bpf: support SKF_NET_OFF and SKF_LL_OFF on skb frags

bpf: support SKF_NET_OFF and SKF_LL_OFF on skb frags

Classic BPF socket filters with SKB_NET_OFF and SKB_LL_OFF fail to
read when these offsets extend into frags.

This has been observed with iwlwifi and reproduced with tun with
IFF_NAPI_FRAGS. The below straightforward socket filter on UDP port,
applied to a RAW socket, will silently miss matching packets.

    const int offset_proto = offsetof(struct ip6_hdr, ip6_nxt);
    const int offset_dport = sizeof(struct ip6_hdr) + offsetof(struct udphdr, dest);
    struct sock_filter filter_code[] = {
            BPF_STMT(BPF_LD  + BPF_B   + BPF_ABS, SKF_AD_OFF + SKF_AD_PKTTYPE),
            BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, PACKET_HOST, 0, 4),
            BPF_STMT(BPF_LD  + BPF_B   + BPF_ABS, SKF_NET_OFF + offset_proto),
            BPF_JUMP(BPF_JMP + BPF_JEQ + BPF_K, IPPROTO_UDP, 0, 2),
            BPF_STMT(BPF_LD  + BPF_H   + BPF_ABS, SKF_NET_OFF + offset_dport),

This is unexpected behavior. Socket filter programs should be
consistent regardless of environment. Silent misses are
particularly concerning as hard to detect.

Use skb_copy_bits for offsets outside linear, same as done for
non-SKF_(LL|NET) offsets.

Offset is always positive after subtracting the reference threshold
SKB_(LL|NET)_OFF, so is always >= skb_(mac|network)_offset. The sum of
the two is an offset against skb->data, and may be negative, but it
cannot point before skb->head, as skb_(mac|network)_offset would too.

This appears to go back to when frag support was introduced to
sk_run_filter in linux-2.4.4, before the introduction of git.

The amount of code change and 8/16/32 bit duplication are unfortunate.
But any attempt I made to be smarter saved very few LoC while
complicating the code.

Fixes: 1da177e4c3f4 ("Linux-2.6.12-rc2")
Link: https://lore.kernel.org/netdev/20250122200402.3461154-1-maze@google.com/
Link: https://elixir.bootlin.com/linux/2.4.4/source/net/core/filter.c#L244
Reported-by: Matt Moeller <moeller.matt@gmail.com>
Co-developed-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://lore.kernel.org/r/20250408132833.195491-2-willemdebruijn.kernel@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>