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=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=ham 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 8D762C35240 for ; Fri, 24 Jan 2020 00:02:46 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 63DB421835 for ; Fri, 24 Jan 2020 00:02:46 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730101AbgAXACp (ORCPT ); Thu, 23 Jan 2020 19:02:45 -0500 Received: from 216-12-86-13.cv.mvl.ntelos.net ([216.12.86.13]:52738 "EHLO brightrain.aerifal.cx" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1729149AbgAXACp (ORCPT ); Thu, 23 Jan 2020 19:02:45 -0500 Received: from dalias by brightrain.aerifal.cx with local (Exim 3.15 #2) id 1iumQt-00039n-00; Fri, 24 Jan 2020 00:02:43 +0000 Date: Thu, 23 Jan 2020 19:02:43 -0500 From: Rich Felker To: linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org, linux-api@vger.kernel.org Cc: Alexander Viro Subject: Proposal to fix pwrite with O_APPEND via pwritev2 flag Message-ID: <20200124000243.GA12112@brightrain.aerifal.cx> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline User-Agent: Mutt/1.5.21 (2010-09-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org There's a longstanding unfixable (due to API stability) bug in the pwrite syscall: http://man7.org/linux/man-pages/man2/pwrite.2.html#BUGS whereby it wrongly honors O_APPEND if set, ignoring the caller-passed offset. Now that there's a pwritev2 syscall that takes a flags argument, it's possible to fix this without breaking stability by adding a new RWF_NOAPPEND flag, which callers that want the fixed behavior can then pass. I have a completely untested patch to add such a flag, but would like to get a feel for whether the concept is acceptable before putting time into testing it. If so, I'll submit this as a proper patch with detailed commit message etc. Draft is below. Rich diff --git a/include/linux/fs.h b/include/linux/fs.h index e0d909d35763..3a769a972f79 100644 --- a/include/linux/fs.h +++ b/include/linux/fs.h @@ -3397,6 +3397,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags) { if (unlikely(flags & ~RWF_SUPPORTED)) return -EOPNOTSUPP; + if (unlikely((flags & RWF_APPEND) && (flags & RWF_NOAPPEND))) + return -EINVAL; if (flags & RWF_NOWAIT) { if (!(ki->ki_filp->f_mode & FMODE_NOWAIT)) @@ -3411,6 +3413,8 @@ static inline int kiocb_set_rw_flags(struct kiocb *ki, rwf_t flags) ki->ki_flags |= (IOCB_DSYNC | IOCB_SYNC); if (flags & RWF_APPEND) ki->ki_flags |= IOCB_APPEND; + if (flags & RWF_NOAPPEND) + ki->ki_flags &= ~IOCB_APPEND; return 0; } diff --git a/include/uapi/linux/fs.h b/include/uapi/linux/fs.h index 379a612f8f1d..591357d9b3c9 100644 --- a/include/uapi/linux/fs.h +++ b/include/uapi/linux/fs.h @@ -299,8 +299,11 @@ typedef int __bitwise __kernel_rwf_t; /* per-IO O_APPEND */ #define RWF_APPEND ((__force __kernel_rwf_t)0x00000010) +/* per-IO negation of O_APPEND */ +#define RWF_NOAPPEND ((__force __kernel_rwf_t)0x00000020) + /* mask of flags supported by the kernel */ #define RWF_SUPPORTED (RWF_HIPRI | RWF_DSYNC | RWF_SYNC | RWF_NOWAIT |\ - RWF_APPEND) + RWF_APPEND | RWF_NOAPPEND) #endif /* _UAPI_LINUX_FS_H */