From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.6 (2021-04-09) on gnuweeb.org X-Spam-Level: X-Spam-Status: No, score=1.0 required=5.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, SPF_HELO_PASS,SPF_PASS autolearn=no autolearn_force=no version=3.4.6 Authentication-Results: gnuweeb.org; dmarc=none (p=none dis=none) header.from=1wt.eu Received-SPF: Pass (mailfrom) identity=mailfrom; client-ip=163.172.96.212; helo=1wt.eu; envelope-from=w@1wt.eu; receiver= Received: from 1wt.eu (ded1.1wt.eu [163.172.96.212]) by gnuweeb.org (Postfix) with ESMTP id 716BC24B21B for ; Tue, 29 Aug 2023 13:22:04 +0700 (WIB) Received: (from willy@localhost) by mail.home.local (8.17.1/8.17.1/Submit) id 37T6Lloe011605; Tue, 29 Aug 2023 08:21:47 +0200 Date: Tue, 29 Aug 2023 08:21:47 +0200 From: Willy Tarreau To: Ammar Faizi Cc: Thomas =?iso-8859-1?Q?Wei=DFschuh?= , Zhangjin Wu , Nicholas Rosenberg , Michael William Jonathan , GNU/Weeb Mailing List , Linux Kernel Mailing List Subject: Re: [PATCH v2 1/1] tools/nolibc: i386: Fix a stack misalign bug on _start Message-ID: References: <20230828070240.1691921-1-ammarfaizi2@gnuweeb.org> <20230828070240.1691921-2-ammarfaizi2@gnuweeb.org> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230828070240.1691921-2-ammarfaizi2@gnuweeb.org> List-Id: Hi Ammar, On Mon, Aug 28, 2023 at 02:02:40PM +0700, Ammar Faizi wrote: > The ABI mandates that the %esp register must be a multiple of 16 when > executing a 'call' instruction. > > Commit 2ab446336b17 ("tools/nolibc: i386: shrink _start with _start_c") > simplified the _start function, but it didn't take care of the %esp > alignment, causing SIGSEGV on SSE and AVX programs that use aligned move > instruction (e.g., movdqa, movaps, and vmovdqa). > > The 'and $-16, %esp' aligns the %esp at a multiple of 16. Then 'push > %eax' will subtract the %esp by 4; thus, it breaks the 16-byte > alignment. Make sure the %esp is correctly aligned after the push by > subtracting 12 before the push. Indeed, good catch! However if we want to do it cleany (i.e not punch a 16 to 28 byes hole in the stack), we should do this instead: add $12, %esp /* the stack must be aligned to 16 ... */ and $-16, %esp /* ... bytes after eax is pushed and ... */ sub $12, %esp /* ... before the call. */ This will only add 0 to 12 bytes depending on the existing alignment. What do you think ? thanks, Willy