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 Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by smtp.lore.kernel.org (Postfix) with ESMTP id 384E6C83F11 for ; Sun, 27 Aug 2023 19:43:01 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230133AbjH0Tmb (ORCPT ); Sun, 27 Aug 2023 15:42:31 -0400 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:46134 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S230046AbjH0Tl5 (ORCPT ); Sun, 27 Aug 2023 15:41:57 -0400 Received: from zeniv.linux.org.uk (zeniv.linux.org.uk [IPv6:2a03:a000:7:0:5054:ff:fe1c:15ff]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 0F8AD12E; Sun, 27 Aug 2023 12:41:52 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=linux.org.uk; s=zeniv-20220401; h=Sender:In-Reply-To:Content-Type: MIME-Version:References:Message-ID:Subject:Cc:To:From:Date:Reply-To: Content-Transfer-Encoding:Content-ID:Content-Description; bh=684ZmibV0iRA7W1nKgqEgsL3aMIsGcfp6Zpl3WdJ4DU=; b=MC2inegbdk323giCAEH+EfdYar 4kXwfSwyy2upf/dK+53+T+qrKyewF5s7dkck4Fy8HiN43ivhIJelmFUnVBY8gTnpZi1xWKTWuXIKw PMxisq0e3KNF97DWVe6B6NWvtgyUOe2N5H03vNuXGmN7U3J3s/iHPYzfZnHTYEtijbR38gB7y6yM+ f7ACCYsTkBZjsnjijsZxvBLbNGKP/mTU09fHGjxl78/It44EuuASOQ2dnSIavl4Uy9jNvEkCB6Mv6 wSB2x3czu7lrhAFvslc5eM2xpF0ak84dfAdNaJZPWXabaeFkiWOblNYIYyhnd8Q9EJERXc2PyYNn0 cSxlywzw==; Received: from viro by zeniv.linux.org.uk with local (Exim 4.96 #2 (Red Hat Linux)) id 1qaLdS-001NqG-1T; Sun, 27 Aug 2023 19:41:22 +0000 Date: Sun, 27 Aug 2023 20:41:22 +0100 From: Al Viro To: Christoph Hellwig Cc: Matthew Wilcox , Jens Axboe , Xiubo Li , Ilya Dryomov , Christian Brauner , Theodore Ts'o , Jaegeuk Kim , Chao Yu , Miklos Szeredi , Andreas Gruenbacher , "Darrick J. Wong" , Trond Myklebust , Anna Schumaker , Damien Le Moal , Andrew Morton , linux-block@vger.kernel.org, ceph-devel@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-ext4@vger.kernel.org, linux-f2fs-devel@lists.sourceforge.net, cluster-devel@redhat.com, linux-xfs@vger.kernel.org, linux-nfs@vger.kernel.org, linux-mm@kvack.org, Hannes Reinecke Subject: Re: [PATCH 03/12] filemap: update ki_pos in generic_perform_write Message-ID: <20230827194122.GA325446@ZenIV> References: <20230601145904.1385409-1-hch@lst.de> <20230601145904.1385409-4-hch@lst.de> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20230601145904.1385409-4-hch@lst.de> Sender: Al Viro Precedence: bulk List-ID: X-Mailing-List: linux-ext4@vger.kernel.org On Thu, Jun 01, 2023 at 04:58:55PM +0200, Christoph Hellwig wrote: > All callers of generic_perform_write need to updated ki_pos, move it into > common code. > @@ -4034,7 +4037,6 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from) > endbyte = pos + status - 1; > err = filemap_write_and_wait_range(mapping, pos, endbyte); > if (err == 0) { > - iocb->ki_pos = endbyte + 1; > written += status; > invalidate_mapping_pages(mapping, > pos >> PAGE_SHIFT, > @@ -4047,8 +4049,6 @@ ssize_t __generic_file_write_iter(struct kiocb *iocb, struct iov_iter *from) > } > } else { > written = generic_perform_write(iocb, from); > - if (likely(written > 0)) > - iocb->ki_pos += written; > } > out: > return written ? written : err; [another late reply, sorry] That part is somewhat fishy - there's a case where you return a positive value and advance ->ki_pos by more than that amount. I really wonder if all callers of ->write_iter() are OK with that. Consider e.g. this: ssize_t ksys_write(unsigned int fd, const char __user *buf, size_t count) { struct fd f = fdget_pos(fd); ssize_t ret = -EBADF; if (f.file) { loff_t pos, *ppos = file_ppos(f.file); if (ppos) { pos = *ppos; ppos = &pos; } ret = vfs_write(f.file, buf, count, ppos); if (ret >= 0 && ppos) f.file->f_pos = pos; fdput_pos(f); } return ret; } ssize_t vfs_write(struct file *file, const char __user *buf, size_t count, loff_t *pos) { ssize_t ret; if (!(file->f_mode & FMODE_WRITE)) return -EBADF; if (!(file->f_mode & FMODE_CAN_WRITE)) return -EINVAL; if (unlikely(!access_ok(buf, count))) return -EFAULT; ret = rw_verify_area(WRITE, file, pos, count); if (ret) return ret; if (count > MAX_RW_COUNT) count = MAX_RW_COUNT; file_start_write(file); if (file->f_op->write) ret = file->f_op->write(file, buf, count, pos); else if (file->f_op->write_iter) ret = new_sync_write(file, buf, count, pos); else ret = -EINVAL; if (ret > 0) { fsnotify_modify(file); add_wchar(current, ret); } inc_syscw(current); file_end_write(file); return ret; } static ssize_t new_sync_write(struct file *filp, const char __user *buf, size_t len, loff_t *ppos) { struct kiocb kiocb; struct iov_iter iter; ssize_t ret; init_sync_kiocb(&kiocb, filp); kiocb.ki_pos = (ppos ? *ppos : 0); iov_iter_ubuf(&iter, ITER_SOURCE, (void __user *)buf, len); ret = call_write_iter(filp, &kiocb, &iter); BUG_ON(ret == -EIOCBQUEUED); if (ret > 0 && ppos) *ppos = kiocb.ki_pos; return ret; } Suppose ->write_iter() ends up doing returning a positive value smaller than the increment of kiocb.ki_pos. What do we get? ret is positive, so kiocb.ki_pos gets copied into *ppos, which is ksys_write's pos and there we copy it into file->f_pos. Is it really OK to have write() return 4096 and advance the file position by 16K? AFAICS, userland wouldn't get any indication of something odd going on - just a short write to a regular file, with followup write of remaining 12K getting quietly written in the range 16K..28K. I don't remember what POSIX says about that, but it would qualify as nasty surprise for any userland program - sure, one can check fsync() results before closing the sucker and see if everything looks fine, but the way it's usually discussed could easily lead to assumption that (synchronous) O_DIRECT writes would not be affected by anything of that sort.