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

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Message-ID: <aLfCa1FkLc3T4QI3@gondor.apana.org.au>
Date: Wed, 3 Sep 2025 12:22:03 +0800
From: Herbert Xu <herbert@...dor.apana.org.au>
To: menglong.dong@...ux.dev
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

On Tue, Sep 02, 2025 at 05:50:32PM +0800, menglong.dong@...ux.dev wrote:
> On 2025/9/2 17:17 Herbert Xu <herbert@...dor.apana.org.au> write:
> > 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.
> 
> Yeah, I understand. The rcu_read_lock() here is totally used to
> suppress the suspicious rcu usage warning, not for the protection.
> So I used it just for the rhltable_lookup() to reduce the impact.
> Maybe I should add some comment for it.

My point is that after a lookup you will be doing some sort of a
dereference on the RCU pointer.  That would cause exactly the same
splat that rhltable_lookup itself generated.

For example, rhl_for_each_entry_rcu should have created the same
warning, but it doesn't because for some reason it is using
rcu_dereference_raw.  I'll need to dig up the history of this
to see if there is a good reason for it to not warn.

Cheers,
-- 
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