Re: [PATCH v3] Fixing directly deferencing a __rcu pointer warning

[Abhinav Singh]

On 10/28/23 15:52, 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
> function rcu_dereference(...) to use when dereferencing a __rcu
> pointer. This function returns the non __rcu tagged pointer which
> can be dereferenced just like a normal pointer.
>
> Signed-off-by: Abhinav Singh <singhabhinav9051571833@xxxxxxxxx>
> ---
> v1 -> v2 : added rcu_dereference(...) at line 2694
> v2 -> v3 : added rcu_dereference(...) at line 2695
>
>   kernel/fork.c | 8 ++++----
>   1 file changed, 4 insertions(+), 4 deletions(-)
>
> diff --git a/kernel/fork.c b/kernel/fork.c
> index 10917c3e1f03..e78649974669 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;
>   	}
> @@ -2690,9 +2690,9 @@ __latent_entropy struct task_struct *copy_process(
>   			 * tasklist_lock with adding child to the process tree
>   			 * for propagate_has_child_subreaper optimization.
>   			 */
> -			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);
> +			p->signal->has_child_subreaper = rcu_dereference(p->real_parent)->signal->has_child_subreaper ||
> +							rcu_dereference(p->real_parent)->signal->is_child_subreaper;
> +			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);

I tested the above with these two config enabled, CONFIG_PROVE_LOCKING 
and CONFIG_PROVE_RCU. In qemu it booted fine without any issues. I then 
checked dmesg log (inside booted qemu envirnoment) for any issues with 
rcu, but didnt get any error or warning.

I didnt receive any warning message "suspicious rcu_dereference_check() 
usage" atleast from sparse tool or in the dmesg log as suggested by Oleg 
Nesterov.