Re: [PATCH] tracing: fprobe: fix suspicious rcu usage in fprobe_entry

[Menglong Dong <menglong.dong@linux.dev>]

On 2025/9/3 12:22 Herbert Xu <herbert@gondor.apana.org.au> write:
> On Tue, Sep 02, 2025 at 05:50:32PM +0800, menglong.dong@linux.dev wrote:
> > On 2025/9/2 17:17 Herbert Xu <herbert@gondor.apana.org.au> write:
> > > Menglong Dong <dongml2@chinatelecom.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.

Yeah, I understand what you mean. I noticed this, and that's why
I added the rcu_read_lock() for rhashtable_lookup() only.

Maybe it is to obtain better performance? Just guess ;)
And hlist_for_each_entry_rcu() also uses rcu_dereference_raw().

Thanks!
Menglong Dong
> 
> Cheers,
> -- 
> Email: Herbert Xu <herbert@gondor.apana.org.au>
> Home Page: http://gondor.apana.org.au/~herbert/
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>