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[35.199.49.16]) by smtp.gmail.com with ESMTPSA id ada2fe7eead31-7cea3d32a48sm6300543137.9.2026.10.28.09.55.02 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 28 Oct 2026 09:55:03 -0700 (PDT) Date: Wed, 28 Oct 2026 12:55:01 -0400 From: Pasha Tatashin 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 , 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-cxl@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 10-27 00:17, 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: Pasha Tatashin > --- > 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 >