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 X-Spam-Level: X-Spam-Status: No, score=-5.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 86C72C433DF for ; Fri, 10 Jul 2020 12:19:27 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 5C67B20720 for ; Fri, 10 Jul 2020 12:19:27 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1727818AbgGJMTZ (ORCPT ); Fri, 10 Jul 2020 08:19:25 -0400 Received: from mx2.suse.de ([195.135.220.15]:38280 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1726774AbgGJMTZ (ORCPT ); Fri, 10 Jul 2020 08:19:25 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id EB156AC46; Fri, 10 Jul 2020 12:19:23 +0000 (UTC) Date: Fri, 10 Jul 2020 14:19:23 +0200 From: Petr Mladek To: Bruno Meneguele Cc: linux-kernel@vger.kernel.org, sergey.senozhatsky@gmail.com, torvalds@linux-foundation.org, Jason@zx2c4.com Subject: Re: [PATCH] doc:kmsg: explictly state the return value in case of SEEK_CUR Message-ID: <20200710121923.GO4751@alley> References: <20200709155415.41207-1-bmeneg@redhat.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20200709155415.41207-1-bmeneg@redhat.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On Thu 2020-07-09 12:54:15, Bruno Meneguele wrote: > The commit 625d3449788f ("Revert "kernel/printk: add kmsg SEEK_CUR > handling"") reverted a change done to the return value in case a SEEK_CUR > operation was performed for kmsg buffer based on the fact that different > userspace apps were handling the new return value (-ESPIPE) in different > ways, breaking them. > > At the same time -ESPIPE was the wrong decision because kmsg /does support/ > seek() but doesn't follow the "normal" behavior userspace is used to. > Because of that and also considering the time -EINVAL has been used, it was > decided to keep this way to avoid more userspace breakage. > > This patch adds an official statement to the kmsg documentation pointing to > the current return value for SEEK_CUR, -EINVAL, thus userspace libraries and > apps can refer to it for a definitive guide on what to expected. > > --- a/Documentation/ABI/testing/dev-kmsg > +++ b/Documentation/ABI/testing/dev-kmsg > @@ -56,6 +56,11 @@ Description: The /dev/kmsg character device node provides userspace access > seek after the last record available at the time > the last SYSLOG_ACTION_CLEAR was issued. > > + Considering that the seek operation is supported but has > + special meaning to the device, any attempt to seek specific > + positions on the buffer (i.e. using SEEK_CUR) results in an > + -EINVAL error returned to userspace. Sigh, I see that devkmsg_llseek() returns -ESPIPE when offset is not zero. This is a real mess. I wonder if we could afford to switch this one to -EINVAL and reduce the mess. Anyway, for a random reader, it might be pretty unclear what is exactly special about /dev/kmsg. I wonder if the following might be more explanatory and strightforward: Other seek operations or offsets are not supported because of the special behavior. The device allows to read or write only whole variable lenght messages that are stored in a ring buffer. Because of the non-standard behavior also the error values are non-standardand. -ESPIPE is returned for non-zero offset. -EINVAL is returned for other operations, e.g. SEEK_CUR. This behavior is historical and could not be modified wihtout the risk of breaking userspace. Finally, only few people read documentation. We should add some warning also to the code. I think about a something like: /* * Be careful when modifying this function!!! * * Only few operations are supported because the device works only with * the entire variable length messages. Non-standard error values are * returned in the other cases. User space applications might depend * on this behavior. */ Best Regards, Petr