selftests/bpf: fix list_del() in arena list

The __list_del fuction doesn't set the previous node's next pointer to
the next node of the node to be deleted. It just updates the local variable
and not the actual pointer in the previous node.

The test was passing up till now because the bpf code is doing bpf_free()
after list_del and therfore reading head->first from the userspace will
read all zeroes. But after arena_list_del() is finished, head->first should
point to NULL;

If you remove the bpf_free() call in arena_list_del(), the test will start
crashing because now the userpsace will read 0x100 (LIST_POISON1) in
head->first and segfault.

Signed-off-by: Puranjay Mohan <[email protected]>
Acked-by: Yonghong Song <[email protected]>

Author: puranjaymohan

tools/testing/selftests/bpf/bpf_arena_list.h

@@ -64,14 +64,12 @@ static inline void list_add_head(arena_list_node_t *n, arena_list_head_t *h)
 
 static inline void __list_del(arena_list_node_t *n)
 {
-	arena_list_node_t *next = n->next, *tmp;
+	arena_list_node_t *next = n->next;
 	arena_list_node_t * __arena *pprev = n->pprev;
 
 	cast_user(next);
 	cast_kern(pprev);
-	tmp = *pprev;
-	cast_kern(tmp);
-	WRITE_ONCE(tmp, next);
+	WRITE_ONCE(*pprev, next);
 	if (next) {
 		cast_user(pprev);
 		cast_kern(next);