bpf, net: Introduce skb_pointer_if_linear().

Network drivers always call skb_header_pointer() with non-null buffer.
Remove !buffer check to prevent accidental misuse of skb_header_pointer().
Introduce skb_pointer_if_linear() instead.

Reported-by: Jakub Kicinski <kuba@kernel.org>
Acked-by: Jakub Kicinski <kuba@kernel.org>
Link: https://lore.kernel.org/r/20230718234021.43640-1-alexei.starovoitov@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

include/linux/skbuff.h

@@ -4023,7 +4023,7 @@ __skb_header_pointer(const struct sk_buff *skb, int offset, int len,
 	if (likely(hlen - offset >= len))
 		return (void *)data + offset;
 
-	if (!skb || !buffer || unlikely(skb_copy_bits(skb, offset, buffer, len) < 0))
+	if (!skb || unlikely(skb_copy_bits(skb, offset, buffer, len) < 0))
 		return NULL;
 
 	return buffer;
@@ -4036,6 +4036,14 @@ skb_header_pointer(const struct sk_buff *skb, int offset, int len, void *buffer)
 				    skb_headlen(skb), buffer);
 }
 
+static inline void * __must_check
+skb_pointer_if_linear(const struct sk_buff *skb, int offset, int len)
+{
+	if (likely(skb_headlen(skb) - offset >= len))
+		return skb->data + offset;
+	return NULL;
+}
+
 /**
  *	skb_needs_linearize - check if we need to linearize a given skb
  *			      depending on the given device features.

kernel/bpf/helpers.c

@@ -2263,7 +2263,10 @@ __bpf_kfunc void *bpf_dynptr_slice(const struct bpf_dynptr_kern *ptr, u32 offset
 	case BPF_DYNPTR_TYPE_RINGBUF:
 		return ptr->data + ptr->offset + offset;
 	case BPF_DYNPTR_TYPE_SKB:
-		return skb_header_pointer(ptr->data, ptr->offset + offset, len, buffer__opt);
+		if (buffer__opt)
+			return skb_header_pointer(ptr->data, ptr->offset + offset, len, buffer__opt);
+		else
+			return skb_pointer_if_linear(ptr->data, ptr->offset + offset, len);
 	case BPF_DYNPTR_TYPE_XDP:
 	{
 		void *xdp_ptr = bpf_xdp_pointer(ptr->data, ptr->offset + offset, len);