bpf, sockmap: Replace zero-length array with flexible-array member

The current codebase makes use of the zero-length array language extension to the C90 standard, but the preferred mechanism to declare variable-length types such as these ones is a flexible array member[1][2], introduced in C99:

struct foo {
        int stuff;
        struct boo array[];
};

By making use of the mechanism above, we will get a compiler warning in case the flexible array does not occur last in the structure, which will help us prevent some kind of undefined behavior bugs from being inadvertently introduced[3] to the codebase from now on.

Also, notice that, dynamic memory allocations won't be affected by this change:

"Flexible array members have incomplete type, and so the sizeof operator may not be applied. As a quirk of the original implementation of zero-length arrays, sizeof evaluates to zero."[1]

This issue was found with the help of Coccinelle.

[1] https://gcc.gnu.org/onlinedocs/gcc/Zero-Length.html
[2] https://github.com/KSPP/linux/issues/21
[3] commit 76497732932f ("cxgb3/l2t: Fix undefined behaviour")

Signed-off-by Gustavo A. R. Silva <[email protected]>

---
 net/core/sock_map.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

net/core/sock_map.c +1 -1

index 085cef5857bb..3a7a96ab088a 100644
@@ -518,7 +518,7 @@ struct bpf_htab_elem {
 	u32 hash;
 	struct sock *sk;
 	struct hlist_node node;
-	u8 key[0];
+	u8 key[];
 };
 
 struct bpf_htab_bucket {

Signature · 1 line

2.25.0