Now that we can create a dynptr to skb metadata, make reads to the metadata
area possible with bpf_dynptr_read() or through a bpf_dynptr_slice(), and
make writes to the metadata area possible with bpf_dynptr_write() or
through a bpf_dynptr_slice_rdwr().

Note that for cloned skbs which share data with the original, we limit the
skb metadata dynptr to be read-only since we don't unclone on a
bpf_dynptr_write to metadata.

Signed-off-by: Jakub Sitnicki <[email protected]>

dynptr for skb metadata behaves the same way as the dynptr for skb data
with one exception - writes to skb_meta dynptr don't invalidate existing
skb and skb_meta slices.

Duplicate those the skb dynptr tests which we can, since
bpf_dynptr_from_skb_meta kfunc can be called only from TC BPF, to cover the
skb_meta dynptr verifier checks.

Also add a couple of new tests (skb_data_valid_*) to ensure we don't
invalidate the slices in the mentioned case, which are specific to skb_meta
dynptr.

Reviewed-by: Jesse Brandeburg <[email protected]>
Signed-off-by: Jakub Sitnicki <[email protected]>

We want to add more test cases to cover different ways to access the
metadata area. Prepare for it. Pull up the skeleton management.

Acked-by: Eduard Zingerman <[email protected]>
Reviewed-by: Jesse Brandeburg <[email protected]>
Signed-off-by: Jakub Sitnicki <[email protected]>

Exercise reading from SKB metadata area in two new ways:
1. indirectly, with bpf_dynptr_read(), and
2. directly, with bpf_dynptr_slice().

Acked-by: Eduard Zingerman <[email protected]>
Reviewed-by: Jesse Brandeburg <[email protected]>
Signed-off-by: Jakub Sitnicki <[email protected]>

Add tests what exercise writes to skb metadata in two ways:
1. indirectly, using bpf_dynptr_write helper,
2. directly, using a read-write dynptr slice.

Acked-by: Eduard Zingerman <[email protected]>
Reviewed-by: Jesse Brandeburg <[email protected]>
Signed-off-by: Jakub Sitnicki <[email protected]>

Demonstrate that, when processing an skb clone, the metadata gets truncated
if the program contains a direct write to either the payload or the
metadata, due to an implicit unclone in the prologue, and otherwise the
dynptr to the metadata is limited to being read-only.

Signed-off-by: Jakub Sitnicki <[email protected]>