From mboxrd@z Thu Jan 1 00:00:00 1970 From: Masami Hiramatsu Subject: Re: [PATCH v3 1/2] kretprobe: produce sane stack traces Date: Sun, 4 Nov 2018 01:34:30 +0900 Message-ID: <20181104013430.9d3e91b8ebbae7dcb6860ef1@kernel.org> References: <20181101083551.3805-1-cyphar@cyphar.com> <20181101083551.3805-2-cyphar@cyphar.com> <20181101204720.6ed3fe37@vmware.local.home> <20181102050509.tw3dhvj5urudvtjl@yavin> <20181102065932.bdt4pubbrkvql4mp@yavin> <20181102091658.1bc979a4@gandalf.local.home> <20181102154325.bt6xoysl4xdl33wd@treble> <20181102121307.32e99414@gandalf.local.home> <20181103220012.55ecd97e671c43e4959c8b62@kernel.org> <20181103091341.3d32683e@vmware.local.home> Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Cc: Josh Poimboeuf , Aleksa Sarai , "Naveen N. Rao" , Anil S Keshavamurthy , "David S. Miller" , Jonathan Corbet , Peter Zijlstra , Ingo Molnar , Arnaldo Carvalho de Melo , Alexander Shishkin , Jiri Olsa , Namhyung Kim , Shuah Khan , Alexei Starovoitov , Daniel Borkmann , Brendan Gregg , Christian Brauner , Aleksa Sarai , netdev@vger.kernel.org, linux-doc@vger.kerne To: Steven Rostedt Return-path: Received: from mail.kernel.org ([198.145.29.99]:56614 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726888AbeKDBqV (ORCPT ); Sat, 3 Nov 2018 21:46:21 -0400 In-Reply-To: <20181103091341.3d32683e@vmware.local.home> Sender: netdev-owner@vger.kernel.org List-ID: On Sat, 3 Nov 2018 09:13:41 -0400 Steven Rostedt wrote: > On Sat, 3 Nov 2018 22:00:12 +0900 > Masami Hiramatsu wrote: > > > On Fri, 2 Nov 2018 12:13:07 -0400 > > Steven Rostedt 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). Yes, I understand it. > 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. Hmm, I doubt that is too complicated and not scalable. I rather like to see the open shadow entry... entry: [[original_retaddr][function][modified_retaddr]] So if there are many users on same function, the entries will be like this [[original_return_address][function][trampoline_A]] [[trampline_A][function][trampoline_B]] [[trampline_B][function][trampoline_C]] And on the top of the stack, there is trampline_C instead of original_return_address. In this case, return to trampoline_C(), it jumps back to trampline_B() and then it jumps back to trampline_A(). And eventually it jumps back to original_return_address. This way, we don't need allocate another bitmap/pages for the shadow stack. We only need a shadow stack for each task. Also, unwinder can easily find the trampline_C from the shadow stack and restores original_return_address. (of course trampline_A,B,C must be registered so that search function can skip it.) Thank you, -- Masami Hiramatsu