io_uring/openclose: fix io_pipe_fixed() slot tracking for specific slots

__io_fixed_fd_install() returns 0 on success for non-alloc mode
(specific slot), not the slot index. io_pipe_fixed() used this return
value directly as the slot index in fds[], which can cause the reported
values returned via copy_to_user() to be incorrect, or the error path
operating on the incorrect direct descriptor.

Fix by computing the actual 0-based slot index (slot - 1) for specific
slot mode, while preserving the existing behavior for auto-alloc mode
where __io_fixed_fd_install() already returns the allocated index.

Cc: stable@vger.kernel.org
Fixes: 53db8a7 ("io_uring: add support for IORING_OP_PIPE")
Signed-off-by: Jens Axboe <axboe@kernel.dk>

Author: axboe

io_uring/openclose.c

@@ -345,31 +345,34 @@ static int io_pipe_fixed(struct io_kiocb *req, struct file **files,
 {
 	struct io_pipe *p = io_kiocb_to_cmd(req, struct io_pipe);
 	struct io_ring_ctx *ctx = req->ctx;
+	bool alloc_slot;
 	int ret, fds[2] = { -1, -1 };
 	int slot = p->file_slot;
 
 	if (p->flags & O_CLOEXEC)
 		return -EINVAL;
 
+	alloc_slot = slot == IORING_FILE_INDEX_ALLOC;
+
 	io_ring_submit_lock(ctx, issue_flags);
 
 	ret = __io_fixed_fd_install(ctx, files[0], slot);
 	if (ret < 0)
 		goto err;
-	fds[0] = ret;
+	fds[0] = alloc_slot ? ret : slot - 1;
 	files[0] = NULL;
 
 	/*
 	 * If a specific slot is given, next one will be used for
 	 * the write side.
 	 */
-	if (slot != IORING_FILE_INDEX_ALLOC)
+	if (!alloc_slot)
 		slot++;
 
 	ret = __io_fixed_fd_install(ctx, files[1], slot);
 	if (ret < 0)
 		goto err;
-	fds[1] = ret;
+	fds[1] = alloc_slot ? ret : slot - 1;
 	files[1] = NULL;
 
 	io_ring_submit_unlock(ctx, issue_flags);