Merge branch 'make-tc-bpf-helpers-preserve-skb-metadata'

Jakub Sitnicki says:

====================
Make TC BPF helpers preserve skb metadata

Changes in v4:
- Fix copy-paste bug in check_metadata() test helper (AI review)
- Add "out of scope" section (at the bottom)
- Link to v3: https://lore.kernel.org/r/20251026-skb-meta-rx-path-v3-0-37cceebb95d3@cloudflare.com

Changes in v3:
- Use the already existing BPF_STREAM_STDERR const in tests (Martin)
- Unclone skb head on bpf_dynptr_write to skb metadata (patch 3) (Martin)
- Swap order of patches 1 & 2 to refer to skb_postpush_data_move() in docs
- Mention in skb_data_move() docs how to move just the metadata
- Note in pskb_expand_head() docs to move metadata after skb_push() (Jakub)
- Link to v2: https://lore.kernel.org/r/20251019-skb-meta-rx-path-v2-0-f9a58f3eb6d6@cloudflare.com

Changes in v2:
- Tweak WARN_ON_ONCE check in skb_data_move() (patch 2)
- Convert all tests to verify skb metadata in BPF (patches 9-10)
- Add test coverage for modified BPF helpers (patches 12-15)
- Link to RFCv1: https://lore.kernel.org/r/20250929-skb-meta-rx-path-v1-0-de700a7ab1cb@cloudflare.com

This patch set continues our work [1] to allow BPF programs and user-space
applications to attach multiple bytes of metadata to packets via the
XDP/skb metadata area.

The focus of this patch set it to ensure that skb metadata remains intact
when packets pass through a chain of TC BPF programs that call helpers
which operate on skb head.

Currently, several helpers that either adjust the skb->data pointer or
reallocate skb->head do not preserve metadata at its expected location,
that is immediately in front of the MAC header. These are:

- bpf_skb_adjust_room
- bpf_skb_change_head
- bpf_skb_change_proto
- bpf_skb_change_tail
- bpf_skb_vlan_pop
- bpf_skb_vlan_push

In TC BPF context, metadata must be moved whenever skb->data changes to
keep the skb->data_meta pointer valid. I don't see any way around
it. Creative ideas how to avoid that would be very welcome.

With that in mind, we can patch the helpers in at least two different ways:

1. Integrate metadata move into header move

   Replace the existing memmove, which follows skb_push/pull, with a helper
   that moves both headers and metadata in a single call. This avoids an
   extra memmove but reduces transparency.

        skb_pull(skb, len);
-       memmove(skb->data, skb->data - len, n);
+       skb_postpull_data_move(skb, len, n);
        skb->mac_header += len;

        skb_push(skb, len)
-       memmove(skb->data, skb->data + len, n);
+       skb_postpush_data_move(skb, len, n);
        skb->mac_header -= len;

2. Move metadata separately

   Add a dedicated metadata move after the header move. This is more
   explicit but costs an additional memmove.

        skb_pull(skb, len);
        memmove(skb->data, skb->data - len, n);
+       skb_metadata_postpull_move(skb, len);
        skb->mac_header += len;

        skb_push(skb, len)
+       skb_metadata_postpush_move(skb, len);
        memmove(skb->data, skb->data + len, n);
        skb->mac_header -= len;

This patch set implements option (1), expecting that "you can have just one
memmove" will be the most obvious feedback, while readability is a,
somewhat subjective, matter of taste, which I don't claim to have ;-)

The structure of the patch set is as follows:

- patches 1-4 prepare ground for safe-proofing the BPF helpers
- patches 5-9 modify the BPF helpers to preserve skb metadata
- patches 10-11 prepare ground for metadata tests with BPF helper calls
- patches 12-16 adapt and expand tests to cover the made changes

Out of scope for this series:
- safe-proofing tunnel & tagging devices - VLAN, GRE, ...
  (next in line, in development preview at [2])
- metadata access after packet foward
  (to do after Rx path - once metadata reliably reaches sk_filter)

Thanks,
-jkbs

[1] https://lore.kernel.org/all/20250814-skb-metadata-thru-dynptr-v7-0-8a39e636e0fb@cloudflare.com/
[2] https://github.com/jsitnicki/linux/commits/skb-meta/safeproof-netdevs/
====================

Link: https://patch.msgid.link/20251105-skb-meta-rx-path-v4-0-5ceb08a9b37b@cloudflare.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>