bpf: Add a check for struct bpf_fib_lookup size

The struct bpf_fib_lookup should not grow outside of its 64 bytes.
Add a static assert to validate this.

Suggested-by: David Ahern <[email protected]>
Signed-off-by: Anton Protopopov <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]
Signed-off-by: Alexei Starovoitov <[email protected]>

net/core/filter.c

@@ -87,6 +87,9 @@
 
 #include "dev.h"
 
+/* Keep the struct bpf_fib_lookup small so that it fits into a cacheline */
+static_assert(sizeof(struct bpf_fib_lookup) == 64, "struct bpf_fib_lookup size check");
+
 static const struct bpf_func_proto *
 bpf_sk_base_func_proto(enum bpf_func_id func_id, const struct bpf_prog *prog);