From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-9.0 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,MENTIONS_GIT_HOSTING,SPF_HELO_NONE,SPF_PASS, URIBL_BLOCKED autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 0FC2CC282DD for ; Thu, 23 May 2019 16:42:13 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id D1E982175B for ; Thu, 23 May 2019 16:42:12 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731156AbfEWQmM (ORCPT ); Thu, 23 May 2019 12:42:12 -0400 Received: from mail-it1-f196.google.com ([209.85.166.196]:53666 "EHLO mail-it1-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730893AbfEWQmM (ORCPT ); Thu, 23 May 2019 12:42:12 -0400 Received: by mail-it1-f196.google.com with SMTP id m141so10831323ita.3 for ; Thu, 23 May 2019 09:42:11 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=6h5Gv6+Yt9f8QV63xDQZtQKCEMQicjrYgqtB5ciLH/o=; b=OvJ8kkaM7BH0a1TWiKuCvGWq6RoblL0kAsFPFanO+MtFkmiHQ9BboniskqpfchhCfb qLKE98s6n2Z5wYcTCGRwMXpBiJ+eDZZknbRHgHzcHUA9Km9zib3WfYrSnzY9+sYI5vNP kbm8zSs9xNqT5OXrSQktZCGrETS2VvS01vT095d7oxVpNcOZvblw/2lZXDwMe8PM8+CA T24o0GxIhESu7SFXWj9IDIjhxnUxLpuXd0iE/DaBLkcTm0L2ageitpb9OTYXDP+n9ewc hw7mtbiCNPKvF219Q035ZUffEbm4odqLHHJJCr4DLR5m39EqSRbxnfX0fL+KFD+ERQJl gJQw== X-Gm-Message-State: APjAAAVkr6Urp+hwpcD0EsjnDQhC9o8mEn1sjnaDS64QrBhIdR2eFVNU o5yVLYt6Mbp/VeCkqd/lTZg/sHWvvCj8K//GGQBr5fyf6NJ7fIXa X-Google-Smtp-Source: APXvYqxTZQkYgbBDIvoFkw/ZTDfJz6cpIlSoNKA34ueI1+BGLDPWDqNKplP2eh4Nb/zUp7AWKX8x3jaCM2nKGrpPFYk= X-Received: by 2002:a24:2e8c:: with SMTP id i134mr13991636ita.9.1558629731329; Thu, 23 May 2019 09:42:11 -0700 (PDT) MIME-Version: 1.0 References: <20190517091044.GM2606@hirez.programming.kicks-ass.net> <20190522140233.GC16275@worktop.programming.kicks-ass.net> <20190522174517.pbdopvookggen3d7@treble> <20190522234635.a47bettklcf5gt7c@treble> <20190523133253.tad6ywzzexks6hrp@treble> <20190523152413.m2pbnamihu3s2c5s@treble> In-Reply-To: <20190523152413.m2pbnamihu3s2c5s@treble> From: Kairui Song Date: Fri, 24 May 2019 00:41:59 +0800 Message-ID: Subject: Re: Getting empty callchain from perf_callchain_kernel() To: Josh Poimboeuf Cc: Alexei Starovoitov , Peter Zijlstra , Song Liu , lkml , Kernel Team , Alexei Starovoitov , Daniel Borkmann , "bpf@vger.kernel.org" Content-Type: text/plain; charset="UTF-8" Sender: bpf-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Thu, May 23, 2019 at 11:24 PM Josh Poimboeuf wrote: > > On Thu, May 23, 2019 at 10:50:24PM +0800, Kairui Song wrote: > > > > Hi Josh, this still won't fix the problem. > > > > > > > > Problem is not (or not only) with ___bpf_prog_run, what actually went > > > > wrong is with the JITed bpf code. > > > > > > There seem to be a bunch of issues. My patch at least fixes the failing > > > selftest reported by Alexei for ORC. > > > > > > How can I recreate your issue? > > > > Hmm, I used bcc's example to attach bpf to trace point, and with that > > fix stack trace is still invalid. > > > > CMD I used with bcc: > > python3 ./tools/stackcount.py t:sched:sched_fork > > I've had problems in the past getting bcc to build, so I was hoping it > was reproducible with a standalone selftest. > > > And I just had another try applying your patch, self test is also failing. > > Is it the same selftest reported by Alexei? > > test_stacktrace_map:FAIL:compare_map_keys stackid_hmap vs. stackmap err -1 errno 2 > > > I'm applying on my local master branch, a few days older than > > upstream, I can update and try again, am I missing anything? > > The above patch had some issues, so with some configs you might see an > objtool warning for ___bpf_prog_run(), in which case the patch doesn't > fix the test_stacktrace_map selftest. > > Here's the latest version which should fix it in all cases (based on > tip/master): > > https://git.kernel.org/pub/scm/linux/kernel/git/jpoimboe/linux.git/commit/?h=bpf-orc-fix Hmm, I still get the failure: test_stacktrace_map:FAIL:compare_map_keys stackid_hmap vs. stackmap err -1 errno 2 And I didn't see how this will fix the issue. As long as ORC need to unwind through the JITed code it will fail. And that will happen before reaching ___bpf_prog_run. > > > > > For frame pointer unwinder, it seems the JITed bpf code will have a > > > > shifted "BP" register? (arch/x86/net/bpf_jit_comp.c:217), so if we can > > > > unshift it properly then it will work. > > > > > > Yeah, that looks like a frame pointer bug in emit_prologue(). > > > > > > > I tried below code, and problem is fixed (only for frame pointer > > > > unwinder though). Need to find a better way to detect and do any > > > > similar trick for bpf part, if this is a feasible way to fix it: > > > > > > > > diff --git a/arch/x86/kernel/unwind_frame.c b/arch/x86/kernel/unwind_frame.c > > > > index 9b9fd4826e7a..2c0fa2aaa7e4 100644 > > > > --- a/arch/x86/kernel/unwind_frame.c > > > > +++ b/arch/x86/kernel/unwind_frame.c > > > > @@ -330,8 +330,17 @@ bool unwind_next_frame(struct unwind_state *state) > > > > } > > > > > > > > /* Move to the next frame if it's safe: */ > > > > - if (!update_stack_state(state, next_bp)) > > > > - goto bad_address; > > > > + if (!update_stack_state(state, next_bp)) { > > > > + // Try again with shifted BP > > > > + state->bp += 5; // see AUX_STACK_SPACE > > > > + next_bp = (unsigned long > > > > *)READ_ONCE_TASK_STACK(state->task, *state->bp); > > > > + // Clean and refetch stack info, it's marked as error outed > > > > + state->stack_mask = 0; > > > > + get_stack_info(next_bp, state->task, > > > > &state->stack_info, &state->stack_mask); > > > > + if (!update_stack_state(state, next_bp)) { > > > > + goto bad_address; > > > > + } > > > > + } > > > > > > > > return true; > > > > > > Nack. > > > > > > > For ORC unwinder, I think the unwinder can't find any info about the > > > > JITed part. Maybe if can let it just skip the JITed part and go to > > > > kernel context, then should be good enough. > > > > > > If it's starting from a fake pt_regs then that's going to be a > > > challenge. > > > > > > Will the JIT code always have the same stack layout? If so then we > > > could hard code that knowledge in ORC. Or even better, create a generic > > > interface for ORC to query the creator of the generated code about the > > > stack layout. > > > > I think yes. > > > > Not sure why we have the BP shift yet, if the prolog code could be > > tweaked to work with frame pointer unwinder it will be good to have. > > But still not for ORC. > > > > Will it be a good idea to have a region reserved for the JITed code? > > Currently it shares the region with "module mapping space". If let it > > have a separate region, when the unwinder meet any code in that region > > it will know it's JITed code and then can do something special about > > it. > > > > This should make it much easier for both frame pointer and ORC unwinder to work. > > There's no need to put special cases in the FP unwinder when we can > instead just fix the frame pointer usage in the JIT code. > > For ORC, I'm thinking we may be able to just require that all generated > code (BPF and others) always use frame pointers. Then when ORC doesn't > recognize a code address, it could try using the frame pointer as a > fallback. Right, this sounds the right way to fix it, I believe this can fix everything well. > > Once I get a reproducer I can do the patches for all that. > > -- > Josh -- Best Regards, Kairui Song