selftests/bpf: Switch SEC("classifier*") usage to a strict SEC("tc")

Convert all SEC("classifier*") uses to a new and strict SEC("tc")
section name. In reference_tracking selftests switch from ambiguous
searching by program title (section name) to non-ambiguous searching by
name in some selftests, getting closer to completely removing
bpf_object__find_program_by_title().

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210928161946.2512801-4-andrii@kernel.org

selftests/bpf: drop custom NULL #define in skb_pkt_end selftest

Now that bpftool generates NULL definition as part of vmlinux.h, drop custom
NULL definition in skb_pkt_end.c.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20210317200510.1354627-3-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

selftests/bpf: Add skb_pkt_end test

Add a test that currently makes LLVM generate assembly code:

$ llvm-objdump -S skb_pkt_end.o
0000000000000000 <main_prog>:
; if (skb_shorter(skb, ETH_IPV4_TCP_SIZE))
0: 61 12 50 00 00 00 00 00 r2 = *(u32 *)(r1 + 80)
1: 61 14 4c 00 00 00 00 00 r4 = *(u32 *)(r1 + 76)
2: bf 43 00 00 00 00 00 00 r3 = r4
3: 07 03 00 00 36 00 00 00 r3 += 54
4: b7 01 00 00 00 00 00 00 r1 = 0
5: 2d 23 02 00 00 00 00 00 if r3 > r2 goto +2 <LBB0_2>
6: 07 04 00 00 0e 00 00 00 r4 += 14
; if (skb_shorter(skb, ETH_IPV4_TCP_SIZE))
7: bf 41 00 00 00 00 00 00 r1 = r4
0000000000000040 <LBB0_2>:
8: b4 00 00 00 ff ff ff ff w0 = -1
; if (!(ip = get_iphdr(skb)))
9: 2d 23 05 00 00 00 00 00 if r3 > r2 goto +5 <LBB0_6>
; proto = ip->protocol;
10: 71 12 09 00 00 00 00 00 r2 = *(u8 *)(r1 + 9)
; if (proto != IPPROTO_TCP)
11: 56 02 03 00 06 00 00 00 if w2 != 6 goto +3 <LBB0_6>
; if (tcp->dest != 0)
12: 69 12 16 00 00 00 00 00 r2 = *(u16 *)(r1 + 22)
13: 56 02 01 00 00 00 00 00 if w2 != 0 goto +1 <LBB0_6>
; return tcp->urg_ptr;
14: 69 10 26 00 00 00 00 00 r0 = *(u16 *)(r1 + 38)
0000000000000078 <LBB0_6>:
; }
15: 95 00 00 00 00 00 00 00 exit

Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/bpf/20201111031213.25109-3-alexei.starovoitov@gmail.com