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Thu, 21 Mar 2024 10:22:30 +0000 From: David Laight To: 'Jiangfeng Xiao' , "Russell King (Oracle)" CC: "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+AaCprEesIWGaAOB7ebFB9uHA Date: Thu, 21 Mar 2024 10:22:30 +0000 Message-ID: References: <1709516385-7778-1-git-send-email-xiaojiangfeng@huawei.com> <1710906278-23851-1-git-send-email-xiaojiangfeng@huawei.com> <84a57ca8-8963-ca24-8bd1-ddc5c33bf4da@huawei.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 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: quoted-printable X-Rspam-User: X-Rspamd-Server: rspam12 X-Rspamd-Queue-Id: 28213C0002 X-Stat-Signature: 5mctnniyx9d9yjxet4fkhym7hbj6tnsz X-HE-Tag: 1711016579-605046 X-HE-Meta: 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 AqN2BbL+ XkeqI6Z1bxR45PCbICCZ2Poy8rhVTqhd4X9Gi3dbA/pheMWIoFhRX9x1/3Q44Sr8TUYJ6eRJKfY+aQyLpanE4RiDywIybks6MSPebRWyIcgSCjFng2Ih3ukd1IGcFk+x0V2b3MEtoT1z36aK5I7vJ3Xa/5DFtfgPivdZPIZO4l720ViO0SHiTg9a6sPU9YAaxS+Tfj5DAS5B5sSDBVeVjWPc7qEgng40Dge0uq2O1ZYtaCLE= X-Bogosity: Ham, tests=bogofilter, spamicity=0.000000, version=1.2.4 Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: List-Unsubscribe: How aggressively does the compiler optimise 'noreturn' functions? Consider: void f(...) { =09... =09if () { =09=09... =09=09noreturn(...); =09} } Without the noreturn() call it is a leaf function. So the compiler doesn't need to save 'lr' on stack (or the save could be deferred to inside the conditional). Since noreturn() doesn't return it can be jumped to. Additionally 'lr' can be used as a scratch register prior to the noreturn() call. So it is likely that inside noreturn() (and anything it might call) you don't have a valid 'lr' chain at all. No amount of picking between 'pc' and 'pc-1' is going to fix that. The only way you can find a return address is by searching the stack and hoping to find something that works. So you need the compiler to 'not believe' the 'noreturn' attribute. Setup a normal call frame and put a faulting instruction after the call in case it returns. That would give you half a chance of generating a backtrace. Without that I suspect you are playing whack-a-mole. =09David - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1= PT, UK Registration No: 1397386 (Wales)