Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754116Ab3GAMwY (ORCPT ); Mon, 1 Jul 2013 08:52:24 -0400 Received: from mail-pa0-f49.google.com ([209.85.220.49]:55038 "EHLO mail-pa0-f49.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752741Ab3GAMwX (ORCPT ); Mon, 1 Jul 2013 08:52:23 -0400 Message-ID: <51D17B76.3060803@gmail.com> Date: Mon, 01 Jul 2013 08:52:06 -0400 From: Dong Fang User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:17.0) Gecko/20130625 Thunderbird/17.0.7 MIME-Version: 1.0 To: Jiaxing Wang CC: 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> In-Reply-To: <1372497096-15914-1-git-send-email-hello.wjx@gmail.com> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 1989 Lines: 59 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/