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=1793090781; b=XMOvJRmPHEeFs9ylBIIKci+nrf2aIzSZu5lByU3KZhmsXJN1GgzMwFilf5j/ORZ/n1Ba biAKzgPHJDuVmkzSeqdA3y7Qf8vnS1UFZHak+E3kSM6mbqAt1aeCSA6SsCwkNMpwPdRz4 Lh5u1Iw1z5V/NdIR238AGzFPEe7eDAuzjiR9qvlNaU3+T32g93FMK9h/0KYtJisQISp8P UQeKRzbcesRAsYz6OXuUHIquwJowdIiz533Ci9tMFQwsCe7X9jwINKo4BftiXdu1DXlej 8HzY3Sdo+Ss0DGK3V7UJkPvJuEHiokkgatTWBMcDgmyy+qgByGpxHkVvW2kixD66/fw== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1793090781; h=Received:DKIM-Signature:Date:From:To:Cc:Subject:Message-ID: References:MIME-Version:Content-Type:Content-Disposition:In-Reply-To; bh=x/YAccCPR+QDBod9TtDEo8/GkjTX2DKJZMvCayiMInU=; b=URvcBNRjTq4mnXTvBIQ+EmZzjn5fKtAkQo/tkN0o6+rv/r2N7sPKg7SEx3cGL7XhyfvH FxRDcou8UW7DKjqzNQIWk38KLnf8dEQdJX1pwqJLKR57VT/rg55qtx1gMssh7yn9OoaW0 W4W8q4QOmi9OPQ/G1cg937mSKzEAhvnOVd3Qbv4wTVdLRg5HaGcLYjHWIbl0YhCNv2DLH V+uUzefuGtKsUIyXLUu+VhQjH5pOIEutXAdXgStbJEVreuG/0n4Jdy5GS7iRht2OZbE17 yIAsqWBKv9irtVm0aO+8mnieMNa3fqmROqWXsh1k7PlPaApkQyMJFo8vj35fdLTTXig== 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=MIHfOMOJ; 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 4CB8160A2614; Tue, 27 Oct 2026 08:46:21 +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 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-Mailing-List: linux-kernel@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=us-ascii Content-Disposition: inline In-Reply-To: <20261027-fixes_core-v1-4-6f74bec9a5bb@kernel.org> 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