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Date: Sun, 4 Nov 2018 11:25:06 +0900 From: Masami Hiramatsu <mhiramat@...nel.org> To: Steven Rostedt <rostedt@...dmis.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: > On Sun, 4 Nov 2018 01:34:30 +0900 > Masami Hiramatsu <mhiramat@...nel.org> 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? > No, I think tramplines are very limited. currently we will only have ftrace and kretprobe trampolines. > > 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(); Ah, that's a good idea. I think we also have to store the called function entry address with the number header, but basically I agree with you. If we have a space to store a data with the function address, that is also good to kretprobe. systemtap heavily uses "entry data" for saving some data at function entry for exit handler. > 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. OK, and could you make it independent from func graph tracer, so that CONFIG_KPROBES=y but CONFIG_FUNCTION_GRAPH_TRACER=n kernel can support kretprobes too. Thank you, -- Masami Hiramatsu <mhiramat@...nel.org>
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