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=-3.8 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI, SPF_HELO_NONE,SPF_PASS,URIBL_BLOCKED autolearn=no 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 76F8AC433E0 for ; Thu, 4 Mar 2021 18:04:36 +0000 (UTC) Received: from desiato.infradead.org (desiato.infradead.org [90.155.92.199]) (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 038F264F5E for ; Thu, 4 Mar 2021 18:04:35 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 038F264F5E 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=desiato.20200630; h=Sender:Content-Transfer-Encoding :Content-Type:List-Subscribe:List-Help:List-Post:List-Archive: List-Unsubscribe:List-Id:In-Reply-To:MIME-Version:References:Message-ID: Subject:Cc: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=ZIOSHtxlYQgQDXTiiryiX1hTdQZoNzioINYkEYSjGJk=; b=DEbfALp7K4OuSxMUQ5p0jl8kS Y9Q2qMHL50zHi2W9NBIuQuLA5mP5nJY+BUPQj9rr+0BSuvQTIqsQFTmWyJtVWB1s6x1F/pYhvCX4C w4EfIIM6gBpbUW9GJwh5ncDSZk9Qu9WXncvn3VZEjYSBC2EheKeLap5ZkbJGmKHVi/eJyA55U2ZPd 5AIs9At7vXffOsKUV/iehNvW2zFWoLV1r9ff7j/6SDlGsZPQg7S0iqU5/KKJ9B5CcJf/YTZnbajgo w6zf+14PEVjsOjhib1itBstVW5vAaWmyXpu3gm+4yW9jlyyBkhbxMxb2qC+lBlUpikLMAqMooQyyQ uSNLaq+Fg==; Received: from localhost ([::1] helo=desiato.infradead.org) by desiato.infradead.org with esmtp (Exim 4.94 #2 (Red Hat Linux)) id 1lHsIz-009W2d-D4; Thu, 04 Mar 2021 18:02:34 +0000 Received: from casper.infradead.org ([2001:8b0:10b:1236::1]) by desiato.infradead.org with esmtps (Exim 4.94 #2 (Red Hat Linux)) id 1lHsIr-009W1F-I4 for linux-arm-kernel@desiato.infradead.org; Thu, 04 Mar 2021 18:02:25 +0000 DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=infradead.org; s=casper.20170209; h=In-Reply-To:Content-Type:MIME-Version: References:Message-ID:Subject:Cc:To:From:Date:Sender:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=Rx9w1y6lR0AXBTE6W2W51QrNrTbTRZVSR3aeHfGZBUY=; b=HxBtCDWb/KU9pYlHrpWEuqPKKz H4w4t6MZ+l3Ac/FvmQrFw73voVOBrij+SQu6/K3wutw81h6enlMXf0kXB+B4DDETl4K4rPiyxep6U n1XOiGXKopIoZVBJeKWVLQBFwdzXvoBouE7n9tfHKG/QraWTZCkNPgqaR2xqT9DhE0VUPi2wq1O4C +GNiGH9T18bit/8e6ZYpCzJsrvyrI8IfGM/x0dv/10Pvx7QOAc+r19KrFvoXbTQ/HEYb/Zjm3K1Yk UMHqaOGirqrUMkcog42yJBGlyDYSVxhVjHTNJTDE/LHZLmp8HWhQDB6svm3PeONQA9oliHxJaCXBi gj3DgZ8A==; Received: from foss.arm.com ([217.140.110.172]) by casper.infradead.org with esmtp (Exim 4.94 #2 (Red Hat Linux)) id 1lHsIU-0086gf-PV for linux-arm-kernel@lists.infradead.org; Thu, 04 Mar 2021 18:02:05 +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 3662731B; Thu, 4 Mar 2021 10:01:59 -0800 (PST) Received: from C02TD0UTHF1T.local (unknown [10.57.53.210]) by usa-sjc-imap-foss1.foss.arm.com (Postfix) with ESMTPSA id 38B273F7D7; Thu, 4 Mar 2021 10:01:57 -0800 (PST) Date: Thu, 4 Mar 2021 18:01:54 +0000 From: Mark Rutland To: Marco Elver Cc: Christophe Leroy , Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , LKML , linuxppc-dev@lists.ozlabs.org, kasan-dev , Catalin Marinas , Will Deacon , Linux ARM , broonie@kernel.org, linux-toolchains@vger.kernel.org Subject: Re: [PATCH v1] powerpc: Include running function as first entry in save_stack_trace() and friends Message-ID: <20210304180154.GD60457@C02TD0UTHF1T.local> References: <1802be3e-dc1a-52e0-1754-a40f0ea39658@csgroup.eu> <20210304145730.GC54534@C02TD0UTHF1T.local> <20210304165923.GA60457@C02TD0UTHF1T.local> MIME-Version: 1.0 Content-Disposition: inline In-Reply-To: X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20210304_180205_956707_EA144218 X-CRM114-Status: GOOD ( 41.50 ) X-BeenThere: linux-arm-kernel@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , 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 On Thu, Mar 04, 2021 at 06:25:33PM +0100, Marco Elver wrote: > On Thu, Mar 04, 2021 at 04:59PM +0000, Mark Rutland wrote: > > On Thu, Mar 04, 2021 at 04:30:34PM +0100, Marco Elver wrote: > > > On Thu, 4 Mar 2021 at 15:57, Mark Rutland wrote: > > > > [adding Mark Brown] > > > > > > > > The bigger problem here is that skipping is dodgy to begin with, and > > > > this is still liable to break in some cases. One big concern is that > > > > (especially with LTO) we cannot guarantee the compiler will not inline > > > > or outline functions, causing the skipp value to be too large or too > > > > small. That's liable to happen to callers, and in theory (though > > > > unlikely in practice), portions of arch_stack_walk() or > > > > stack_trace_save() could get outlined too. > > > > > > > > Unless we can get some strong guarantees from compiler folk such that we > > > > can guarantee a specific function acts boundary for unwinding (and > > > > doesn't itself get split, etc), the only reliable way I can think to > > > > solve this requires an assembly trampoline. Whatever we do is liable to > > > > need some invasive rework. > > > > > > Will LTO and friends respect 'noinline'? > > > > I hope so (and suspect we'd have more problems otherwise), but I don't > > know whether they actually so. > > > > I suspect even with 'noinline' the compiler is permitted to outline > > portions of a function if it wanted to (and IIUC it could still make > > specialized copies in the absence of 'noclone'). > > > > > One thing I also noticed is that tail calls would also cause the stack > > > trace to appear somewhat incomplete (for some of my tests I've > > > disabled tail call optimizations). > > > > I assume you mean for a chain A->B->C where B tail-calls C, you get a > > trace A->C? ... or is A going missing too? > > Correct, it's just the A->C outcome. I'd assumed that those cases were benign, e.g. for livepatching what matters is what can be returned to, so B disappearing from the trace isn't a problem there. Is the concern debugability, or is there a functional issue you have in mind? > > > Is there a way to also mark a function non-tail-callable? > > > > I think this can be bodged using __attribute__((optimize("$OPTIONS"))) > > on a caller to inhibit TCO (though IIRC GCC doesn't reliably support > > function-local optimization options), but I don't expect there's any way > > to mark a callee as not being tail-callable. > > I don't think this is reliable. It'd be > __attribute__((optimize("-fno-optimize-sibling-calls"))), but doesn't > work if applied to the function we do not want to tail-call-optimize, > but would have to be applied to the function that does the tail-calling. Yup; that's what I meant then I said you could do that on the caller but not the callee. I don't follow why you'd want to put this on the callee, though, so I think I'm missing something. Considering a set of functions in different compilation units: A->B->C->D->E->F->G->H->I->J->K ... if K were marked in this way, and J was compiled with visibility of this, J would stick around, but J's callers might not, and so the a trace might see: A->J->K ... do you just care about the final caller, i.e. you just need certainty that J will be in the trace? If so, we can somewhat bodge that by having K have an __always_inline wrapper which has a barrier() or similar after the real call to K, so the call couldn't be TCO'd. Otherwise I'd expect we'd probably need to disable TCO generally. > So it's a bit backwards, even if it worked. > > > Accoding to the GCC documentation, GCC won't TCO noreturn functions, but > > obviously that's not something we can use generally. > > > > https://gcc.gnu.org/onlinedocs/gcc/Common-Function-Attributes.html#Common-Function-Attributes > > Perhaps we can ask the toolchain folks to help add such an attribute. Or > maybe the feature already exists somewhere, but hidden. > > +Cc linux-toolchains@vger.kernel.org > > > > But I'm also not sure if with all that we'd be guaranteed the code we > > > want, even though in practice it might. > > > > True! I'd just like to be on the least dodgy ground we can be. > > It's been dodgy for a while, and I'd welcome any low-cost fixes to make > it less dodgy in the short-term at least. :-) :) Thanks, Mark. _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel