- 
Mathieu Desnoyers
EfficiOS Inc.
https://www.efficios.com