From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1793056750; b=wf61Rm/UytH0wpUuqHHpbZCEkho591b3jYhcHh9tYyrH2XHP3w8kYzb3XYBHyUAW3kCr FhNAm/4lm1ccQc8DwLsqi+k09/27RiGL5pEZ50eSHGqjXrSVjq8KnJmyvHNEFnY3uapLd ug3ycvm2tzIt/xG2ppcOI0GLl6RLbpjlJ82tO7KIFJdlV2cLr8xXk2EK1jSZJDqg4xhlc J4fYsmIr6WDSmeya7mBckBz5uiZJCC1FmXTN7YW+mcxsat3Vvm+Y2LulcSuIOoETUYJnI NISRd9Crcg0+1hp8dDmGzvCyN6cvkjE+PV4xf3eurypf8X8qbK9jZzAS3G3DDm/BfKA== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793056750; h=Received:DKIM-Signature:From:Date:Subject:MIME-Version:Content-Type: Content-Transfer-Encoding:Message-Id:References:In-Reply-To:To:Cc: X-Mailer:X-Developer-Signature:X-Developer-Key; bh=DWZmK6+Hk4tl7852WKNp5JuN3J4xZu+CGZlmoPNLfsM=; b=efET5dJ8RinDDidzsn2MWc+W/uyt99P2Jn255GeyuJsLrYFY31SI4/f6gGZ7S1G5XCQU 9nr5hbCMvldUGrmOgOsq4C8ExLcxhyhk1CJ6ew6Lqu43HKIg8RCGHneoL3X9IOIhJl7Oo ZxJhzHEKdshPec2Tg1iM+8DZGklSWl0qBDG1nVs2ZIMldz+LEoZJc7DqQhCgboqx6iQud 9X35rrqBJuJhCCiOR4dqrrWp/VI9vqNhOWfL6DZWMfr0xRbrWmqUU5IsII440kTNGRk1L UaVAjMA7PHdgZJxXUH9Iw197z6Iyg/j6swUaia2v5dFPVPaTssQCukRiX2eR3rPeKOw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=gW6pTedF; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 0F9C3606A31D; Mon, 26 Oct 2026 23:19:09 +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=gW6pTedF Received: by smtp.kernel.org (Postfix) with ESMTPSA id D00311F01825; Mon, 26 Oct 2026 23:18:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793056749; bh=DWZmK6+Hk4tl7852WKNp5JuN3J4xZu+CGZlmoPNLfsM=; h=From:Date:Subject:References:In-Reply-To:To:Cc; b=gW6pTedF1Y7FmSKqp6sywEFrR/GjpR7B5QIg6CrYe1nEA3VmOP0NtVG8z9JRGHzse wyIuNXfKfNWFMZzzDKdZrGRpWxLMWQ3Mk5wl/UwvUtuL6tZR3haafCOYhfTAXUOBa+ ykPd8a/9Rt7kNXN1je3zkKwW2OAtPi0XosqEZAanIOKsd9uaafcic+W6y9YbiJCR2n 51e1aLAJPNZLMU9AiT8S3Hxy/ekTnAU3DkUr6fRsaQKewqAhqqkmPoLRuFAG1QsL7N VoFnPtRTe1FmbBVii22XYC+xC11jd1g13RX61M35thSsmZ03giXHXe8kbmcMCRNyNi ewBR1byALxIOw== From: "David Hildenbrand (Arm)" Date: Tue, 27 Oct 2026 00:17:24 +0100 Subject: [PATCH 04/46] mm/memfd: make f_pos accesses atomic X-Mailing-List: linux-fsdevel@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 7bit Message-Id: <20261027-fixes_core-v1-4-6f74bec9a5bb@kernel.org> References: <20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org> In-Reply-To: <20261027-fixes_core-v1-0-6f74bec9a5bb@kernel.org> To: Andrew Morton , Johannes Weiner , Michal Hocko , Qi Zheng , Shakeel Butt , Lorenzo Stoakes , 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 Cc: 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, "David Hildenbrand (Arm)" X-Mailer: b4 0.16.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=2295; i=david@kernel.org; h=from:subject:message-id; bh=TCrZs4qHRVL9zqVdabp5tYlE4exCzu0PLt/vPmw9c2I=; b=owGbwMvMwCF2/OVpCdfQ3ZWMp9WSGLLu359s9Y75wVP1yesrJE4rS6+5pRS8vjb4u9PBBj+tY +vmxa/M7yhlYRDjYJAVU2QJCr5v8PLlutpqNz5mmDmsTCBDGLg4BWAigmsY/od/lHvkEXT9sr25 V3GMO08dU8PRpU/aWTXFfNlORC7YocjI8FUyfLqt1oU7Jb3OuiW+56LKtxznl5I2Kfu34rwPy9E 8JgA= X-Developer-Key: i=david@kernel.org; a=openpgp; fpr=1BD9CAAD735C4C3A460DFCCA4DDE10F700FF835A 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) --- 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