bpf: Bpf_skc_to_* casting helpers require a NULL check on sk

The bpf_skc_to_* type casting helpers are available to
BPF_PROG_TYPE_TRACING.  The traced PTR_TO_BTF_ID may be NULL.
For example, the skb->sk may be NULL.  Thus, these casting helpers
need to check "!sk" also and this patch fixes them.

Fixes: 0d4fad3e ("bpf: Add bpf_skc_to_udp6_sock() helper")
Fixes: 478cfbdf ("bpf: Add bpf_skc_to_{tcp, tcp_timewait, tcp_request}_sock() helpers")
Fixes: af7ec138 ("bpf: Add bpf_skc_to_tcp6_sock() helper")
Signed-off-by: Martin KaFai Lau <kafai@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20200915182959.241101-1-kafai@fb.com

net/core/filter.c

@@ -9522,7 +9522,7 @@ BPF_CALL_1(bpf_skc_to_tcp6_sock, struct sock *, sk)
 	 * trigger an explicit type generation here.
 	 */
 	BTF_TYPE_EMIT(struct tcp6_sock);
-	if (sk_fullsock(sk) && sk->sk_protocol == IPPROTO_TCP &&
+	if (sk && sk_fullsock(sk) && sk->sk_protocol == IPPROTO_TCP &&
 	    sk->sk_family == AF_INET6)
 		return (unsigned long)sk;
 
@@ -9540,7 +9540,7 @@ const struct bpf_func_proto bpf_skc_to_tcp6_sock_proto = {
 
 BPF_CALL_1(bpf_skc_to_tcp_sock, struct sock *, sk)
 {
-	if (sk_fullsock(sk) && sk->sk_protocol == IPPROTO_TCP)
+	if (sk && sk_fullsock(sk) && sk->sk_protocol == IPPROTO_TCP)
 		return (unsigned long)sk;
 
 	return (unsigned long)NULL;
@@ -9558,12 +9558,12 @@ const struct bpf_func_proto bpf_skc_to_tcp_sock_proto = {
 BPF_CALL_1(bpf_skc_to_tcp_timewait_sock, struct sock *, sk)
 {
 #ifdef CONFIG_INET
-	if (sk->sk_prot == &tcp_prot && sk->sk_state == TCP_TIME_WAIT)
+	if (sk && sk->sk_prot == &tcp_prot && sk->sk_state == TCP_TIME_WAIT)
 		return (unsigned long)sk;
 #endif
 
 #if IS_BUILTIN(CONFIG_IPV6)
-	if (sk->sk_prot == &tcpv6_prot && sk->sk_state == TCP_TIME_WAIT)
+	if (sk && sk->sk_prot == &tcpv6_prot && sk->sk_state == TCP_TIME_WAIT)
 		return (unsigned long)sk;
 #endif
 
@@ -9582,12 +9582,12 @@ const struct bpf_func_proto bpf_skc_to_tcp_timewait_sock_proto = {
 BPF_CALL_1(bpf_skc_to_tcp_request_sock, struct sock *, sk)
 {
 #ifdef CONFIG_INET
-	if (sk->sk_prot == &tcp_prot  && sk->sk_state == TCP_NEW_SYN_RECV)
+	if (sk && sk->sk_prot == &tcp_prot && sk->sk_state == TCP_NEW_SYN_RECV)
 		return (unsigned long)sk;
 #endif
 
 #if IS_BUILTIN(CONFIG_IPV6)
-	if (sk->sk_prot == &tcpv6_prot && sk->sk_state == TCP_NEW_SYN_RECV)
+	if (sk && sk->sk_prot == &tcpv6_prot && sk->sk_state == TCP_NEW_SYN_RECV)
 		return (unsigned long)sk;
 #endif
 
@@ -9609,7 +9609,7 @@ BPF_CALL_1(bpf_skc_to_udp6_sock, struct sock *, sk)
 	 * trigger an explicit type generation here.
 	 */
 	BTF_TYPE_EMIT(struct udp6_sock);
-	if (sk_fullsock(sk) && sk->sk_protocol == IPPROTO_UDP &&
+	if (sk && sk_fullsock(sk) && sk->sk_protocol == IPPROTO_UDP &&
 	    sk->sk_type == SOCK_DGRAM && sk->sk_family == AF_INET6)
 		return (unsigned long)sk;