[PATCH 6/6] io_uring: enable IORING_ENTER_EXT_ARG_REG back

[Pavel Begunkov <asml.silence@gmail.com>]

We can reenable IORING_ENTER_EXT_ARG_REG, which is now backed by
parameter regions.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 io_uring/io_uring.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/io_uring/io_uring.c b/io_uring/io_uring.c
index c640b8a4ceee..8b4b866b7761 100644
--- a/io_uring/io_uring.c
+++ b/io_uring/io_uring.c
@@ -3195,7 +3195,16 @@ void __io_uring_cancel(bool cancel_all)
 static struct io_uring_reg_wait *io_get_ext_arg_reg(struct io_ring_ctx *ctx,
 			const struct io_uring_getevents_arg __user *uarg)
 {
-	return ERR_PTR(-EFAULT);
+	unsigned long size = sizeof(struct io_uring_reg_wait);
+	unsigned long offset = (uintptr_t)uarg;
+	unsigned long end;
+
+	/* also protects from NULL ->cq_wait_arg as the size would be 0 */
+	if (unlikely(check_add_overflow(offset, size, &end) ||
+		     end >= ctx->cq_wait_size))
+		return ERR_PTR(-EFAULT);
+
+	return ctx->cq_wait_arg + offset;
 }
 
 static int io_validate_ext_arg(struct io_ring_ctx *ctx, unsigned flags,
-- 
2.46.0