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 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.lore.kernel.org (Postfix) with ESMTPS id 149F6C54E68 for ; Thu, 21 Mar 2024 14:38:21 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender: Content-Transfer-Encoding:Content-Type:List-Subscribe:List-Help:List-Post: List-Archive:List-Unsubscribe:List-Id:MIME-Version:In-Reply-To:References: Message-ID:Date:Subject:CC:To:From:Reply-To:Content-ID:Content-Description: Resent-Date:Resent-From:Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID: List-Owner; bh=/RT2Zbp4DLoHmA7elhN5PEpO+ibiPpKJHJIQAA5+qqQ=; b=Zm3H4g4nZzOr3Q nsQr2b+PSRywYreFUQNyVZiR7Ezr0n3QY5EDr6GxY42mbzNFxTR26I/dJiL3QztDtyLJOV7xx49OG kwKqBOMxMEGQgQzSz4XyWt08RAyxeaGxfmJ9C5Odg5U7WJfbQx7yvSJhFaexSWqINndLTDaTC7rzw UCgCrSM4sK832VO0wklW7gyYJtEKcnWjJprv82ckIa/jUvuSPD7fiQXod6fVbn3NhsROw6al1Bzi3 Lrd4LHBKV9Ctj18/ZSGdt8viwfZWatsPvDpTDae+shTC7Hw4LDffGfarn5yAVPTyFu4vvJLRBwswO aunoqy57LXY2imYh5CLg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.97.1 #2 (Red Hat Linux)) id 1rnJYS-00000003IW0-1cHp; Thu, 21 Mar 2024 14:38:04 +0000 Received: from eu-smtp-delivery-151.mimecast.com ([185.58.86.151]) by bombadil.infradead.org with esmtps (Exim 4.97.1 #2 (Red Hat Linux)) id 1rnJYN-00000003IUs-1bVj for linux-arm-kernel@lists.infradead.org; Thu, 21 Mar 2024 14:38:02 +0000 Received: from AcuMS.aculab.com (156.67.243.121 [156.67.243.121]) by relay.mimecast.com with ESMTP with both STARTTLS and AUTH (version=TLSv1.2, cipher=TLS_ECDHE_RSA_WITH_AES_256_CBC_SHA384) id uk-mta-187-6fRfpWVCPDm3cO9Kixse_g-1; Thu, 21 Mar 2024 14:37:54 +0000 X-MC-Unique: 6fRfpWVCPDm3cO9Kixse_g-1 Received: from AcuMS.Aculab.com (10.202.163.6) by AcuMS.aculab.com (10.202.163.6) with Microsoft SMTP Server (TLS) id 15.0.1497.48; Thu, 21 Mar 2024 14:37:28 +0000 Received: from AcuMS.Aculab.com ([::1]) by AcuMS.aculab.com ([::1]) with mapi id 15.00.1497.048; Thu, 21 Mar 2024 14:37:28 +0000 From: David Laight To: 'Russell King' CC: Ard Biesheuvel , 'Jiangfeng Xiao' , "arnd@arndb.de" , "keescook@chromium.org" , "haibo.li@mediatek.com" , "angelogioacchino.delregno@collabora.com" , "amergnat@baylibre.com" , "akpm@linux-foundation.org" , "dave.hansen@linux.intel.com" , "douzhaolei@huawei.com" , "gustavoars@kernel.org" , "jpoimboe@kernel.org" , "kepler.chenxin@huawei.com" , "kirill.shutemov@linux.intel.com" , "linux-hardening@vger.kernel.org" , "linux-kernel@vger.kernel.org" , "linux-mm@kvack.org" , "linux-arm-kernel@lists.infradead.org" , "nixiaoming@huawei.com" , "peterz@infradead.org" , "wangbing6@huawei.com" , "wangfangpeng1@huawei.com" , "jannh@google.com" , "willy@infradead.org" Subject: RE: [PATCH v2] ARM: unwind: improve unwinders for noreturn case Thread-Topic: [PATCH v2] ARM: unwind: improve unwinders for noreturn case Thread-Index: AQHae3ROEuI+AaCprEesIWGaAOB7ebFB9uHAgAAWtoCAAAVW8IAACy4AgAADrmCAAAjqAIAAC+ng Date: Thu, 21 Mar 2024 14:37:28 +0000 Message-ID: <9d6057b110034c04b6b590522c8c69cc@AcuMS.aculab.com> References: <1710906278-23851-1-git-send-email-xiaojiangfeng@huawei.com> <84a57ca8-8963-ca24-8bd1-ddc5c33bf4da@huawei.com> <0fd55e156195440bb1d815dd8300894b@AcuMS.aculab.com> In-Reply-To: Accept-Language: en-GB, en-US X-MS-Has-Attach: X-MS-TNEF-Correlator: x-ms-exchange-transport-fromentityheader: Hosted x-originating-ip: [10.202.205.107] MIME-Version: 1.0 X-Mimecast-Spam-Score: 0 X-Mimecast-Originator: aculab.com Content-Language: en-US X-CRM114-Version: 20100106-BlameMichelson ( TRE 0.8.0 (BSD) ) MR-646709E3 X-CRM114-CacheID: sfid-20240321_073759_756307_EDC8D38D X-CRM114-Status: GOOD ( 21.43 ) 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 From: Russell King > Sent: 21 March 2024 13:08 > > On Thu, Mar 21, 2024 at 12:57:07PM +0000, David Laight wrote: > > From: Russell King > > > Sent: 21 March 2024 12:23 > > ... > > > > That might mean you can get the BL in the middle of a function > > > > but where the following instruction is for the 'no stack frame' > > > > side of the branch. > > > > That is very likely to break any stack offset calculations. > > > > > > No it can't. At any one point in the function, the stack has to be in > > > a well defined state, so that access to local variables can work, and > > > also the stack can be correctly unwound. If there exists a point in > > > the function body which can be reached where the stack could be in two > > > different states, then the stack can't be restored to the parent > > > context. > > > > Actually you can get there with a function that has a lot of args. > > So you can have: > > if (...) { > > push x > > bl func > > add %sp, #8 > > } > > code; > > which is fine. > > No you can't.... and that isn't even Arm code. Arm doesn't use %sp. > Moreover, that "bl" will stomp over the link register, meaning this > function can not return. With 9+ arguments they spill to see https://godbolt.org/z/Yj3ovd8bY Where the compiler generates: f9: cmp w0, 0 ble .L2 sub sp, sp, #32 mov w7, w0 mov w6, w0 mov w5, w0 mov w4, w0 mov w3, w0 stp x29, x30, [sp, 16] add x29, sp, 16 mov w2, w0 mov w1, w0 str w0, [sp] bl f .L2: ret A traceback from inside f() definitely needs to use LR-4 for the stack offset. (arm64 doesn't seem to support -mno-sched-prolog). I've failed to get different sized stack frames for the true/false sides of the branch. The compiler seems to pre-allocate the space for extra args rather than using 'push' type instructions. This was certainly better for some x86 cpu (p-pro?) but has now gone out of fashion. David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales) _______________________________________________ linux-arm-kernel mailing list linux-arm-kernel@lists.infradead.org http://lists.infradead.org/mailman/listinfo/linux-arm-kernel