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Tue, 27 Oct 2026 10:39:27 +0000 (UTC) Received: from dovecot-director2.suse.de ([2a07:de40:b281:106:10:150:64:167]) by imap1.dmz-prg2.suse.org with ESMTPSA id wIn6B19/4GprGgAAD6G6ig (envelope-from ); Tue, 27 Oct 2026 10:39:27 +0000 Date: Tue, 27 Oct 2026 10:39:25 +0000 From: Pedro Falcato 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 , 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> X-Rspamd-Queue-Id: 06B761F7CE X-Spam-Level: X-Rspamd-Server: rspamd2.dmz-prg2.suse.org X-Rspamd-Action: no action X-Spam-Flag: NO X-Spamd-Result: default: False [-12.01 / 50.00]; SUSE_INTERNAL_BYPASS(-9.00)[]; BAYES_HAM(-3.00)[100.00%]; SUSPICIOUS_RECIPS(1.50)[]; NEURAL_HAM_LONG(-1.00)[-1.000]; NEURAL_HAM_SHORT(-0.20)[-1.000]; R_DKIM_ALLOW(-0.20)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; MIME_GOOD(-0.10)[text/plain]; MX_GOOD(-0.01)[]; FREEMAIL_CC(0.00)[linux-foundation.org,cmpxchg.org,kernel.org,linux.dev,tencent.com,google.com,linux.alibaba.com,infradead.org,suse.com,soleen.com,suse.cz,nvidia.com,surriel.com,arm.com,intel.com,redhat.com,gmail.com,suse.de,huaweicloud.com,lge.com,gourry.net,sk.com,ziepe.ca,kvack.org,vger.kernel.org,lists.infradead.org,toxicpanda.com]; RCVD_TLS_ALL(0.00)[]; MIME_TRACE(0.00)[0:+]; ARC_NA(0.00)[]; RBL_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:104:10:150:64:97:from]; DKIM_SIGNED(0.00)[suse.de:s=susede2_rsa,suse.de:s=susede2_ed25519]; DKIM_TRACE(0.00)[suse.de:+]; FROM_HAS_DN(0.00)[]; MISSING_XM_UA(0.00)[]; TO_MATCH_ENVRCPT_SOME(0.00)[]; RCVD_COUNT_TWO(0.00)[2]; FROM_EQ_ENVFROM(0.00)[]; TO_DN_SOME(0.00)[]; DNSWL_BLOCKED(0.00)[2a07:de40:b281:106:10:150:64:167:received]; RECEIVED_SPAMHAUS_BLOCKED_OPENRESOLVER(0.00)[2a07:de40:b281:106:10:150:64:167:received]; RCPT_COUNT_GT_50(0.00)[65]; RCVD_VIA_SMTP_AUTH(0.00)[]; TAGGED_RCPT(0.00)[]; DBL_BLOCKED_OPENRESOLVER(0.00)[toxicpanda.com:email,suse.de:dkim,suse.de:email,imap1.dmz-prg2.suse.org:rdns,imap1.dmz-prg2.suse.org:helo] X-Spam-Score: -12.01 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: Pedro Falcato -- Pedro