Re: [PATCH v2] Fixing directly deferencing a __rcu pointer warning

From: "Michael S. Tsirkin" <mst@redhat.com>
To: Abhinav Singh <singhabhinav9051571833@gmail.com>
Cc: brauner@kernel.org, mjguzik@gmail.com, peterz@infradead.org,
	linux-kernel-mentees@lists.linuxfoundation.org,
	linux-kernel@vger.kernel.org, npiggin@gmail.com,
	mathieu.desnoyers@efficios.com, shakeelb@google.com,
	akpm@linux-foundation.org, surenb@google.com,
	michael.christie@oracle.com
Subject: Re: [PATCH v2] Fixing directly deferencing a __rcu pointer warning
Date: Thu, 26 Oct 2023 09:14:21 -0400	[thread overview]
Message-ID: <20231026091222-mutt-send-email-mst@kernel.org> (raw)
In-Reply-To: <20231026121621.358388-1-singhabhinav9051571833@gmail.com>

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@gmail.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.

> ---
>  kernel/fork.c | 4 ++--
>  1 file changed, 2 insertions(+), 2 deletions(-)
> 
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 10917c3e1f03..802b7bbe3d92 100644
> --- a/kernel/fork.c
> +++ b/kernel/fork.c
> @@ -2369,7 +2369,7 @@ __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 &&
> +		if (rcu_dereference(p->real_cred)->user != INIT_USER &&
>  		    !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN))
>  			goto bad_fork_cleanup_count;
>  	}
> @@ -2692,7 +2692,7 @@ __latent_entropy struct task_struct *copy_process(
>  			 */
>  			p->signal->has_child_subreaper = p->real_parent->signal->has_child_subreaper ||
>  							 p->real_parent->signal->is_child_subreaper;
> -			list_add_tail(&p->sibling, &p->real_parent->children);
> +			list_add_tail(&p->sibling, &(rcu_dereference(p->real_parent)->children));
>  			list_add_tail_rcu(&p->tasks, &init_task.tasks);
>  			attach_pid(p, PIDTYPE_TGID);
>  			attach_pid(p, PIDTYPE_PGID);
> -- 
> 2.39.2

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