io_uring: accept directly into fixed file table

As done with open opcodes, allow accept to skip installing fd into
processes' file tables and put it directly into io_uring's fixed file
table. Same restrictions and design as for open.
Suggested-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>
Link: https://lore.kernel.org/r/6d16163f376fac7ac26a656de6b42199143e9721.1629888991.git.asml.silence@gmail.comSigned-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -515,6 +515,7 @@ struct io_accept {
 	struct sockaddr __user		*addr;
 	int __user			*addr_len;
 	int				flags;
+	u32				file_slot;
 	unsigned long			nofile;
 };
 
@@ -4803,7 +4804,7 @@ static int io_accept_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 
 	if (unlikely(req->ctx->flags & IORING_SETUP_IOPOLL))
 		return -EINVAL;
-	if (sqe->ioprio || sqe->len || sqe->buf_index || sqe->splice_fd_in)
+	if (sqe->ioprio || sqe->len || sqe->buf_index)
 		return -EINVAL;
 
 	accept->addr = u64_to_user_ptr(READ_ONCE(sqe->addr));
@@ -4811,6 +4812,10 @@ static int io_accept_prep(struct io_kiocb *req, const struct io_uring_sqe *sqe)
 	accept->flags = READ_ONCE(sqe->accept_flags);
 	accept->nofile = rlimit(RLIMIT_NOFILE);
 
+	accept->file_slot = READ_ONCE(sqe->file_index);
+	if (accept->file_slot && ((req->open.how.flags & O_CLOEXEC) ||
+				  (accept->flags & SOCK_CLOEXEC)))
+		return -EINVAL;
 	if (accept->flags & ~(SOCK_CLOEXEC | SOCK_NONBLOCK))
 		return -EINVAL;
 	if (SOCK_NONBLOCK != O_NONBLOCK && (accept->flags & SOCK_NONBLOCK))
@@ -4823,28 +4828,35 @@ static int io_accept(struct io_kiocb *req, unsigned int issue_flags)
 	struct io_accept *accept = &req->accept;
 	bool force_nonblock = issue_flags & IO_URING_F_NONBLOCK;
 	unsigned int file_flags = force_nonblock ? O_NONBLOCK : 0;
+	bool fixed = !!accept->file_slot;
 	struct file *file;
 	int ret, fd;
 
 	if (req->file->f_flags & O_NONBLOCK)
 		req->flags |= REQ_F_NOWAIT;
 
-	fd = __get_unused_fd_flags(accept->flags, accept->nofile);
-	if (unlikely(fd < 0))
-		return fd;
-
+	if (!fixed) {
+		fd = __get_unused_fd_flags(accept->flags, accept->nofile);
+		if (unlikely(fd < 0))
+			return fd;
+	}
 	file = do_accept(req->file, file_flags, accept->addr, accept->addr_len,
 			 accept->flags);
 	if (IS_ERR(file)) {
+		if (!fixed)
+			put_unused_fd(fd);
 		ret = PTR_ERR(file);
 		if (ret == -EAGAIN && force_nonblock)
 			return -EAGAIN;
 		if (ret == -ERESTARTSYS)
 			ret = -EINTR;
 		req_set_fail(req);
-	} else {
+	} else if (!fixed) {
 		fd_install(fd, file);
 		ret = fd;
+	} else {
+		ret = io_install_fixed_file(req, file, issue_flags,
+					    accept->file_slot - 1);
 	}
 	__io_req_complete(req, issue_flags, ret, 0);
 	return 0;