From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1754771AbbAWXJo (ORCPT ); Fri, 23 Jan 2015 18:09:44 -0500 Received: from mail.linuxfoundation.org ([140.211.169.12]:41845 "EHLO mail.linuxfoundation.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751737AbbAWXJk (ORCPT ); Fri, 23 Jan 2015 18:09:40 -0500 Date: Fri, 23 Jan 2015 15:09:39 -0800 From: Andrew Morton To: Mike Crowe Cc: linux-kernel@vger.kernel.org, Arnd Bergmann , Kay Sievers , Greg Kroah-Hartman Subject: Re: kmsg: lseek errors confuse glibc's dprintf Message-Id: <20150123150939.f96d28a380fc5ec1f3894d3a@linux-foundation.org> In-Reply-To: <20150115173132.GA7486@mcrowe.com> References: <20150115173132.GA7486@mcrowe.com> X-Mailer: Sylpheed 3.4.1 (GTK+ 2.24.23; x86_64-pc-linux-gnu) Mime-Version: 1.0 Content-Type: text/plain; charset=US-ASCII Content-Transfer-Encoding: 7bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu, 15 Jan 2015 17:31:32 +0000 Mike Crowe wrote: > glibc's dprintf implementation does not work correctly with /dev/kmsg file > descriptors because glibc treats receiving EBADF and EINVAL from lseek when > trying to determine the current file position as errors. See > https://sourceware.org/bugzilla/show_bug.cgi?id=17830 > > >From what I can tell prior to Kay Sievers printk record commit > e11fea92e13fb91c50bacca799a6131c81929986, calling lseek(fd, 0, SEEK_CUR) > with such a file descriptor would not return an error. > > Prior to Kay's change, Arnd Bergmann's commit > 6038f373a3dc1f1c26496e60b6c40b164716f07e seemed to go to some lengths to > preserve the successful return code rather than returning (the perhaps more > logical) -ESPIPE. > > glibc is happy with either a successful return or -ESPIPE. > > For maximum compatibility it seems that success should be returned but > given Kay's new seek interface perhaps this isn't helpful. > > This patch ensures that such a seek succeeds: > > diff --git a/kernel/printk/printk.c b/kernel/printk/printk.c > index 02d6b6d..b3ff6f0 100644 > --- a/kernel/printk/printk.c > +++ b/kernel/printk/printk.c > @@ -693,7 +693,7 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence) > loff_t ret = 0; > > if (!user) > - return -EBADF; > + return (whence == SEEK_CUR) ? 0 : -EBADF; What's actually going on here? What is the significance of file->private_data==NULL and why does this code treat it as an error? > if (offset) > return -ESPIPE; > > @@ -718,6 +718,11 @@ static loff_t devkmsg_llseek(struct file *file, loff_t offset, int whence) > user->idx = log_next_idx; > user->seq = log_next_seq; > break; > + case SEEK_CUR: > + /* For compatibility with userspace requesting the > + * current file position. */ > + ret = 0; > + break; Can we actually do something useful here? Return some value which can be fed back into SEEK_SET to restore the file position? > default: > ret = -EINVAL; > }