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Date:   Tue, 24 May 2022 19:23:01 -0400
From:   Steven Rostedt <rostedt@goodmis.org>
To:     Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        Jiri Olsa <jolsa@kernel.org>, linux-kernel@vger.kernel.org,
        netdev@vger.kernel.org, bpf@vger.kernel.org
Subject: Re: [PATCH] rethook: Reject getting a rethook if RCU is not
 watching
Message-ID: <20220524192301.0c2ab08a@gandalf.local.home>
In-Reply-To: <165189881197.175864.14757002789194211860.stgit@devnote2>
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On Sat,  7 May 2022 13:46:52 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

Is this expected to go through the BPF tree?

-- Steve


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