net: warn if mac header was not set

Make sure skb_mac_header(), skb_mac_offset() and skb_mac_header_len() uses
are not fooled if the mac header has not been set.

These checks are enabled if CONFIG_DEBUG_NET=y

This commit will likely expose existing bugs in linux networking stacks.
Signed-off-by: Eric Dumazet <edumazet@google.com>
Link: https://lore.kernel.org/r/20220620093017.3366713-1-eric.dumazet@gmail.comSigned-off-by: Paolo Abeni <pabeni@redhat.com>

include/linux/skbuff.h

@@ -2763,8 +2763,14 @@ static inline void skb_set_network_header(struct sk_buff *skb, const int offset)
 	skb->network_header += offset;
 }
 
+static inline int skb_mac_header_was_set(const struct sk_buff *skb)
+{
+	return skb->mac_header != (typeof(skb->mac_header))~0U;
+}
+
 static inline unsigned char *skb_mac_header(const struct sk_buff *skb)
 {
+	DEBUG_NET_WARN_ON_ONCE(!skb_mac_header_was_set(skb));
 	return skb->head + skb->mac_header;
 }
 
@@ -2775,14 +2781,10 @@ static inline int skb_mac_offset(const struct sk_buff *skb)
 
 static inline u32 skb_mac_header_len(const struct sk_buff *skb)
 {
+	DEBUG_NET_WARN_ON_ONCE(!skb_mac_header_was_set(skb));
 	return skb->network_header - skb->mac_header;
 }
 
-static inline int skb_mac_header_was_set(const struct sk_buff *skb)
-{
-	return skb->mac_header != (typeof(skb->mac_header))~0U;
-}
-
 static inline void skb_unset_mac_header(struct sk_buff *skb)
 {
 	skb->mac_header = (typeof(skb->mac_header))~0U;