samples/bpf: fix sockex2 example

since llvm commit "Do not expand UNDEF SDNode during insn selection lowering"
llvm will generate code that uses uninitialized registers for cases
where C code is actually uses uninitialized data.
So this sockex2 example is technically broken.
Fix it by initializing on the stack variable fully.
Also increase verifier buffer limit, since verifier output
may not fit in 64k for this sockex2 code depending on llvm version.
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>

samples/bpf/libbpf.h

@@ -18,7 +18,7 @@ int bpf_prog_load(enum bpf_prog_type prog_type,
 int bpf_obj_pin(int fd, const char *pathname);
 int bpf_obj_get(const char *pathname);
 
-#define LOG_BUF_SIZE 65536
+#define LOG_BUF_SIZE (256 * 1024)
 extern char bpf_log_buf[LOG_BUF_SIZE];
 
 /* ALU ops on registers, bpf_add|sub|...: dst_reg += src_reg */

samples/bpf/sockex2_kern.c

@@ -198,7 +198,7 @@ struct bpf_map_def SEC("maps") hash_map = {
 SEC("socket2")
 int bpf_prog2(struct __sk_buff *skb)
 {
-	struct bpf_flow_keys flow;
+	struct bpf_flow_keys flow = {};
 	struct pair *value;
 	u32 key;