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

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Date:   Sat, 3 Nov 2018 09:13:41 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Masami Hiramatsu <mhiramat@...nel.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>,
        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 Sat, 3 Nov 2018 22:00:12 +0900
Masami Hiramatsu <mhiramat@...nel.org> wrote:

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

> > 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);
> }
>

Something like that. But since it's not a single mapping between shadow
entries and handlers, that is we have multiple tasks with multiple
shadow entries and multiple handlers, we can't use a link list (two
different parents per handler).

I was thinking of a bitmask that represents the handlers, and use that
to map which handler gets called for which shadow entry for a
particular task.

-- Steve