io_uring: don't use 'fd' for openat/openat2/statx

We currently make some guesses as when to open this fd, but in reality
we have no business (or need) to do so at all. In fact, it makes certain
things fail, like O_PATH.

Remove the fd lookup from these opcodes, we're just passing the 'fd' to
generic helpers anyway. With that, we can also remove the special casing
of fd values in io_req_needs_file(), and the 'fd_non_neg' check that
we have. And we can ensure that we only read sqe->fd once.

This fixes O_PATH usage with openat/openat2, and ditto statx path side
oddities.

Cc: stable@vger.kernel.org: # v5.6
Reported-by: Max Kellermann <mk@cm4all.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -680,8 +680,6 @@ struct io_op_def {
 	unsigned		needs_mm : 1;
 	/* needs req->file assigned */
 	unsigned		needs_file : 1;
-	/* needs req->file assigned IFF fd is >= 0 */
-	unsigned		fd_non_neg : 1;
 	/* hash wq insertion if file is a regular file */
 	unsigned		hash_reg_file : 1;
 	/* unbound wq insertion if file is a non-regular file */
@@ -784,8 +782,6 @@ static const struct io_op_def io_op_defs[] = {
 		.needs_file		= 1,
 	},
 	[IORING_OP_OPENAT] = {
-		.needs_file		= 1,
-		.fd_non_neg		= 1,
 		.file_table		= 1,
 		.needs_fs		= 1,
 	},
@@ -799,8 +795,6 @@ static const struct io_op_def io_op_defs[] = {
 	},
 	[IORING_OP_STATX] = {
 		.needs_mm		= 1,
-		.needs_file		= 1,
-		.fd_non_neg		= 1,
 		.needs_fs		= 1,
 		.file_table		= 1,
 	},
@@ -837,8 +831,6 @@ static const struct io_op_def io_op_defs[] = {
 		.buffer_select		= 1,
 	},
 	[IORING_OP_OPENAT2] = {
-		.needs_file		= 1,
-		.fd_non_neg		= 1,
 		.file_table		= 1,
 		.needs_fs		= 1,
 	},
@@ -5368,15 +5360,6 @@ static void io_wq_submit_work(struct io_wq_work **workptr)
 	io_steal_work(req, workptr);
 }
 
-static int io_req_needs_file(struct io_kiocb *req, int fd)
-{
-	if (!io_op_defs[req->opcode].needs_file)
-		return 0;
-	if ((fd == -1 || fd == AT_FDCWD) && io_op_defs[req->opcode].fd_non_neg)
-		return 0;
-	return 1;
-}
-
 static inline struct file *io_file_from_index(struct io_ring_ctx *ctx,
 					      int index)
 {
@@ -5414,14 +5397,11 @@ static int io_file_get(struct io_submit_state *state, struct io_kiocb *req,
 }
 
 static int io_req_set_file(struct io_submit_state *state, struct io_kiocb *req,
-			   int fd, unsigned int flags)
+			   int fd)
 {
 	bool fixed;
 
-	if (!io_req_needs_file(req, fd))
-		return 0;
-
-	fixed = (flags & IOSQE_FIXED_FILE);
+	fixed = (req->flags & REQ_F_FIXED_FILE) != 0;
 	if (unlikely(!fixed && req->needs_fixed_file))
 		return -EBADF;
 
@@ -5798,7 +5778,7 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 		       struct io_submit_state *state, bool async)
 {
 	unsigned int sqe_flags;
-	int id, fd;
+	int id;
 
 	/*
 	 * All io need record the previous position, if LINK vs DARIN,
@@ -5850,8 +5830,10 @@ static int io_init_req(struct io_ring_ctx *ctx, struct io_kiocb *req,
 					IOSQE_ASYNC | IOSQE_FIXED_FILE |
 					IOSQE_BUFFER_SELECT | IOSQE_IO_LINK);
 
-	fd = READ_ONCE(sqe->fd);
-	return io_req_set_file(state, req, fd, sqe_flags);
+	if (!io_op_defs[req->opcode].needs_file)
+		return 0;
+
+	return io_req_set_file(state, req, READ_ONCE(sqe->fd));
 }
 
 static int io_submit_sqes(struct io_ring_ctx *ctx, unsigned int nr,