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

[Masami Hiramatsu (Google) <mhiramat@kernel.org>]

On Thu, 26 May 2022 16:49:26 +0200
Jiri Olsa <olsajiri@gmail.com> wrote:

> On Thu, May 26, 2022 at 11:25:30PM +0900, Masami Hiramatsu wrote:
> > On Tue, 24 May 2022 19:23:01 -0400
> > Steven Rostedt <rostedt@goodmis.org> wrote:
> > 
> > > On Sat,  7 May 2022 13:46:52 +0900
> > > Masami Hiramatsu <mhiramat@kernel.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?
> 
> sure I'll test it.. can't see the original email,
> perhaps I wasn't cc-ed.. but I'll find it

Here it is. I Cc-ed your @kernel.org address.
https://lore.kernel.org/all/165189881197.175864.14757002789194211860.stgit@devnote2/T/#u

> 
> is this also related to tracing 'idle' functions,
> as discussed in here?
>   https://lore.kernel.org/bpf/20220515203653.4039075-1-jolsa@kernel.org/

Ah, yes. So this may not happen with the above patch, but for the
hardening (ensuring it is always safe), I would like to add this.

> 
> because that's the one I can reproduce.. but I can
> certainly try that with your change as well

Thank you!

> 
> jirka
> 
> > 
> > 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@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;
> > > 
> > 
> > 
> > -- 
> > Masami Hiramatsu (Google) <mhiramat@kernel.org>


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