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=-11.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,INCLUDES_PATCH,MAILING_LIST_MULTI, NICE_REPLY_A,SPF_HELO_NONE,SPF_PASS 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 A7118C433E0 for ; Fri, 12 Mar 2021 03:23:28 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 8149364FB3 for ; Fri, 12 Mar 2021 03:23:28 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229737AbhCLDWy (ORCPT ); Thu, 11 Mar 2021 22:22:54 -0500 Received: from mail.kernel.org ([198.145.29.99]:35878 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231573AbhCLDWo (ORCPT ); Thu, 11 Mar 2021 22:22:44 -0500 Received: by mail.kernel.org (Postfix) with ESMTPSA id 68A3064F8D; Fri, 12 Mar 2021 03:22:41 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1615519364; bh=tFRlENPzzzTji5RdOHauaisevYkYYed7N5Ib325hWXw=; h=Date:From:To:Cc:Subject:In-Reply-To:References:From; b=cD9v2FT0NxDRc/hLAa6Omm7XYH14xA968UpBADEJU/O5bZzk77YTm099OQA9KaIeT J3kAytlQ9VLaRFwnUYzxi4Zo99Z1sUtqIsI3Wvb3LM+ESZwQM1m8VgsrqeKLNc8yLw qMVO0tsZVm/GomUvSin+tWQmtN1Q+1GUkCBopIWW4S1T7i/ZXtgmUUUycX0wM4ZpCn WPAaE80YflaDS7e5LdjWJWcBy5mQiXiaew9n5wzd6ID2f2X6IfSIRhnXkOL3JEolsb Xt+17B3urfLmK0D/ogGGHuHEiDDSEFMVVZ9LlamiVAvZ96PyB+1xbV7ea8nzJuIZK4 MJUNLkoze6H5A== Date: Fri, 12 Mar 2021 12:22:39 +0900 From: Masami Hiramatsu To: Josh Poimboeuf 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: <20210312122239.12fb22d3259e198f72caf7ea@kernel.org> In-Reply-To: <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> <20210311165110.y22uyne6ax4qgryf@treble> X-Mailer: Sylpheed 3.7.0 (GTK+ 2.24.32; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Thu, 11 Mar 2021 10:51:10 -0600 Josh Poimboeuf wrote: > 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); Hmm, but that ip is for the "next" frame, isn't it? [stack] 0x040 kretprobe_trampoline+0x25 -+ 0x048 r15 | ... /* pt_regs */ +- ORC sp_offset at the kretprobe_trampoline+0x24 0x0d8 flags -+ 0x0e0 rsp (=0x0e0) /* will be replaced by the real return address */ Your idea seems replacing stack@0x040 with the real return address. In that case, may orc_find() returns the wrong ORC info? > 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. But that is what the kretprobe_trampoline actually does. I would rather like to see the real information from stacktrace. > 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). As I said, I would like to see the kretprobe_trampoline+0x25 on my stacktrace, because kretprobe doesn't replace the kretprobe_trampoline but the target_func on the stack. > > > > > 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. OK, anyway try to do that and see what happen. Thank you, -- Masami Hiramatsu