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 E063E606A321 for ; Tue, 27 Oct 2026 08:15:33 +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=TohUlqMB Received: by kanga.kvack.org (Postfix) id ECBC46B060E; Tue, 27 Oct 2026 04:15:32 -0400 (EDT) Received: by kanga.kvack.org (Postfix, from userid 40) id EA44A6B0611; Tue, 27 Oct 2026 04:15:32 -0400 (EDT) X-Delivered-To: int-list-linux-mm@kvack.org Received: by kanga.kvack.org (Postfix, from userid 63042) id DBA216B0613; Tue, 27 Oct 2026 04:15:32 -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 B54D26B060E for ; Tue, 27 Oct 2026 04:15:32 -0400 (EDT) Received: from smtpin21.hostedemail.com (lb01a-stub [10.200.18.249]) by unirelay06.hostedemail.com (Postfix) with ESMTP id 52096A8629 for ; Tue, 27 Oct 2026 08:15:32 +0000 (UTC) X-FDA: 85367696904.21.0432C89 Received: from tor.source.kernel.org (tor.source.kernel.org [172.105.4.254]) by imf24.hostedemail.com (Postfix) with ESMTP id B948E180008 for ; Tue, 27 Oct 2026 08:15:30 +0000 (UTC) Authentication-Results: imf24.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=TohUlqMB; spf=pass (imf24.hostedemail.com: domain of rppt@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=rppt@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=1793088930; 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=5kxHiEVY3AkdeOmkiyMicYxXOYN4StpWVAs8/ArHz0o=; b=XBcM0VRbdLFJuUWwMJqQPwPFgidL5binm4LY+sc+XhAOFnSbizvUhXpPVlCYVDL9vbZtJO 208BwmTvmFSilk0SkT5VIyal7xQ/Fqemt+/5jGDfWOJYmZ89CW/sJmLl4QUn1+Yinhb8b/ 5weX4ySDVYyECoUVPrcYCYApQsGsECY= ARC-Authentication-Results: i=1; imf24.hostedemail.com; dkim=pass header.d=kernel.org header.s=k20260515 header.b=TohUlqMB; spf=pass (imf24.hostedemail.com: domain of rppt@kernel.org designates 172.105.4.254 as permitted sender) smtp.mailfrom=rppt@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=1793088930; b=adXQ7r1YZ/MIVscG3vDrWNEk1Du488Z6NHPzDaQbUMB2c3PRzdgNL6mX5afZZC5kjJCCuv /ja9EJheteEyadkgWSw85lmAIzMe8CDi7XOi+6+RmAvzFNelNVOLgTXz4NLzDyrVRKoPcz g/wxw3BZzYaYEnJomYLc3gs1tvJ0k4I= Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by tor.source.kernel.org (Postfix) with ESMTP id EB7F9601F0; Tue, 27 Oct 2026 08:15:29 +0000 (UTC) Received: by smtp.kernel.org (Postfix) with ESMTPSA id 6AC241F01825; Tue, 27 Oct 2026 08:15:11 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=kernel.org; s=k20260515; t=1793088929; bh=5kxHiEVY3AkdeOmkiyMicYxXOYN4StpWVAs8/ArHz0o=; h=Date:From:To:Cc:Subject:References:In-Reply-To; b=TohUlqMBXvTV6SqJFuiC+dqJjW/ij05cgrKtKEOfYxV88J5WLixwsYKmPfpSGBA2B C+OmR9z5ZQPvnP70HVJyU7RC/CroJIV4UeBShTJu8eqdqUxUaoqOarZFm1dzPQXqFd 9tm6W7ZBJjPSXKIvNDqJIqzVvCngxMhyCkgQuyeGljrVxcZZ6+6MOELxqy231soY3X GqAKIoz4S+Xnh6OouG5YutTdPxrxAOYEgd5VJ3AkybBIYVwdQ4Zz9O/MVH8vdiCTjr p4b/83kRK763X0IwAQe+MCH3KJ3GHP5MK8a903LLNXFYeAbx5wprUZo5Jf2y+jOZpP WoD4aFVO/XgLQ== Date: Tue, 27 Oct 2026 10:15:07 +0200 From: Mike Rapoport To: "David Hildenbrand (Arm)" Cc: 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 , 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-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: B948E180008 X-Stat-Signature: ku1nn6huc9ijfsq47md4b7raox5horwk X-Rspam-User: 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: 1793088930-68679 X-HE-Meta: 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 2JsLstIV la+L3QQE8sANX/dspj82mQJ3LV9wvmQ9rA4vG2tfo8s3LIICSCkd/YRjxkbK6LxvVTj5RWEvgTdl+57Zsa9ktOESWjdMDGixPcAy6TH66WEFJ3k/6K+J2uNzgpzCdbEIvYuXTMj0shjakHPYpuELyuf2sek1MSnDOCr4W2RtwnciY68v63+kq/EgtyEL8zoCXb5r+/aPAm7uXlgS9h29RV5B2w6Yb5azK3Dg+4IeJ4O8tnj6EiRDtsqS6/TlEghfvdIVYDQl2S0ozt20cyFDs7LemGRAHWCV1kp8wbBVu2IfKQQBrX8P1lMLMyQzgq/PgFX2C/fe+delYZfnPn0cj9dbsZ5QLofmx7O4QbnuHrv/hwBhRRb3f2cjdBKWbv8qUUt9LAySjrxNjuVV1Wq8o49PpfN6UbuQFFfob79kUWNve1+LQbncjvBF9Mj/gg2U+KHBfDUgBLt089SA= 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) Reviewed-by: Mike Rapoport (Microsoft) > --- > 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 > -- Sincerely yours, Mike.