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

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Date:   Wed, 25 Oct 2023 16:50:02 -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 04:57:42 +0530 Abhinav Singh <singhabhinav9051571833@...il.com> wrote:

> On 10/26/23 04:08, Andrew Morton wrote:
> >> +++ 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.

That would be adding code which has no effect?

> 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 ?

I don't see why.  rcu_dereference(p) cannot return NULL if `p' is non-NULL?