io_uring: handle -EOPNOTSUPP on path resolution

Any attempt to do path resolution on /proc/self from an async worker will
yield -EOPNOTSUPP. We can safely do that resolution from the task itself,
and without blocking, so retry it from there.

Ideally io_uring would know this upfront and not have to go through the
worker thread to find out, but that doesn't currently seem feasible.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -478,6 +478,7 @@ struct io_sr_msg {
 struct io_open {
 	struct file			*file;
 	int				dfd;
+	bool				ignore_nonblock;
 	struct filename			*filename;
 	struct open_how			how;
 	unsigned long			nofile;
@@ -3796,6 +3797,7 @@ static int __io_openat_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe
 		return ret;
 	}
 	req->open.nofile = rlimit(RLIMIT_NOFILE);
+	req->open.ignore_nonblock = false;
 	req->flags |= REQ_F_NEED_CLEANUP;
 	return 0;
 }
@@ -3839,7 +3841,7 @@ static int io_openat2(struct io_kiocb *req, bool force_nonblock)
 	struct file *file;
 	int ret;
 
-	if (force_nonblock)
+	if (force_nonblock && !req->open.ignore_nonblock)
 		return -EAGAIN;
 
 	ret = build_open_flags(&req->open.how, &op);
@@ -3854,6 +3856,21 @@ static int io_openat2(struct io_kiocb *req, bool force_nonblock)
 	if (IS_ERR(file)) {
 		put_unused_fd(ret);
 		ret = PTR_ERR(file);
+		/*
+		 * A work-around to ensure that /proc/self works that way
+		 * that it should - if we get -EOPNOTSUPP back, then assume
+		 * that proc_self_get_link() failed us because we're in async
+		 * context. We should be safe to retry this from the task
+		 * itself with force_nonblock == false set, as it should not
+		 * block on lookup. Would be nice to know this upfront and
+		 * avoid the async dance, but doesn't seem feasible.
+		 */
+		if (ret == -EOPNOTSUPP && io_wq_current_is_worker()) {
+			req->open.ignore_nonblock = true;
+			refcount_inc(&req->refs);
+			io_req_task_queue(req);
+			return 0;
+		}
 	} else {
 		fsnotify_open(file);
 		fd_install(ret, file);