linux-kernel - Re: [PATCH] Fixing warning of directly dereferencing _

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Message-ID: <c4688c53-4206-4d54-aed9-f877697e8073@gmail.com>
Date:   Thu, 26 Oct 2023 04:57:42 +0530
From:   Abhinav Singh <singhabhinav9051571833@...il.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     brauner@...nel.org, surenb@...gle.com, mst@...hat.com,
        michael.christie@...cle.com, mathieu.desnoyers@...icios.com,
        mjguzik@...il.com, npiggin@...il.com, shakeelb@...gle.com,
        peterz@...radead.org, linux-kernel@...r.kernel.org,
        linux-kernel-mentees@...ts.linuxfoundation.org
Subject: Re: [PATCH] Fixing warning of directly dereferencing __rcu tagged

On 10/26/23 04:08, Andrew Morton wrote:
> On Thu, 26 Oct 2023 03:58:11 +0530 Abhinav Singh <singhabhinav9051571833@...il.com> 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.
> 
> Seems sensible.
> 
>> Like normal pointer there should be a check for null case when
>> further dereferencing the returned dereferenced __rcu pointer.
> 
> Why is this?
> 
>> --- a/kernel/fork.c
>> +++ b/kernel/fork.c
>> @@ -2369,7 +2369,9 @@ __latent_entropy struct task_struct *copy_process(
>>   
>>   	retval = -EAGAIN;
>>   	if (is_rlimit_overlimit(task_ucounts(p), UCOUNT_RLIMIT_NPROC, rlimit(RLIMIT_NPROC))) {
>> -		if (p->real_cred->user != INIT_USER &&
>> +		const struct cred *real_cred = rcu_dereference(p->real_cred);
>> +
>> +		if (real_cred && real_cred->user != INIT_USER &&
>>   		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
>>   			goto bad_fork_cleanup_count;
> 
> The old code assumes that p->read_cred cannot be NULL and the new code
> does nothing to make it possible that `real_cred' can be NULL?
> 
> In other words, I see no reason to add this new check for NULL?

Thank you for the response!

I thought it will be better to have check before accessing it, just so 
we dont have any segmentation fault in future.

Also I just noticed there are two more places where direct dereferencing 
of __rcu pointer is done in this same file. Should I do those changes in 
this patch ?