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=-10.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED,USER_AGENT_SANE_1 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 EE712C433C1 for ; Fri, 19 Mar 2021 19:02:48 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id BE2EB61931 for ; Fri, 19 Mar 2021 19:02:48 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S229974AbhCSTCS (ORCPT ); Fri, 19 Mar 2021 15:02:18 -0400 Received: from mail.kernel.org ([198.145.29.99]:40264 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230473AbhCSTCK (ORCPT ); Fri, 19 Mar 2021 15:02:10 -0400 Received: by mail.kernel.org (Postfix) with ESMTPSA id B762361931; Fri, 19 Mar 2021 19:02:08 +0000 (UTC) Date: Fri, 19 Mar 2021 19:02:06 +0000 From: Catalin Marinas To: Mark Rutland Cc: linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org, Chen Jun , Marco Elver , Mark Brown , Will Deacon Subject: Re: [PATCH] arm64: stacktrace: don't trace arch_stack_walk() Message-ID: <20210319190205.GI6832@arm.com> References: <20210319184106.5688-1-mark.rutland@arm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210319184106.5688-1-mark.rutland@arm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Fri, Mar 19, 2021 at 06:41:06PM +0000, Mark Rutland wrote: > We recently converted arm64 to use arch_stack_walk() in commit: > > 5fc57df2f6fd ("arm64: stacktrace: Convert to ARCH_STACKWALK") > > The core stacktrace code expects that (when tracing the current task) > arch_stack_walk() starts a trace at its caller, and does not include > itself in the trace. However, arm64's arch_stack_walk() includes itself, > and so traces include one more entry than callers expect. The core > stacktrace code which calls arch_stack_walk() tries to skip a number of > entries to prevent itself appearing in a trace, and the additional entry > prevents skipping one of the core stacktrace functions, leaving this in > the trace unexpectedly. > > We can fix this by having arm64's arch_stack_walk() begin the trace with > its caller. The first value returned by the trace will be > __builtin_return_address(0), i.e. the caller of arch_stack_walk(). The > first frame record to be unwound will be __builtin_frame_address(1), > i.e. the caller's frame record. To prevent surprises, arch_stack_walk() > is also marked noinline. > > While __builtin_frame_address(1) is not safe in portable code, local GCC > developers have confirmed that it is safe on arm64. To find the caller's > frame record, the builtin can safely dereference the current function's > frame record or (in theory) could stash the original FP into another GPR > at function entry time, neither of which are problematic. > > Prior to this patch, the tracing code would unexpectedly show up in > traces of the current task, e.g. > > | # cat /proc/self/stack > | [<0>] stack_trace_save_tsk+0x98/0x100 > | [<0>] proc_pid_stack+0xb4/0x130 > | [<0>] proc_single_show+0x60/0x110 > | [<0>] seq_read_iter+0x230/0x4d0 > | [<0>] seq_read+0xdc/0x130 > | [<0>] vfs_read+0xac/0x1e0 > | [<0>] ksys_read+0x6c/0xfc > | [<0>] __arm64_sys_read+0x20/0x30 > | [<0>] el0_svc_common.constprop.0+0x60/0x120 > | [<0>] do_el0_svc+0x24/0x90 > | [<0>] el0_svc+0x2c/0x54 > | [<0>] el0_sync_handler+0x1a4/0x1b0 > | [<0>] el0_sync+0x170/0x180 > > After this patch, the tracing code will not show up in such traces: > > | # cat /proc/self/stack > | [<0>] proc_pid_stack+0xb4/0x130 > | [<0>] proc_single_show+0x60/0x110 > | [<0>] seq_read_iter+0x230/0x4d0 > | [<0>] seq_read+0xdc/0x130 > | [<0>] vfs_read+0xac/0x1e0 > | [<0>] ksys_read+0x6c/0xfc > | [<0>] __arm64_sys_read+0x20/0x30 > | [<0>] el0_svc_common.constprop.0+0x60/0x120 > | [<0>] do_el0_svc+0x24/0x90 > | [<0>] el0_svc+0x2c/0x54 > | [<0>] el0_sync_handler+0x1a4/0x1b0 > | [<0>] el0_sync+0x170/0x180 > > Erring on the side of caution, I've given this a spin with a bunch of > toolchains, verifying the output of /proc/self/stack and checking that > the assembly looked sound. For GCC (where we require version 5.1.0 or > later) I tested with the kernel.org crosstool binares for versions > 5.5.0, 6.4.0, 6.5.0, 7.3.0, 7.5.0, 8.1.0, 8.3.0, 8.4.0, 9.2.0, and > 10.1.0. For clang (where we require version 10.0.1 or later) I tested > with the llvm.org binary releases of 11.0.0, and 11.0.1. > > Fixes: 5fc57df2f6fd ("arm64: stacktrace: Convert to ARCH_STACKWALK") > Signed-off-by: Mark Rutland > Cc: Catalin Marinas > Cc: Chen Jun > Cc: Marco Elver > Cc: Mark Brown > Cc: Will Deacon Thanks Mark. I think we should add a cc stable, just with Fixes doesn't always seem to end up in a stable kernel: Cc: # 5.10.x With that: Reviewed-by: Catalin Marinas