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=-9.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_PATCH, MAILING_LIST_MULTI,SIGNED_OFF_BY,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 E255AC43467 for ; Thu, 8 Oct 2020 10:17:38 +0000 (UTC) Received: from merlin.infradead.org (merlin.infradead.org [205.233.59.134]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id 6230E2076B for ; Thu, 8 Oct 2020 10:17:38 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=lists.infradead.org header.i=@lists.infradead.org header.b="ssYWDxOD" DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 6230E2076B Authentication-Results: mail.kernel.org; dmarc=fail (p=none dis=none) header.from=arm.com Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=merlin.20170209; h=Sender:Content-Transfer-Encoding: Content-Type:Cc:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:To:From:Date:Reply-To:Content-ID:Content-Description:Resent-Date: Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Owner; bh=XPHA5ISgl+IHTvMBb9OVKm63k2e+tLZ4QIFW5VAJ7Po=; b=ssYWDxOD7WJIJAeSxKIlA/zRh E5Ow3sOCEIAK/uoS5yYbIS6gmZUcozTr+Ykbt0RH+Nat8oNged4mlWK7AyZ3PCI6b3ffJsU26Wi9H 39BIj/51z4ZImAVkBw41pxbV7uLsUtZ2EhvFBVKwLi9ejl3tZ4qQ/BCPUh/SVfcKTCLYNUiJjHBP7 acWHC43yVFKVsu00XoAq67ONVfqKnadraWCgMlRgpEXntIqexk95BbHRgN3+x4Vjgs/n9RFVrbXC4 rATYfF1oYb2wmEq+s5qbHRg94ACyQWaLdQMi7E2Rx2t+QcIfAQBqiJ2Y5AvRFmG5P3hlSvp5SG7Nd Iwbtl50Kg==; Received: from localhost ([::1] helo=merlin.infradead.org) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kQSy5-0007a2-8x; Thu, 08 Oct 2020 10:16:13 +0000 Received: from foss.arm.com ([217.140.110.172]) by merlin.infradead.org with esmtp (Exim 4.92.3 #3 (Red Hat Linux)) id 1kQSxz-0007ZU-DP for linux-arm-kernel@lists.infradead.org; Thu, 08 Oct 2020 10:16:11 +0000 Received: from usa-sjc-imap-foss1.foss.arm.com (unknown [10.121.207.14]) by usa-sjc-mx-foss1.foss.arm.com (Postfix) with ESMTP id A3FAC31B; Thu, 8 Oct 2020 03:16:04 -0700 (PDT) Received: from C02TD0UTHF1T.local (unknown [10.57.52.79]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id A75FF3F70D; Thu, 8 Oct 2020 03:16:03 -0700 (PDT) Date: Thu, 8 Oct 2020 11:15:58 +0100 From: Mark Rutland To: Mark Brown Subject: Re: [PATCH] arm64: stacktrace: Clarify documentation of EL0 entry frame skipping Message-ID: <20201008101558.GA72325@C02TD0UTHF1T.local> References: <20201007171138.36015-1-broonie@kernel.org> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: <20201007171138.36015-1-broonie@kernel.org> X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20201008_061607_535408_28B9F23F X-CRM114-Status: GOOD ( 34.31 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: Catalin Marinas , Will Deacon , Ard Biesheuvel , linux-arm-kernel@lists.infradead.org Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit Sender: "linux-arm-kernel" Errors-To: linux-arm-kernel-bounces+linux-arm-kernel=archiver.kernel.org@lists.infradead.org Hi Mark, On Wed, Oct 07, 2020 at 06:11:38PM +0100, Mark Brown wrote: > The unwinder suppresses reporting of frames created on entry from EL0 where > both FP and PC are NULL in order to avoid having the final NULL PC > cluttering traces and potentially creating confusion. There is a comment > explaining this which doesn't explicitly call out the case where we have > a valid PC but NULL FP, do so in order to ensure that things are as > clear as possible to readers unfamiliar with the code. > > Signed-off-by: Mark Brown > --- > arch/arm64/kernel/stacktrace.c | 5 +++-- > 1 file changed, 3 insertions(+), 2 deletions(-) > > diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c > index fa56af1a59c3..0c474d81d4e9 100644 > --- a/arch/arm64/kernel/stacktrace.c > +++ b/arch/arm64/kernel/stacktrace.c > @@ -107,8 +107,9 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame) > /* > * Frames created upon entry from EL0 have NULL FP and PC values, so > * don't bother reporting these. Frames created by __noreturn functions > - * might have a valid FP even if PC is bogus, so only terminate where > - * both are NULL. > + * might have a valid FP even if PC is bogus and the last frame in a > + * normal stack will have a valid PC but NULL FP, so only terminate > + * early where both are NULL. > */ I reckon we should remove the special case entirely rather than trying to shore up the comment here -- the related comment in entry.S is stale and misleading, and this will be a mystery to unfamiliar readers anyhow. Does the below patch look ok to you? I suspect the explicit check for the terminal case may help with the reliable stacktrace stuff? e.g. we could make that return -ENOENT to distinguish a terminal case from a broken frame record. Thanks, Mark. ---->8---- >From 61cb79bd9d926744190a43a5e1ec9aefda7ba052 Mon Sep 17 00:00:00 2001 From: Mark Rutland Date: Thu, 8 Oct 2020 10:47:59 +0100 Subject: [PATCH] arm64: remove EL0 exception frame record When entering an exception from EL0, the entry code creates a synthetic frame record with a NULL PC. This was used by the code introduced in commit: 7326749801396105 ("arm64: unwind: reference pt_regs via embedded stack frame") ... to discover exception entries on the stack and dump the associated pt_regs. Since the NULL PC was undesirable for the stacktrace, we added a special case to unwind_frame() to prevent the NULL PC from being logged. Since commit: a25ffd3a6302a678 ("arm64: traps: Don't print stack or raw PC/LR values in backtraces") ... we no longer try to dump the pt_regs as part of a stacktrace, and hence no longer need the synthetic exception record. This patch removes the synthetic exception record and the associated special case in unwind_frame(). Instead, EL0 exceptions set the FP to NULL, as is the case for other terminal records (e.g. when a kernel thread starts). The synthetic record for exceptions from EL1 is retrained as this has useful unwind information for the interrupted context. To make the terminal case a bit clearer, an explicit check is added to the start of unwind_frame(). This would otherwise be caught implicitly by the on_accessible_stack() checks. Signed-off-by: Mark Rutland Reported-by: Mark Brown Cc: Ard Biesheuvel Cc: Catalin Marinas Cc: Will Deacon --- arch/arm64/kernel/entry.S | 10 +++++----- arch/arm64/kernel/stacktrace.c | 13 ++++--------- 2 files changed, 9 insertions(+), 14 deletions(-) diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S index f30007dff35f7..8af811b6b2259 100644 --- a/arch/arm64/kernel/entry.S +++ b/arch/arm64/kernel/entry.S @@ -228,16 +228,16 @@ alternative_else_nop_endif stp lr, x21, [sp, #S_LR] /* - * In order to be able to dump the contents of struct pt_regs at the - * time the exception was taken (in case we attempt to walk the call - * stack later), chain it together with the stack frames. + * For exceptions from EL0, terminate the callchain here. + * For exceptions from EL1, create a synthetic frame record so the + * interrupted code shows up in the backtrace. */ .if \el == 0 - stp xzr, xzr, [sp, #S_STACKFRAME] + mov x29, xzr .else stp x29, x22, [sp, #S_STACKFRAME] - .endif add x29, sp, #S_STACKFRAME + .endif #ifdef CONFIG_ARM64_SW_TTBR0_PAN alternative_if_not ARM64_HAS_PAN diff --git a/arch/arm64/kernel/stacktrace.c b/arch/arm64/kernel/stacktrace.c index fa56af1a59c39..0fb42129b4691 100644 --- a/arch/arm64/kernel/stacktrace.c +++ b/arch/arm64/kernel/stacktrace.c @@ -44,6 +44,10 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame) unsigned long fp = frame->fp; struct stack_info info; + /* Terminal record; nothing to unwind */ + if (!fp) + return -EINVAL; + if (fp & 0xf) return -EINVAL; @@ -104,15 +108,6 @@ int notrace unwind_frame(struct task_struct *tsk, struct stackframe *frame) frame->pc = ptrauth_strip_insn_pac(frame->pc); - /* - * Frames created upon entry from EL0 have NULL FP and PC values, so - * don't bother reporting these. Frames created by __noreturn functions - * might have a valid FP even if PC is bogus, so only terminate where - * both are NULL. - */ - if (!frame->fp && !frame->pc) - return -EINVAL; - return 0; } NOKPROBE_SYMBOL(unwind_frame); -- 2.11.0 _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel