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);
