selftests/bpf: update tcp_custom_syncookie to use scalar packet offset

This commit updates tcp_custom_syncookie.c:tcp_parse_option() to use
explicit packet offset (ctx->off) for packet access instead of ever
moving pointer (ctx->ptr), this reduces verification complexity:
- the tcp_parse_option() is passed as a callback to bpf_loop();
- suppose a checkpoint is created each time at function entry;
- the ctx->ptr is tracked by verifier as PTR_TO_PACKET;
- the ctx->ptr is incremented in tcp_parse_option(),
thus umax_value field tracked for it is incremented as well;
- on each next iteration of tcp_parse_option()
checkpoint from a previous iteration can't be reused
for state pruning, because PTR_TO_PACKET registers are
considered equivalent only if old->umax_value >= cur->umax_value;
- on the other hand, the ctx->off is a SCALAR,
subject to widen_imprecise_scalars();
- it's exact bounds are eventually forgotten and it is tracked as
unknown scalar at entry to tcp_parse_option();
- hence checkpoints created at the start of the function eventually
converge.

The change is similar to one applied in [0] to xdp_synproxy_kern.c.

Comparing before and after with veristat yields following results:

File Insns (A) Insns (B) Insns (DIFF)
------------------------------- --------- --------- -----------------
test_tcp_custom_syncookie.bpf.o 466657 12423 -454234 (-97.34%)

[0] commit 977bc146d4eb ("selftests/bpf: track tcp payload offset as scalar in xdp_synproxy")

Acked-by: Yonghong Song <yonghong.song@linux.dev>
Signed-off-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20240222150300.14909-2-eddyz87@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftest: bpf: Test bpf_sk_assign_tcp_reqsk().

This commit adds a sample selftest to demonstrate how we can use
bpf_sk_assign_tcp_reqsk() as the backend of SYN Proxy.

The test creates IPv4/IPv6 x TCP connections and transfer messages
over them on lo with BPF tc prog attached.

The tc prog will process SYN and returns SYN+ACK with the following
ISN and TS. In a real use case, this part will be done by other
hosts.

MSB LSB
ISN: | 31 ... 8 | 7 6 | 5 | 4 | 3 2 1 0 |
| Hash_1 | MSS | ECN | SACK | WScale |

TS: | 31 ... 8 | 7 ... 0 |
| Random | Hash_2 |

WScale in SYN is reused in SYN+ACK.

The client returns ACK, and tc prog will recalculate ISN and TS
from ACK and validate SYN Cookie.

If it's valid, the prog calls kfunc to allocate a reqsk for skb and
configure the reqsk based on the argument created from SYN Cookie.

Later, the reqsk will be processed in cookie_v[46]_check() to create
a connection.

Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Link: https://lore.kernel.org/r/20240115205514.68364-7-kuniyu@amazon.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>