[RFC 11/16] io_uring/region: introduce io_region_slice

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

Add a new helper function that returns a sub-slice of a region memory
from an {offset,size} pair and performs a bunch of extra sanitisation.
It'll be used later for (slow path) ring setup.

Signed-off-by: Pavel Begunkov <asml.silence@gmail.com>
---
 io_uring/memmap.c | 12 ++++++++++++
 io_uring/memmap.h |  2 ++
 2 files changed, 14 insertions(+)

diff --git a/io_uring/memmap.c b/io_uring/memmap.c
index b329cee8d6e8..83faef350b9d 100644
--- a/io_uring/memmap.c
+++ b/io_uring/memmap.c
@@ -232,6 +232,18 @@ int io_create_region(struct io_ring_ctx *ctx, struct io_mapped_region *mr,
 	return ret;
 }
 
+void *io_region_slice(struct io_mapped_region *mr, size_t off, size_t size)
+{
+	if (WARN_ON_ONCE(!size) || !io_region_is_set(mr))
+		return NULL;
+
+	size = size_add(off, size);
+	if (size == SIZE_MAX || size > io_region_size(mr))
+		return NULL;
+
+	return io_region_get_ptr(mr) + off;
+}
+
 static struct io_mapped_region *io_mmap_get_region(struct io_ring_ctx *ctx,
 						   loff_t pgoff)
 {
diff --git a/io_uring/memmap.h b/io_uring/memmap.h
index d4b8b6363a7d..fa7a45cdb6dd 100644
--- a/io_uring/memmap.h
+++ b/io_uring/memmap.h
@@ -21,6 +21,8 @@ int io_create_region(struct io_ring_ctx *ctx, struct io_mapped_region *mr,
 		     struct io_uring_region_desc *reg,
 		     unsigned long mmap_offset);
 
+void *io_region_slice(struct io_mapped_region *mr, size_t off, size_t size);
+
 static inline void *io_region_get_ptr(struct io_mapped_region *mr)
 {
 	return mr->ptr;
-- 
2.49.0