linux-kernel - Re: [PATCH] rethook: Reject getting a rethook if RCU is not watching

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Date:   Thu, 26 May 2022 23:25:30 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Andrii Nakryiko <andrii@...nel.org>,
        Jiri Olsa <jolsa@...nel.org>, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH] rethook: Reject getting a rethook if RCU is not
 watching

On Tue, 24 May 2022 19:23:01 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Sat,  7 May 2022 13:46:52 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> Is this expected to go through the BPF tree?
> 

Yes, since rethook (fprobe) is currently used only from eBPF.
Jiri, can you check this is good for your test case?

Thank you,


> -- 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@...nel.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;
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>