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

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Date:   Sat, 3 Nov 2018 13:33:29 -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 13:30:21 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> What I was thinking was to store a count and the functions to be called:
> 
> 
> 	[original_return_address]
> 	[function_A]
> 	[function_B]
> 	[function_C]
> 	[ 3 ]
> 
> Then the trampoline that processes the return codes for ftrace (and
> kretprobes and everyone else) can simply do:
> 
> 	count = pop_shadow_stack();
> 	for (i = 0; i < count; i++) {
> 		func = pop_shadow_stack();
> 		func(...);
> 	}
> 	return_address = pop_shadow_stack();
> 
> That way we only need to register a function to the return handler and
> it will be called, without worrying about making trampolines. There
> will just be a single trampoline that handles all the work.

And since the most common case is a single function to call, instead of
using a count, we can take advantage that kernel functions are negative
numbers and do:

	[original_return_address]
	[function_A]

	----

	long count;

	count = pop_shadow_stack();
	if (count < 0) {
		func = (void *)count;
		func();
	} else {
		for (i = 0; i < count; i++) {
			[...]

The unwinder will just need to know how to handle all this :-)

-- Steve