lib/generic-radix-tree.c: Don't overflow in peek()

When we started spreading new inode numbers throughout most of the 64
bit inode space, that triggered some corner case bugs, in particular
some integer overflows related to the radix tree code. Oops.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

include/linux/generic-radix-tree.h

@@ -38,6 +38,7 @@
 
 #include <asm/page.h>
 #include <linux/bug.h>
+#include <linux/limits.h>
 #include <linux/log2.h>
 #include <linux/math.h>
 #include <linux/types.h>
@@ -184,6 +185,12 @@ void *__genradix_iter_peek(struct genradix_iter *, struct __genradix *, size_t);
 static inline void __genradix_iter_advance(struct genradix_iter *iter,
 					   size_t obj_size)
 {
+	if (iter->offset + obj_size < iter->offset) {
+		iter->offset	= SIZE_MAX;
+		iter->pos	= SIZE_MAX;
+		return;
+	}
+
 	iter->offset += obj_size;
 
 	if (!is_power_of_2(obj_size) &&

lib/generic-radix-tree.c

@@ -166,6 +166,10 @@ void *__genradix_iter_peek(struct genradix_iter *iter,
 	struct genradix_root *r;
 	struct genradix_node *n;
 	unsigned level, i;
+
+	if (iter->offset == SIZE_MAX)
+		return NULL;
+
 restart:
 	r = READ_ONCE(radix->root);
 	if (!r)
@@ -184,10 +188,17 @@ void *__genradix_iter_peek(struct genradix_iter *iter,
 			(GENRADIX_ARY - 1);
 
 		while (!n->children[i]) {
+			size_t objs_per_ptr = genradix_depth_size(level);
+
+			if (iter->offset + objs_per_ptr < iter->offset) {
+				iter->offset	= SIZE_MAX;
+				iter->pos	= SIZE_MAX;
+				return NULL;
+			}
+
 			i++;
-			iter->offset = round_down(iter->offset +
-					   genradix_depth_size(level),
-					   genradix_depth_size(level));
+			iter->offset = round_down(iter->offset + objs_per_ptr,
+						  objs_per_ptr);
 			iter->pos = (iter->offset >> PAGE_SHIFT) *
 				objs_per_page;
 			if (i == GENRADIX_ARY)