linux-kernel - Re: [PATCH] tracing: fprobe: fix suspicious rcu usage in fprobe

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Message-ID: <aLa2D4It1Zxc7bs0@gondor.apana.org.au>
Date: Tue, 2 Sep 2025 17:17:03 +0800
From: Herbert Xu <herbert@...dor.apana.org.au>
To: Menglong Dong <dongml2@...natelecom.cn>
Cc: mhiramat@...nel.org, rostedt@...dmis.org,
	mathieu.desnoyers@...icios.com, linux-kernel@...r.kernel.org,
	linux-trace-kernel@...r.kernel.org, oliver.sang@...el.com
Subject: Re: [PATCH] tracing: fprobe: fix suspicious rcu usage in fprobe_entry

Menglong Dong <dongml2@...natelecom.cn> wrote:
>
> diff --git a/kernel/trace/fprobe.c b/kernel/trace/fprobe.c
> index fb127fa95f21..fece0f849c1c 100644
> --- a/kernel/trace/fprobe.c
> +++ b/kernel/trace/fprobe.c
> @@ -269,7 +269,9 @@ static int fprobe_entry(struct ftrace_graph_ent *trace, struct fgraph_ops *gops,
>        if (WARN_ON_ONCE(!fregs))
>                return 0;
> 
> +       rcu_read_lock();
>        head = rhltable_lookup(&fprobe_ip_table, &func, fprobe_rht_params);
> +       rcu_read_unlock();
>        reserved_words = 0;
>        rhl_for_each_entry_rcu(node, pos, head, hlist) {
>                if (node->addr != func)

Actually this isn't quite right.  I know that it is a false-positive
so that it's actually safe, but if you're going to mark it with
rcu_read_lock, it should cover both the lookup as well as the
dereference which happens in the loop rhl_for_each_entry_rcu.

Thanks,
-- 
Email: Herbert Xu <herbert@...dor.apana.org.au>
Home Page: http://gondor.apana.org.au/~herbert/
PGP Key: http://gondor.apana.org.au/~herbert/pubkey.txt