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Date:   Tue, 7 Jun 2022 01:02:29 +0900
From:   Masami Hiramatsu (Google) <mhiramat@kernel.org>
To:     Andrii Nakryiko <andrii.nakryiko@gmail.com>
Cc:     Jiri Olsa <olsajiri@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Andrii Nakryiko <andrii@kernel.org>,
        open list <linux-kernel@vger.kernel.org>,
        Networking <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
        "Paul E. McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH] rethook: Reject getting a rethook if RCU is not
 watching
Message-Id: <20220607010229.5e75445aedb12c99cae2cd51@kernel.org>
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On Fri, 3 Jun 2022 12:21:19 -0700
Andrii Nakryiko <andrii.nakryiko@gmail.com> wrote:

> On Fri, May 27, 2022 at 6:19 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
> >
> > On Sat, 28 May 2022 00:10:08 +0200
> > Jiri Olsa <olsajiri@gmail.com> wrote:
> >
> > > On Fri, May 27, 2022 at 01:14:34AM +0900, Masami Hiramatsu wrote:
> > > > 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!
> > >
> > > it did not help the idle warning as expected, but I did not
> > > see any problems running bpf tests on top of this
> >
> > Oops, right. I forgot this is only for the rethook, not protect the
> > fprobe handlers, since fprobe code doesn't involve the RCU code (it
> > depends on ftrace's check). Sorry about that.
> > Hmm, I need to add a test code for this issue, but that could be
> > solved by your noninstr patch.
> >
> 
> 
> Masami,
> 
> It's not clear to me, do you intend to send a new revision with some
> more tests or this patch as is ready to go into bpf tree?

OK, let me make a test code against this issue. This may need a raw
fprobe test code (not a test case because it depends on that we can
trace the "arch_cpu_idle()"), but that test code won't work after
the "arch_cpu_idle()" is marked as noinstr (thus the test code will
only for the kernel which doesn't have the noinstr patch).
I want to add this check for the case if someone accidentally add
a function which is not covered by RCU and that is tracable by
fprobe (ftrace).
Thus this is a kind of preventative fix.

Thank you,

> 
> 
> > Thank you,
> >
> > >
> > > jirka
> > >
> > > >
> > > > >
> > > > > 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>
> >
> >
> > --
> > Masami Hiramatsu (Google) <mhiramat@kernel.org>


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