From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from kanga.kvack.org (kanga.kvack.org [205.233.56.17]) by smtp.subspace.kernel.org (Postfix) with ESMTP id 3424F606A325 for ; Tue, 27 Oct 2026 08:46:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=MIHfOMOJ Received: by kanga.kvack.org (Postfix) id 127216B0633; Tue, 27 Oct 2026 04:46:24 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id 0B0DE6B0635; Tue, 27 Oct 2026 04:46:24 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id E6C326B0636; Tue, 27 Oct 2026 04:46:23 -0400 (EDT) X-Delivered-To: linux-mm@kvack.org Received: from relay.hostedemail.com (smtprelay0016.hostedemail.com [216.40.44.16]) by kanga.kvack.org (Postfix) with ESMTP id B5C656B0633 for ; Tue, 27 Oct 2026 04:46:23 -0400 (EDT) Received: from smtpin12.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay09.hostedemail.com (Postfix) with ESMTP id 4258E80719 for ; Tue, 27 Oct 2026 08:46:23 +0000 (UTC) X-FDA: 85367774646.12.DEC32DC Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf29.hostedemail.com (Postfix) with ESMTP id A2847120004 for ; Tue, 27 Oct 2026 08:46:21 +0000 (UTC) Authentication-Results: imf29.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=MIHfOMOJ; spf=pass (imf29.hostedemail.com: domain of ljs@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=ljs@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Seal: i=1; a=rsa-sha256; d=hostedemail.com; s=arc-20220608; cv=none; t=1793090781; b=ML+fFAFlh2LJcyXRBYdRI/j91u5MLLLH1YrMsSGp4JS+iWzHyoMAmE2icuUITEVTeMYIuJ zZ7Giwawxwef3a91UVICt5Gb4QFSWM19dUPLt+j0gd8VH59L6xzGghnPkvs5sFKF6nLljl 75I0NfvCrB/R3MpO5ZIn5vz3U5Nb7vg= ARC-Authentication-Results: i=1; imf29.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=MIHfOMOJ; spf=pass (imf29.hostedemail.com: domain of ljs@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=ljs@kernel.org; dmarc=pass (policy=quarantine) header.from=kernel.org ARC-Message-Signature: i=1; a=rsa-sha256; c=relaxed/relaxed; d=hostedemail.com; s=arc-20220608; t=1793090781; h=from:from:sender:reply-to:subject:subject:date:date: message-id:message-id:to:to:cc:cc:mime-version:mime-version: content-type:content-type:content-transfer-encoding: in-reply-to:in-reply-to:references:references:dkim-signature; bh=x/YAccCPR+QDBod9TtDEo8/GkjTX2DKJZMvCayiMInU=; b=zg43jNma7meZz3i3emzbhSizzusyl7WEBZU0f+5NwEqrRZyWe7kKpLyll5icMeWH6bjNkD U+Asakxutw3A+hH34jYKOjVmxtSfACuCPnuzFBduZ5JQqlBc0x+pjEBfYZhFXoAkjJ1J6j R14gExD+7K0wdyftjn6uailTS9xymyU= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id 3EF5A60210; Tue, 27 Oct 2026 08:46:21 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 3898B1F01825; Tue, 27 Oct 2026 08:46:07 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793090780; bh=x/YAccCPR+QDBod9TtDEo8/GkjTX2DKJZMvCayiMInU=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=MIHfOMOJNGtvK/bOljqxWLAHcLixYnsRYXCAHtUQcunjVDLnNyeo97KX3xAMoJr0Z 5SfJAbFdkjKREw+q9Mtl0ffuKASWKcn7N0F0BmgFbcVUy46FHcanqfHr21qQWcr5jW pN5brAKAoAMh9KQ2wKajox7TxwfMs/QXMXRrm6/pCm731qJU0brAVO4+ppYe1VSs4W QVZYWrlfndxZlKOun9t4YN97qlLu70ioNm4z4BQTX+PWQAgemaKean1qRUpkEbbfK2 Ylrgvy+1T3A3hOJGk4nx9jTZUzT20nRhHgy1MV+6HzvLQExySYnOpZQXYHoPDsoTcO 4j7djgWhTTvDQ== Date: Tue, 27 Oct 2026 08:46:04 +0000 From: "Lorenzo Stoakes (ARM)" To: "David Hildenbrand (Arm)" Cc: Andrew Morton , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Kairui Song , Barry Song , Axel Rasmussen , Yuanchu Xie , Wei Xu , Baoquan He , Baolin Wang , "Liam R. Howlett" , Vlastimil Babka , Mike Rapoport , Suren Baghdasaryan , Michal Hocko , Kees Cook , Peter Zijlstra , Jann Horn , Pedro Falcato , Hugh Dickins , Pasha Tatashin , Pratyush Yadav , "Matthew Wilcox (Oracle)" , Jan Kara , Zi Yan , Lance Yang , Rik van Riel , Harry Yoo , Alistair Popple , Nico Pache , Ryan Roberts , Dev Jain , Usama Arif , Kiryl Shutsemau , Matthew Brost , John Hubbard , Peter Xu , Will Deacon , "Aneesh Kumar K.V" , Nick Piggin , Oscar Salvador , Minchan Kim , Muchun Song , Xu Xin , Chengming Zhou , Chris Li , Kemeng Shi , Nhat Pham , Youngjun Park , Gregory Price , Joshua Hahn , Rakie Kim , Ying Huang , Jason Gunthorpe , linux-mm@kvack.org, linux-kernel@vger.kernel.org, kexec@lists.infradead.org, linux-fsdevel@vger.kernel.org, linux-arch@vger.kernel.org, linux-cxl@vger.kernel.org, Josef Bacik , stable@vger.kernel.org Subject: Re: [PATCH 04/46] mm/memfd: make f_pos accesses atomic Message-ID: References: <20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org> <20261027-fixes_core-v1-4-6f74bec9a5bb@kernel.org> X-Archived-By: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20261027-fixes_core-v1-4-6f74bec9a5bb@kernel.org> X-Rspam-User: X-Rspamd-Queue-Id: A2847120004 X-Stat-Signature: ohqh798t3otqgwdr4gq4pai1j8sjmft6 X-Rspamd-Server: rspam01 X-Spam-Status: No, score=-16.1 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_PASS, SPF_PASS autolearn=unavailable autolearn_force=no version=4.0.1 X-HE-Tag: 1793090781-220741 X-HE-Meta: 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 FDijYuWE n+Wq+YcemZks4sVipGQpwRWr5dloAz/VqgD77n7gQF5hPUXN7UbGJdYoU1fvOYwcTe/hJQEdYt6yn2OuR3L/Ttch8qQ22us2T51VSthdbxKlw2V48JKTEDwUvK45FyTrWCpzlq7UFKjCzuOA7me1LtE6dlDjCYXTDvSsE7hdmoRkqtndjA+DSFIB1X0KszlbJtIiLfbSr08gmoyOlCLFHY8mc6jlOgwrH60Ubz685mxiKm/JL7QSmexEM/MnBKPgyf//X3fQYTICx2dnu2KYnfXZr4chuPojaNoMeRNbouq7r8y50wjEBrbGyJlNWwKOh5LIk1hm4nYVITMKmnwfHEFQSREF3Qd1X+Ms3NDV5T7qUicqBT741Kx7cS0Psn+mvfzc2wki1a/rE4eiLSYTgSbLEu2AeMf6jJXU3HyziufB7EFyKWI1DPG0doA1KHkFRbvRtEFnmPGUPxUc= Sender: owner-linux-mm@kvack.org Precedence: bulk X-Loop: owner-majordomo@kvack.org List-ID: List-Subscribe: X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 On Tue, Oct 27, 2026 at 12:17:24AM +0100, David Hildenbrand (Arm) wrote: > From: Josef Bacik > > Commit 9c225f2655e3 ("vfs: atomic f_pos accesses as per POSIX") made > read(), write() and lseek() on a shared open file description of a regular > file serialize on ->f_pos_lock. The flag that asks for it, > FMODE_ATOMIC_POS, is set in do_dentry_open(). A memfd is a regular file > too, but its struct file comes from alloc_file_pseudo() and never passes > through do_dentry_open(), so it does not get the flag: > > CPU0 CPU1 > write(memfd, "a", 1) write(memfd, "b", 1) > pos = file->f_pos (= 0) > pos = file->f_pos (= 0) > vfs_write(..., &pos) > vfs_write(..., &pos) > file->f_pos = pos (= 1) > file->f_pos = pos (= 1) > > One of the two bytes is overwritten. Two threads writing 100000 bytes > each, one at a time, to a memfd end up with a file that is far shorter than > 200000 bytes, while the same program on a tmpfs file never loses a byte. > > Set the flag when the memfd is created. As for any other regular file the > lock is only taken if the file is reachable in more than one way, and > nothing a memfd does in read or write waits for another operation on the > same file. This cannot be done for all alloc_file_pseudo() files: anon > inode files such as eventfd are S_IFREG as well, and a blocked read holding > ->f_pos_lock would lock out the write that is supposed to wake it. > > Fix generated by an LLM and sent by Josef Bacik in private. > > Fixes: 9183df25fe7b ("shm: add memfd_create() syscall") > Cc: stable@vger.kernel.org > Signed-off-by: Josef Bacik > [ david: minor description tweaks ] > Signed-off-by: David Hildenbrand (Arm) Acked-by: Lorenzo Stoakes (ARM) > --- > mm/memfd.c | 2 ++ > 1 file changed, 2 insertions(+) > > diff --git a/mm/memfd.c b/mm/memfd.c > index 7d2da23157003..6f5fbce74f21e 100644 > --- a/mm/memfd.c > +++ b/mm/memfd.c > @@ -484,6 +484,8 @@ struct file *memfd_alloc_file(const char *name, unsigned int flags) > } > > file->f_mode |= FMODE_LSEEK | FMODE_PREAD | FMODE_PWRITE; > + /* a regular file like any other: POSIX.1-2008/SUSv4 Section XSI 2.9.7 */ > + file->f_mode |= FMODE_ATOMIC_POS; > file->f_flags |= O_LARGEFILE; > > if (flags & MFD_NOEXEC_SEAL) { > > -- > 2.43.0 > -- Cheers, Lorenzo