Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1755987Ab3GBAoI (ORCPT ); Mon, 1 Jul 2013 20:44:08 -0400 Received: from mail-pa0-f41.google.com ([209.85.220.41]:44214 "EHLO mail-pa0-f41.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1755204Ab3GBAoE (ORCPT ); Mon, 1 Jul 2013 20:44:04 -0400 Message-ID: <51D2224E.8090700@gmail.com> Date: Tue, 02 Jul 2013 08:43:58 +0800 From: Jiaxing Wang User-Agent: Mozilla/5.0 (X11; Linux i686; rv:17.0) Gecko/20130623 Thunderbird/17.0.7 MIME-Version: 1.0 To: fangdong CC: Jiaxing Wang , viro@zeniv.linux.org.uk, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org Subject: Re: [PATCH] seq_file:update file->f_pos when lseek() to m->read_pos References: <1372497096-15914-1-git-send-email-hello.wjx@gmail.com> <51D178FA.4020601@pipul.org> In-Reply-To: <51D178FA.4020601@pipul.org> Content-Type: text/plain; charset=ISO-8859-1 Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 2552 Lines: 64 On 07/01/2013 08:41 PM, fangdong wrote: > On 06/29/2013 05:11 AM, Jiaxing Wang wrote: >> After pread(), file->f_pos and m->read_pos get different, >> and lseek() to m->read_pos did not update file->f_pos, then >> a subsequent read may read from a wrong position, the following >> program shows the problem: >> >> char str1[32] = { 0 }; >> char str2[32] = { 0 }; >> int poffset = 10; >> int count = 20; >> >> /*open any seq file*/ >> int fd = open("/proc/modules", O_RDONLY); >> >> pread(fd, str1, count, poffset); >> printf("pread:%s\n", str1); >> >> /*seek to where m->read_pos is*/ >> lseek(fd, poffset+count, SEEK_SET); >> >> /*supposed to read from poffset+count, but this read from position 0*/ >> read(fd, str2, count); >> printf("read:%s\n", str2); >> >> Signed-off-by: Jiaxing Wang >> --- >> fs/seq_file.c | 3 ++- >> 1 file changed, 2 insertions(+), 1 deletion(-) >> >> diff --git a/fs/seq_file.c b/fs/seq_file.c >> index 774c1eb..4b22b26 100644 >> --- a/fs/seq_file.c >> +++ b/fs/seq_file.c >> @@ -328,7 +328,8 @@ loff_t seq_lseek(struct file *file, loff_t offset, int whence) >> m->read_pos = offset; >> retval = file->f_pos = offset; >> } >> - } >> + } else >> + file->f_pos = offset; >> } >> file->f_version = m->version; >> mutex_unlock(&m->lock); >> > This does not appear to be a problem, in linux man page, the behaver seems clearly defined: > > DESCRIPTION > pread() reads up to count bytes from file descriptor fd at offset off-set (from the start of the file) into the buffer starting at buf. The file offset is not changed. > > pwrite() writes up to count bytes from the buffer starting at buf to the file descriptor fd at offset offset. The file offset is not changed. > > -- > To unsubscribe from this list: send the line "unsubscribe linux-kernel" in > the body of a message to majordomo@vger.kernel.org > More majordomo info at http://vger.kernel.org/majordomo-info.html > Please read the FAQ at http://www.tux.org/lkml/ There's no problem that pread() don't change file->f_pos, but I think lseek() should have changed it. Any comments from other people, and Alexander? -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/