Re: [PATCH bpf v2 2/2] rethook: Reject getting a rethook if RCU is not watching

[Steven Rostedt <rostedt@goodmis.org>]

On Wed,  8 Jun 2022 01:11:12 +0900
"Masami Hiramatsu (Google)" <mhiramat@kernel.org> wrote:

> From: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> 
> Since the rethook_recycle() will involve the call_rcu() for reclaiming
> the rethook_instance, the rethook must be set up at the RCU available
> context (non idle). This rethook_recycle() in the rethook trampoline
> handler is inevitable, thus the RCU available check must be done before
> setting the rethook trampoline.
> 
> This adds a rcu_is_watching() check in the rethook_try_get() so that
> it will return NULL if it is called when !rcu_is_watching().
> 
> Fixes: 54ecbe6f1ed5 ("rethook: Add a generic return hook")
> Signed-off-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
> ---
>  kernel/trace/rethook.c |    9 +++++++++
>  1 file changed, 9 insertions(+)
> 
> diff --git a/kernel/trace/rethook.c b/kernel/trace/rethook.c
> index b56833700d23..c69d82273ce7 100644
> --- a/kernel/trace/rethook.c
> +++ b/kernel/trace/rethook.c
> @@ -154,6 +154,15 @@ struct rethook_node *rethook_try_get(struct rethook *rh)
>  	if (unlikely(!handler))
>  		return NULL;
>  
> +	/*
> +	 * This expects the caller will set up a rethook on a function entry.
> +	 * When the function returns, the rethook will eventually be reclaimed
> +	 * or released in the rethook_recycle() with call_rcu().
> +	 * This means the caller must be run in the RCU-availabe context.
> +	 */
> +	if (unlikely(!rcu_is_watching()))
> +		return NULL;

Acked-by: Steven Rostedt (Google) <rostedt@goodmis.org>

-- Steve

> +
>  	fn = freelist_try_get(&rh->pool);
>  	if (!fn)
>  		return NULL;