overflow.h: Add flex_array_size() helper
Add flex_array_size() helper for the calculation of the size, in bytes,
of a flexible array member contained within an enclosing structure.
Example of usage:
struct something {
size_t count;
struct foo items[];
};
struct something *instance;
instance = kmalloc(struct_size(instance, items, count), GFP_KERNEL);
instance->count = count;
memcpy(instance->items, src, flex_array_size(instance, items, instance->count));
The helper returns SIZE_MAX on overflow instead of wrapping around.
Additionally replaces parameter "n" with "count" in struct_size() helper
for greater clarity and unification.
Signed-off-by: Gustavo A. R. Silva <gustavoars@kernel.org>
Link: https://lore.kernel.org/r/20200609012233.GA3371@embeddedor
Signed-off-by: Kees Cook <keescook@chromium.org>
diff --git a/include/linux/overflow.h b/include/linux/overflow.h
index 6590450..93fcef1 100644
--- a/include/linux/overflow.h
+++ b/include/linux/overflow.h
@@ -304,16 +304,33 @@
* struct_size() - Calculate size of structure with trailing array.
* @p: Pointer to the structure.
* @member: Name of the array member.
- * @n: Number of elements in the array.
+ * @count: Number of elements in the array.
*
* Calculates size of memory needed for structure @p followed by an
- * array of @n @member elements.
+ * array of @count number of @member elements.
*
* Return: number of bytes needed or SIZE_MAX on overflow.
*/
-#define struct_size(p, member, n) \
- __ab_c_size(n, \
+#define struct_size(p, member, count) \
+ __ab_c_size(count, \
sizeof(*(p)->member) + __must_be_array((p)->member),\
sizeof(*(p)))
+/**
+ * flex_array_size() - Calculate size of a flexible array member
+ * within an enclosing structure.
+ *
+ * @p: Pointer to the structure.
+ * @member: Name of the flexible array member.
+ * @count: Number of elements in the array.
+ *
+ * Calculates size of a flexible array of @count number of @member
+ * elements, at the end of structure @p.
+ *
+ * Return: number of bytes needed or SIZE_MAX on overflow.
+ */
+#define flex_array_size(p, member, count) \
+ array_size(count, \
+ sizeof(*(p)->member) + __must_be_array((p)->member))
+
#endif /* __LINUX_OVERFLOW_H */
