netdev - Re: [PATCH v3 1/2] kretprobe: produce sane stack traces

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Date:   Sat, 3 Nov 2018 22:00:12 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Josh Poimboeuf <jpoimboe@...hat.com>,
        Aleksa Sarai <cyphar@...har.com>,
        "Naveen N. Rao" <naveen.n.rao@...ux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@...el.com>,
        "David S. Miller" <davem@...emloft.net>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Jonathan Corbet <corbet@....net>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>,
        Namhyung Kim <namhyung@...nel.org>,
        Shuah Khan <shuah@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Daniel Borkmann <daniel@...earbox.net>,
        Brendan Gregg <bgregg@...flix.com>,
        Christian Brauner <christian@...uner.io>,
        Aleksa Sarai <asarai@...e.de>, netdev@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        linux-kselftest@...r.kernel.org
Subject: Re: [PATCH v3 1/2] kretprobe: produce sane stack traces

On Fri, 2 Nov 2018 12:13:07 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Fri, 2 Nov 2018 10:43:26 -0500
> Josh Poimboeuf <jpoimboe@...hat.com> wrote:
> 
> > > I'll hopefully have a prototype ready by plumbers.  
> > 
> > Why do we need multiple users?  It would be a lot simpler if we could
> > just enforce a single user per fgraphed/kretprobed function (and return
> > -EBUSY if it's already being traced/probed).
> 
> Because that means if function graph tracer is active, then you can't
> do a kretprobe, and vice versa. I'd really like to have it working for
> multiple users, then we could trace different graph functions and store
> them in different buffers. It would also allow for perf to use function
> graph tracer too.

Steve, how woul you allow multiple users on it? Something like this?

ret_trampoline_multiple(){
   list_for_each(handler, &shadow_entry[i].handlers, list)
	handler(shadow_entry[i]);
   restore_retval_and_jump_to(shadow_entry[i].orig);
}


> > > And this too will require each architecture to probably change. As a
> > > side project to this, I'm going to try to consolidate the function
> > > graph code among all the architectures as well. Not an easy task.  
> > 
> > Do you mean implementing HAVE_FUNCTION_GRAPH_RET_ADDR_PTR for all the
> > arches?  If so, I think have an old crusty patch which attempted to
> > that.  I could try to dig it up if you're interested.
> > 
> 
> I'd like to have that, but it still requires some work. But I'd just
> the truly architecture dependent code be in the architecture (basically
> the asm code), and have the ability to move most of the duplicate code
> out of the archs.

I will also do that for kretprobe handlers.

Thank you,

-- 
Masami Hiramatsu <mhiramat@...nel.org>