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 mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id D0F1BC433FE for ; Sun, 24 Oct 2021 17:29:23 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id B80C160EDF for ; Sun, 24 Oct 2021 17:29:23 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S231786AbhJXRbm (ORCPT ); Sun, 24 Oct 2021 13:31:42 -0400 Received: from wtarreau.pck.nerim.net ([62.212.114.60]:44527 "EHLO 1wt.eu" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231290AbhJXRbf (ORCPT ); Sun, 24 Oct 2021 13:31:35 -0400 Received: (from willy@localhost) by pcw.home.local (8.15.2/8.15.2/Submit) id 19OHSYEs018038; Sun, 24 Oct 2021 19:28:34 +0200 From: Willy Tarreau To: "Paul E . McKenney" Cc: Bedirhan KURT , Louvian Lyndal , Ammar Faizi , Peter Cordes , linux-kernel@vger.kernel.org, stable@vger.kernel.org Subject: [PATCH 1/3] tools/nolibc: x86-64: Fix startup code bug Date: Sun, 24 Oct 2021 19:28:14 +0200 Message-Id: <20211024172816.17993-2-w@1wt.eu> X-Mailer: git-send-email 2.17.5 In-Reply-To: <20211024172816.17993-1-w@1wt.eu> References: <20211024172816.17993-1-w@1wt.eu> Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org From: Ammar Faizi Before this patch, the `_start` function looks like this: ``` 0000000000001170 <_start>: 1170: pop %rdi 1171: mov %rsp,%rsi 1174: lea 0x8(%rsi,%rdi,8),%rdx 1179: and $0xfffffffffffffff0,%rsp 117d: sub $0x8,%rsp 1181: call 1000
1186: movzbq %al,%rdi 118a: mov $0x3c,%rax 1191: syscall 1193: hlt 1194: data16 cs nopw 0x0(%rax,%rax,1) 119f: nop ``` Note the "and" to %rsp with $-16, it makes the %rsp be 16-byte aligned, but then there is a "sub" with $0x8 which makes the %rsp no longer 16-byte aligned, then it calls main. That's the bug! What actually the x86-64 System V ABI mandates is that right before the "call", the %rsp must be 16-byte aligned, not after the "call". So the "sub" with $0x8 here breaks the alignment. Remove it. An example where this rule matters is when the callee needs to align its stack at 16-byte for aligned move instruction, like `movdqa` and `movaps`. If the callee can't align its stack properly, it will result in segmentation fault. x86-64 System V ABI also mandates the deepest stack frame should be zero. Just to be safe, let's zero the %rbp on startup as the content of %rbp may be unspecified when the program starts. Now it looks like this: ``` 0000000000001170 <_start>: 1170: pop %rdi 1171: mov %rsp,%rsi 1174: lea 0x8(%rsi,%rdi,8),%rdx 1179: xor %ebp,%ebp # zero the %rbp 117b: and $0xfffffffffffffff0,%rsp # align the %rsp 117f: call 1000
1184: movzbq %al,%rdi 1188: mov $0x3c,%rax 118f: syscall 1191: hlt 1192: data16 cs nopw 0x0(%rax,%rax,1) 119d: nopl (%rax) ``` Cc: Bedirhan KURT Cc: Louvian Lyndal Reported-by: Peter Cordes Signed-off-by: Ammar Faizi [wt: I did this on purpose due to a misunderstanding of the spec, other archs will thus have to be rechecked, particularly i386] Cc: stable@vger.kernel.org Signed-off-by: Willy Tarreau --- tools/include/nolibc/nolibc.h | 10 ++++++++-- 1 file changed, 8 insertions(+), 2 deletions(-) diff --git a/tools/include/nolibc/nolibc.h b/tools/include/nolibc/nolibc.h index 3430667b0d24..96b6d56acb57 100644 --- a/tools/include/nolibc/nolibc.h +++ b/tools/include/nolibc/nolibc.h @@ -399,14 +399,20 @@ struct stat { }) /* startup code */ +/* + * x86-64 System V ABI mandates: + * 1) %rsp must be 16-byte aligned right before the function call. + * 2) The deepest stack frame should be zero (the %rbp). + * + */ asm(".section .text\n" ".global _start\n" "_start:\n" "pop %rdi\n" // argc (first arg, %rdi) "mov %rsp, %rsi\n" // argv[] (second arg, %rsi) "lea 8(%rsi,%rdi,8),%rdx\n" // then a NULL then envp (third arg, %rdx) - "and $-16, %rsp\n" // x86 ABI : esp must be 16-byte aligned when - "sub $8, %rsp\n" // entering the callee + "xor %ebp, %ebp\n" // zero the stack frame + "and $-16, %rsp\n" // x86 ABI : esp must be 16-byte aligned before call "call main\n" // main() returns the status code, we'll exit with it. "movzb %al, %rdi\n" // retrieve exit code from 8 lower bits "mov $60, %rax\n" // NR_exit == 60 -- 2.17.5