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

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Date:   Wed, 25 Oct 2023 15:38:07 -0700
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Abhinav Singh <singhabhinav9051571833@...il.com>
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 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?