From 5426cd43a4d28f1041c9ca45955121b921f29548 Mon Sep 17 00:00:00 2001 From: Josef Bacik Date: Sat, 31 Oct 2026 10:39:33 +0100 Subject: [PATCH] mm/memfd: make f_pos accesses atomic 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 ] Reviewed-by: Rik van Riel Reviewed-by: Gregory Price (Meta) Tested-by: Gregory Price (Meta) Signed-off-by: Gregory Price (Meta) Reviewed-by: Mike Rapoport (Microsoft) Acked-by: Lorenzo Stoakes (ARM) Reviewed-by: Pedro Falcato Reviewed-by: Kiryl Shutsemau (Meta) Reviewed-by: Pasha Tatashin Tested-by: Muhammad Usama Anjum Link: https://patch.msgid.link/20261031-fixes_core-v3-4-d6437171ecc3@kernel.org 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 0f6fff004f5e..0e3ff3eebfa8 100644 --- a/mm/memfd.c +++ b/mm/memfd.c @@ -482,6 +482,8 @@ 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) {