From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753246Ab1IMMFR (ORCPT ); Tue, 13 Sep 2011 08:05:17 -0400 Received: from www17.your-server.de ([213.133.104.17]:40391 "EHLO www17.your-server.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752924Ab1IMMFP (ORCPT ); Tue, 13 Sep 2011 08:05:15 -0400 Subject: a question regarding sys_poll() on x86_64 via tha ia32 layer From: Thomas Meyer To: Linux Kernel Mailing List Date: Tue, 13 Sep 2011 14:04:02 +0200 Content-Type: text/plain; charset="UTF-8" X-Mailer: Evolution 3.0.3 (3.0.3-1.fc15) Content-Transfer-Encoding: 7bit Message-ID: <1315915448.6545.44.camel@localhost.localdomain> Mime-Version: 1.0 X-Authenticated-Sender: thomas@m3y3r.de Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Hello, the ia32 poll system call is routed through the "standard" function sys_poll(). This function is defined as: SYSCALL_DEFINE3(poll, struct pollfd __user *, ufds, unsigned int, nfds, long, timeout_msecs) in fs/select.c timeout_msecs is of type long which is AFAIK is 4 bytes on x86 and 8 bytes on x86_64. the test for sign (i.e. < 0) in the objdump is done against the 64 bit register (here %rbx): ffffffff811313e0 : ffffffff811313e0: 55 push %rbp ffffffff811313e1: 48 89 e5 mov %rsp,%rbp ffffffff811313e4: 48 83 ec 30 sub $0x30,%rsp ffffffff811313e8: 48 89 5d e8 mov %rbx,-0x18(%rbp) ffffffff811313ec: 48 89 d3 mov %rdx,%rbx ffffffff811313ef: 31 d2 xor %edx,%edx ffffffff811313f1: 48 85 db test %rbx,%rbx ffffffff811313f4: 4c 89 65 f0 mov %r12,-0x10(%rbp) ffffffff811313f8: 4c 89 6d f8 mov %r13,-0x8(%rbp) ffffffff811313fc: 41 89 f4 mov %esi,%r12d ffffffff811313ff: 49 89 fd mov %rdi,%r13 ffffffff81131402: 78 42 js ffffffff81131446 on an x86 kernel the test is done against %ebx so when the system call is called with %rbx = 00000000ffffffff (i.e. -1 from %ebx) on an x86_64 kernel via the ia32 layer the test for sign will fail and the timer will be set. btw. seems to define the function as extern int poll (struct pollfd *__fds, nfds_t __nfds, int __timeout); what am I overloking? mfg thomas