From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from bombadil.infradead.org (bombadil.infradead.org [198.137.202.133]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1) server-digest SHA256) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id D8FFF606A324 for ; Mon, 26 Oct 2026 23:19:10 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key, unprotected) header.d=lists.infradead.org header.i=@lists.infradead.org header.a=rsa-sha256 header.s=bombadil.20210309 header.b=B3lHtmgu; dkim=pass (2048-bit key, unprotected) header.d=kernel.org header.i=@kernel.org header.a=rsa-sha256 header.s=k20260515 header.b=gW6pTedF DKIM-Signature: v=1; a=rsa-sha256; q=dns/txt; c=relaxed/relaxed; d=lists.infradead.org; s=bombadil.20210309; h=Sender:List-Subscribe:List-Help :List-Post:List-Archive:List-Unsubscribe:List-Id:Cc:To:In-Reply-To:References :Message-Id:Content-Transfer-Encoding:Content-Type:MIME-Version:Subject:Date: From:Reply-To:Content-ID:Content-Description:Resent-Date:Resent-From: Resent-Sender:Resent-To:Resent-Cc:Resent-Message-ID:List-Unsubscribe-Post: List-Owner; bh=DWZmK6+Hk4tl7852WKNp5JuN3J4xZu+CGZlmoPNLfsM=; b=B3lHtmgufwfAzB V2cZaffE6xeVnqh3QrCc0gwyHY0yx3cCOGKnXRKXoZg6MIAlOAI69ljB4ePiM8pWFoBQYbEZRsVzH 130l3Mc0ysdT9tEwxg2Anhns0YjzYH89R7HvoyI4FTjpHl8QKMSO5T9OcSRd7rJ8Hs1RddsQDiRn9 Ezk3kls471zsFg+EI+sdIKklHN8grRuftZwDe3qgLWV25dASlmIKWaEjb67qBZPmaMWKKemsTASoE DL8pUQhc7Ms/sqrns8rKDdkqnBKy/PaIpHlAlUko2ydNHLc06av7aUNCNlu4L5+ZQZE5/NdhfxBG7 Ww8nZO7kH/GakmJn8mwg==; Received: from localhost ([::1] helo=bombadil.infradead.org) by bombadil.infradead.org with esmtp (Exim 4.100.1 #2 (Red Hat Linux)) id 1xLTyA-00000003PAl-2Vro; Mon, 26 Oct 2026 23:19:10 +0000 Received: from sea.source.kernel.org ([172.234.252.31]) by bombadil.infradead.org with esmtps (Exim 4.100.1 #2 (Red Hat Linux)) id 1xLTyA-00000003PAf-04YC for kexec@lists.infradead.org; Mon, 26 Oct 2026 23:19:10 +0000 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) by sea.source.kernel.org (Postfix) with ESMTP id BCC3540338; Mon, 26 Oct 2026 23:19:09 +0000 (UTC) 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-Archived-By: 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 X-BeenThere: kexec@lists.infradead.org X-Mailman-Version: 2.1.34 Precedence: list List-Id: List-Archive: List-Post: List-Help: List-Subscribe: , Sender: "kexec" X-Spam-Status: No, score=-19.2 required=3.0 tests=BAYES_00,DKIMWL_WL_HIGH, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,DKIM_VALID_EF, HEADER_FROM_DIFFERENT_DOMAINS,INCLUDES_CR_TRAILER,INCLUDES_PATCH, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_GIT autolearn=ham autolearn_force=no version=4.0.1 X-Spam-Checker-Version: SpamAssassin 4.0.1 (2024-03-26) on aws-us-west-2-korg-listserv-alma10-1 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