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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-Rspamd-Server: rspam05 X-Rspamd-Queue-Id: 70D4540004 X-Stat-Signature: 4ikk3kd7dao1yhkmgk41pz87dihk469x X-Rspam-User: X-Spam-Status: No, score=-19.1 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,USER_AGENT_GIT autolearn=unavailable autolearn_force=no version=4.0.1 X-HE-Tag: 1793056750-943782 X-HE-Meta: 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 6V0v4MGq uHq8lZmVRdk7lthj9J8/xi5HOIjLkATvqu4aJ9aNlB+XlVy4jyUq3P3ZylvGFGZm062z93cYaCSwAZIYbb7n58fW9r2+NlhYvXtb3PkXkPcRvurkkITLg69YqAvOwrWp2iz8UIb3HOF1jpsy9LiVUMrmHWgFFsaUE78DP8xkidRKENZZerHgowB2vME+OZlVfZAR+gkyqBpPlxP2TaRiHWTF9NeHIc2r8Swx94xGdAa9ZDJ47YWz2IhDdmaQipuNWLHtZZZwfxzWQowZOz3+HJ4me1GGIlQLUic+fG6TUL8OS0YPWigqjmO0lSS3xDWyvJJHXQfIUzKhWIfYDaodEyDA/8FX/HkzYbB6IjF2ddxooenhi/zAY72QfG9ECb/36D1aRbjTSVe2WfU8iMkyM6SeopgyIaebXQG+f97FKozH+uP0= 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 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