Re: [PATCH 1/3] rcu: Return early if callback is not specified

[Joel Fernandes <joelagnelf@nvidia.com>]

On 6/10/2025 1:34 PM, Uladzislau Rezki (Sony) wrote:
> Currently the call_rcu() API does not check whether a callback
> pointer is NULL. If NULL is passed, rcu_core() will try to invoke
> it, resulting in NULL pointer dereference and a kernel crash.
> 
> To prevent this and improve debuggability, this patch adds a check
> for NULL and emits a kernel stack trace to help identify a faulty
> caller.
> 
> Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>

Reviewed-by: Joel Fernandes <joelagnelf@nvidia.com>

I will add this first one (only this one since we're discussing the others) to a
new rcu/fixes-for-6.16 branch, but let me know if any objections.

Will push that branch out during -rc2 or -rc3 after sufficient testing.

thanks,

 - Joel

> ---
>  kernel/rcu/tree.c | 4 ++++
>  1 file changed, 4 insertions(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index e8a4b720d7d2..14d4499c6fc3 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -3072,6 +3072,10 @@ __call_rcu_common(struct rcu_head *head, rcu_callback_t func, bool lazy_in)
>  	/* Misaligned rcu_head! */
>  	WARN_ON_ONCE((unsigned long)head & (sizeof(void *) - 1));
>  
> +	/* Avoid NULL dereference if callback is NULL. */
> +	if (WARN_ON_ONCE(!func))
> +		return;
> +
>  	if (debug_rcu_head_queue(head)) {
>  		/*
>  		 * Probable double call_rcu(), so leak the callback.