9p/trans_fd: Annotate data-racy writes to file::f_flags

syzbot reported:

| BUG: KCSAN: data-race in p9_fd_create / p9_fd_create
|
| read-write to 0xffff888130fb3d48 of 4 bytes by task 15599 on cpu 0:
| p9_fd_open net/9p/trans_fd.c:842 [inline]
| p9_fd_create+0x210/0x250 net/9p/trans_fd.c:1092
| p9_client_create+0x595/0xa70 net/9p/client.c:1010
| v9fs_session_init+0xf9/0xd90 fs/9p/v9fs.c:410
| v9fs_mount+0x69/0x630 fs/9p/vfs_super.c:123
| legacy_get_tree+0x74/0xd0 fs/fs_context.c:611
| vfs_get_tree+0x51/0x190 fs/super.c:1519
| do_new_mount+0x203/0x660 fs/namespace.c:3335
| path_mount+0x496/0xb30 fs/namespace.c:3662
| do_mount fs/namespace.c:3675 [inline]
| __do_sys_mount fs/namespace.c:3884 [inline]
| [...]
|
| read-write to 0xffff888130fb3d48 of 4 bytes by task 15563 on cpu 1:
| p9_fd_open net/9p/trans_fd.c:842 [inline]
| p9_fd_create+0x210/0x250 net/9p/trans_fd.c:1092
| p9_client_create+0x595/0xa70 net/9p/client.c:1010
| v9fs_session_init+0xf9/0xd90 fs/9p/v9fs.c:410
| v9fs_mount+0x69/0x630 fs/9p/vfs_super.c:123
| legacy_get_tree+0x74/0xd0 fs/fs_context.c:611
| vfs_get_tree+0x51/0x190 fs/super.c:1519
| do_new_mount+0x203/0x660 fs/namespace.c:3335
| path_mount+0x496/0xb30 fs/namespace.c:3662
| do_mount fs/namespace.c:3675 [inline]
| __do_sys_mount fs/namespace.c:3884 [inline]
| [...]
|
| value changed: 0x00008002 -> 0x00008802

Within p9_fd_open(), O_NONBLOCK is added to f_flags of the read and
write files. This may happen concurrently if e.g. mounting process
modifies the fd in another thread.

Mark the plain read-modify-writes as intentional data-races, with the
assumption that the result of executing the accesses concurrently will
always result in the same result despite the accesses themselves not
being atomic.

Reported-by: [e-mail]
Signed-off-by: Marco Elver <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Dominique Martinet <[e-mail]>
Message-ID: <[e-mail]>

net/9p/trans_fd.c

···
 		goto out_free_ts;
 	if (!(ts->rd->f_mode & FMODE_READ))
 		goto out_put_rd;
-	/* prevent workers from hanging on IO when fd is a pipe */
-	ts->rd->f_flags |= O_NONBLOCK;
+	/* Prevent workers from hanging on IO when fd is a pipe.
+	 * It's technically possible for userspace or concurrent mounts to
+	 * modify this flag concurrently, which will likely result in a
+	 * broken filesystem. However, just having bad flags here should
+	 * not crash the kernel or cause any other sort of bug, so mark this
+	 * particular data race as intentional so that tooling (like KCSAN)
+	 * can allow it and detect further problems.
+	 */
+	data_race(ts->rd->f_flags |= O_NONBLOCK);
 	ts->wr = fget(wfd);
 	if (!ts->wr)
 		goto out_put_rd;
 	if (!(ts->wr->f_mode & FMODE_WRITE))
 		goto out_put_wr;
-	ts->wr->f_flags |= O_NONBLOCK;
+	data_race(ts->wr->f_flags |= O_NONBLOCK);
 
 	client->trans = ts;
 	client->status = Connected;