From mboxrd@z Thu Jan 1 00:00:00 1970 From: Steven Rostedt Subject: Re: [PATCH v3 1/2] kretprobe: produce sane stack traces Date: Sat, 3 Nov 2018 13:30:21 -0400 Message-ID: <20181103133021.6676708c@vmware.local.home> 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> <20181104013430.9d3e91b8ebbae7dcb6860ef1@kernel.org> 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: Masami Hiramatsu Return-path: In-Reply-To: <20181104013430.9d3e91b8ebbae7dcb6860ef1@kernel.org> Sender: linux-kernel-owner@vger.kernel.org List-Id: netdev.vger.kernel.org On Sun, 4 Nov 2018 01:34:30 +0900 Masami Hiramatsu wrote: > > > 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... It can scale and not too complex (I already played a little with it). But that said, I'm not committed to it, and using the shadow stack is also an interesting idea. > > 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. Where are trampolines A, B, and C made? Do we also need to dynamically create them? If I register multiple function tracing ones, each one will need its own trampoline? > > 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.) 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. -- Steve