Re: [PATCH v2 1/3] bpf: Allow NULL buffers in bpf_dynptr_slice(_rw)

[Jakub Kicinski <kuba@kernel.org>]

On Tue, 18 Jul 2023 13:34:06 -0700 Alexei Starovoitov wrote:
> > Direct packet access via skb->data is there for those who want high
> > speed 🤷️  
> 
> skb->data/data_end approach unfortunately doesn't work that well.
> Too much verifier fighting. That's why dynptr was introduced.

I wish Daniel told us more about the use case.

> > My worry is that people will think that whether the buffer is needed or
> > not depends on _their program_, rather than on the underlying platform.
> > So if it works in testing without the buffer - the buffer must not be
> > required for their use case.  
> 
> Are you concerned about bpf progs breaking this way?

Both, BPF progs breaking and netdev code doing things which don't make
sense. But I won't argue too hard about the former, i.e. the BPF API.

> I thought you're worried about the driver misusing
> skb_header_pointer() with buffer==NULL.
> 
> We can remove !buffer check as in the attached patch,
> but I don't quite see how it would improve driver quality.

The drivers may not be pretty but they aren't buggy AFAICT.

> [0001-bpf-net-Introduce-skb_pointer_if_linear.patch  application/octet-stream (2873 bytes)] 

Or we can simply pretend we don't have the skb:

diff --git a/include/linux/skbuff.h b/include/linux/skbuff.h
index 91ed66952580..217447f01d56 100644
--- a/include/linux/skbuff.h
+++ b/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;
diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index 9e80efa59a5d..8bc4622cc1df 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -2239,7 +2239,13 @@ __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);
+	{
+		const struct sk_buff *skb = ptr->data;
+
+		return __skb_header_pointer(NULL, ptr->offset + offset, len,
+					    skb->data, skb_headlen(skb),
+					    buffer__opt);
+	}
 	case BPF_DYNPTR_TYPE_XDP:
 	{
 		void *xdp_ptr = bpf_xdp_pointer(ptr->data, ptr->offset + offset, len);