path: root/fs/file_table.c
diff options
authorLinus Torvalds <torvalds@linux-foundation.org>2014-03-03 09:36:58 -0800
committerAl Viro <viro@zeniv.linux.org.uk>2014-03-10 11:44:41 -0400
commit9c225f2655e36a470c4f58dbbc99244c5fc7f2d4 (patch)
tree7cb89dbc82ee1b533ff2d097fed6a4248374bd4b /fs/file_table.c
parentocfs2 syncs the wrong range... (diff)
vfs: atomic f_pos accesses as per POSIX
Our write() system call has always been atomic in the sense that you get the expected thread-safe contiguous write, but we haven't actually guaranteed that concurrent writes are serialized wrt f_pos accesses, so threads (or processes) that share a file descriptor and use "write()" concurrently would quite likely overwrite each others data. This violates POSIX.1-2008/SUSv4 Section XSI 2.9.7 that says: "2.9.7 Thread Interactions with Regular File Operations All of the following functions shall be atomic with respect to each other in the effects specified in POSIX.1-2008 when they operate on regular files or symbolic links: [...]" and one of the effects is the file position update. This unprotected file position behavior is not new behavior, and nobody has ever cared. Until now. Yongzhi Pan reported unexpected behavior to Michael Kerrisk that was due to this. This resolves the issue with a f_pos-specific lock that is taken by read/write/lseek on file descriptors that may be shared across threads or processes. Reported-by: Yongzhi Pan <panyongzhi@gmail.com> Reported-by: Michael Kerrisk <mtk.manpages@gmail.com> Cc: Al Viro <viro@zeniv.linux.org.uk> Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org> Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Diffstat (limited to 'fs/file_table.c')
1 files changed, 1 insertions, 0 deletions
diff --git a/fs/file_table.c b/fs/file_table.c
index 5fff9030be34..5b24008ea4f6 100644
--- a/fs/file_table.c
+++ b/fs/file_table.c
@@ -135,6 +135,7 @@ struct file *get_empty_filp(void)
atomic_long_set(&f->f_count, 1);
+ mutex_init(&f->f_pos_lock);
/* f->f_version: 0 */
return f;