From mboxrd@z Thu Jan  1 00:00:00 1970
From: Steven Rostedt <rostedt@goodmis.org>
Subject: Re: [PATCH v3 1/2] kretprobe: produce sane stack traces
Date: Sat, 3 Nov 2018 13:33:29 -0400
Message-ID: <20181103133329.2160bfd7@vmware.local.home>
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        <20181102154325.bt6xoysl4xdl33wd@treble>
        <20181102121307.32e99414@gandalf.local.home>
        <20181103220012.55ecd97e671c43e4959c8b62@kernel.org>
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Cc: Josh Poimboeuf <jpoimboe@redhat.com>,
        Aleksa Sarai <cyphar@cyphar.com>,
        "Naveen N. Rao" <naveen.n.rao@linux.vnet.ibm.com>,
        Anil S Keshavamurthy <anil.s.keshavamurthy@intel.com>,
        "David S. Miller" <davem@davemloft.net>,
        Jonathan Corbet <corbet@lwn.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Arnaldo Carvalho de Melo <acme@kernel.org>,
        Alexander Shishkin <alexander.shishkin@linux.intel.com>,
        Jiri Olsa <jolsa@redhat.com>,
        Namhyung Kim <namhyung@kernel.org>,
        Shuah Khan <shuah@kernel.org>,
        Alexei Starovoitov <ast@kernel.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Brendan Gregg <bgregg@netflix.com>,
        Christian Brauner <christian@brauner.io>,
        Aleksa Sarai <asarai@suse.de>, netdev@vger.kernel.org,
        linux-doc@vger.kerne
To: Masami Hiramatsu <mhiramat@kernel.org>
Return-path: <linux-kernel-owner@vger.kernel.org>
In-Reply-To: <20181103133021.6676708c@vmware.local.home>
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On Sat, 3 Nov 2018 13:30:21 -0400
Steven Rostedt <rostedt@goodmis.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