Re: [PATCH] Fixing warning of directly dereferencing __rcu tagged

[Abhinav Singh]

On 10/26/23 04:08, Andrew Morton wrote:
> On Thu, 26 Oct 2023 03:58:11 +0530 Abhinav Singh <singhabhinav9051571833@xxxxxxxxx> 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 ?