io_uring/net: punt IORING_OP_BIND async if it needs file create

For two reasons:

1) An opcode cannot block inside io_uring_enter() doing submissions, as
   it'll stall the submission side pipeline.

2) Ending up in sb_start_write() -> __sb_start_write() ->
   percpu_down_read_freezable() introduces a new lockdep edge, which it
   correctly complains about.

Check if the socket type is AF_UNIX and has a non-empty pathname. If it
does, mark it REQ_F_FORCE_ASYNC to punt the submission to io-wq rather
than attempt to do it inline.

Fixes: 7481fd9 ("io_uring: Introduce IORING_OP_BIND")
Reviewed-by: Gabriel Krisman Bertazi <krisman@suse.de>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

io_uring/net.c

@@ -4,6 +4,7 @@
 #include <linux/file.h>
 #include <linux/slab.h>
 #include <linux/net.h>
+#include <linux/un.h>
 #include <linux/compat.h>
 #include <net/compat.h>
 #include <linux/io_uring.h>
@@ -1799,11 +1800,29 @@ int io_connect(struct io_kiocb *req, unsigned int issue_flags)
 	return IOU_COMPLETE;
 }
 
+/*
+ * Check if bind request would potentially end up with filename_create(),
+ * which in turn end up in mnt_want_write() which will grab the fs
+ * percpu start write sem. This can trigger a lockdep warning.
+ */
+static int io_bind_file_create(const struct io_async_msghdr *io, int addr_len)
+{
+	const struct sockaddr_un *sun;
+
+	if (io->addr.ss_family != AF_UNIX)
+		return 0;
+	if (addr_len <= offsetof(struct sockaddr_un, sun_path))
+		return 0;
+	sun = (const struct sockaddr_un *) &io->addr;
+	return sun->sun_path[0] != '\0';
+}
+
 int io_bind_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 {
 	struct io_bind *bind = io_kiocb_to_cmd(req, struct io_bind);
 	struct sockaddr __user *uaddr;
 	struct io_async_msghdr *io;
+	int ret;
 
 	if (sqe->len || sqe->buf_index || sqe->rw_flags || sqe->splice_fd_in)
 		return -EINVAL;
@@ -1814,7 +1833,12 @@ int io_bind_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	io = io_msg_alloc_async(req);
 	if (unlikely(!io))
 		return -ENOMEM;
-	return move_addr_to_kernel(uaddr, bind->addr_len, &io->addr);
+	ret = move_addr_to_kernel(uaddr, bind->addr_len, &io->addr);
+	if (unlikely(ret))
+		return ret;
+	if (io_bind_file_create(io, bind->addr_len))
+		req->flags |= REQ_F_FORCE_ASYNC;
+	return 0;
 }
 
 int io_bind(struct io_kiocb *req, unsigned int issue_flags)