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=-11.0 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS autolearn=unavailable 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 B569DC433E6 for ; Thu, 11 Mar 2021 16:51:59 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 6DC0164FF1 for ; Thu, 11 Mar 2021 16:51:59 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229624AbhCKQv0 (ORCPT ); Thu, 11 Mar 2021 11:51:26 -0500 Received: from us-smtp-delivery-124.mimecast.com ([63.128.21.124]:32365 "EHLO us-smtp-delivery-124.mimecast.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S229605AbhCKQvY (ORCPT ); Thu, 11 Mar 2021 11:51:24 -0500 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1615481483; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: in-reply-to:in-reply-to:references:references; bh=OO/tPG5G1RczqaGF/GiOfsdnXsSo9S9cqaZfFmpZNqM=; b=L8w8Dd0ZHJZL7/PSiHjzB7zciuZgy3Q5cSorzqTVqt/cVD/c8UURNqo/V6W7ISTH/Tu9E/ MI6oGUppGft8qNPcfYKtLxuFrRBHefI8XknUdQxXyv2Mmth7uNg9PZB9TOTx1rM6yvBiJX /nLkvjQb9uswXZzrx2lGEap7VsMAErM= Received: from mimecast-mx01.redhat.com (mimecast-mx01.redhat.com [209.132.183.4]) (Using TLS) by relay.mimecast.com with ESMTP id us-mta-500-ATpMhlmYOjyp7FIOUtFhnQ-1; Thu, 11 Mar 2021 11:51:19 -0500 X-MC-Unique: ATpMhlmYOjyp7FIOUtFhnQ-1 Received: from smtp.corp.redhat.com (int-mx02.intmail.prod.int.phx2.redhat.com [10.5.11.12]) (using TLSv1.2 with cipher AECDH-AES256-SHA (256/256 bits)) (No client certificate requested) by mimecast-mx01.redhat.com (Postfix) with ESMTPS id DA1EF1858F2D; Thu, 11 Mar 2021 16:51:17 +0000 (UTC) Received: from treble (ovpn-116-225.rdu2.redhat.com [10.10.116.225]) by smtp.corp.redhat.com (Postfix) with ESMTPS id EE0A9610AF; Thu, 11 Mar 2021 16:51:13 +0000 (UTC) Date: Thu, 11 Mar 2021 10:51:10 -0600 From: Josh Poimboeuf To: Masami Hiramatsu Cc: Daniel Xu , Steven Rostedt , Ingo Molnar , X86 ML , linux-kernel@vger.kernel.org, bpf@vger.kernel.org, kuba@kernel.org, mingo@redhat.com, ast@kernel.org, tglx@linutronix.de, kernel-team@fb.com, yhs@fb.com Subject: Re: [PATCH -tip 0/5] kprobes: Fix stacktrace in kretprobes Message-ID: <20210311165110.y22uyne6ax4qgryf@treble> References: <20210307212333.7jqmdnahoohpxabn@maharaja.localdomain> <20210308115210.732f2c42bf347c15fbb2a828@kernel.org> <20210309011945.ky7v3pnbdpxhmxkh@treble> <20210310185734.332d9d52a26780ba02d09197@kernel.org> <20210310150845.7kctaox34yrfyjxt@treble> <20210311005509.0a1a65df0d2d6c7da73a9288@kernel.org> <20210310183113.xxverwh4qplr7xxb@treble> <20210311092018.2d0e54d2c891850e549d16fe@kernel.org> <20210311010615.7pemfngxx7cy42fe@treble> <20210311105438.cca15ed7645c454294dc3e1f@kernel.org> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <20210311105438.cca15ed7645c454294dc3e1f@kernel.org> X-Scanned-By: MIMEDefang 2.79 on 10.5.11.12 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, Mar 11, 2021 at 10:54:38AM +0900, Masami Hiramatsu wrote: > On Wed, 10 Mar 2021 19:06:15 -0600 > Josh Poimboeuf wrote: > > > On Thu, Mar 11, 2021 at 09:20:18AM +0900, Masami Hiramatsu wrote: > > > > > bool unwind_next_frame(struct unwind_state *state) > > > > > { > > > > > unsigned long ip_p, sp, tmp, orig_ip = state->ip, prev_sp = state->sp; > > > > > @@ -536,6 +561,18 @@ bool unwind_next_frame(struct unwind_state *state) > > > > > > > > > > state->ip = ftrace_graph_ret_addr(state->task, &state->graph_idx, > > > > > state->ip, (void *)ip_p); > > > > > + /* > > > > > + * There are special cases when the stack unwinder is called > > > > > + * from the kretprobe handler or the interrupt handler which > > > > > + * occurs in the kretprobe trampoline code. In those cases, > > > > > + * %sp is shown on the stack instead of the return address. > > > > > + * Or, when the unwinder find the return address is replaced > > > > > + * by kretprobe_trampoline. > > > > > + * In those cases, correct address can be found in kretprobe. > > > > > + */ > > > > > + if (state->ip == sp || > > > > > > > > Why is the 'state->ip == sp' needed? > > > > > > As I commented above, until kretprobe_trampoline writes back the real > > > address to the stack, sp value is there (which has been pushed by the > > > 'pushq %rsp' at the entry of kretprobe_trampoline.) > > > > > > ".type kretprobe_trampoline, @function\n" > > > "kretprobe_trampoline:\n" > > > /* We don't bother saving the ss register */ > > > " pushq %rsp\n" // THIS > > > " pushfq\n" > > > > > > Thus, from inside the kretprobe handler, like ftrace, you'll see > > > the sp value instead of the real return address. > > > > I see. If you change is_kretprobe_trampoline_address() to include the > > entire function, like: > > > > static bool is_kretprobe_trampoline_address(unsigned long ip) > > { > > return (void *)ip >= kretprobe_trampoline && > > (void *)ip < kretprobe_trampoline_end; > > } > > > > then the unwinder won't ever read the bogus %rsp value into state->ip, > > and the 'state->ip == sp' check can be removed. > > Hmm, I couldn't get your point. Since sp is the address of stack, > it always out of text address. When unwinding from trampoline_handler(), state->ip will point to the instruction after the call: call trampoline_handler movq %rax, 19*8(%rsp) <-- state->ip points to this insn But then, the above version of is_kretprobe_trampoline_address() is true, so state->ip gets immediately replaced with the real return address: if (is_kretprobe_trampoline_address(state->ip)) state->ip = orc_kretprobe_correct_ip(state); so the unwinder skips over the kretprobe_trampoline() frame and goes straight to the frame of the real return address. Thus it never reads this bogus return value into state->ip: pushq %rsp which is why the weird 'state->ip == sp' check is no longer needed. The only "downside" is that the unwinder skips the kretprobe_trampoline() frame. (note that downside wouldn't exist in the case of UNWIND_HINT_REGS + valid regs->ip). > > > > And it would make the unwinder just work automatically when unwinding > > > > from the handler using the regs. > > > > > > > > It would also work when unwinding from the handler's stack, if we put an > > > > UNWIND_HINT_REGS after saving the regs. > > > > > > At that moment, the real return address is not identified. So we can not > > > put it. > > > > True, at the time the regs are originally saved, the real return address > > isn't available. But by the time the user handler is called, the return > > address *is* available. So if the real return address were placed in > > regs->ip before calling the handler, the unwinder could find it there, > > when called from the handler. > > OK, but this is not arch independent specification. I can make a hack > only for x86, but that is not clean implementation, hmm. > > > > > Then we wouldn't need the call to orc_kretprobe_correct_ip() in > > __unwind_start(). > > What about the ORC implementation in other architecture? Is that for > x86 only? ORC is x86 only. > > But maybe it's not possible due to the regs->ip expectations of legacy > > handlers? > > Usually, the legacy handlers will ignore it, the official way to access > the correct return address is kretprobe_instance.ret_addr. Because it is > arch independent. > > Nowadays there are instruction_pointer() and instruction_pointer_set() APIs > in many (not all) architecutre, so I can try to replace to use it instead > of the kretprobe_instance.ret_addr. > (and it will break the out-of-tree codes) That sounds better to me, though I don't have an understanding of what it would break. -- Josh