Since commit 0108a4e9 ("bpf: ensure main program has an extable"),
prog->aux->func[0]->kallsyms is left as uninitialized. For BPF programs
with subprogs, the symbol for the main program is missing just as shown
in the output of perf script below:

 ffffffff81284b69 qp_trie_lookup_elem+0xb9 ([kernel.kallsyms])
 ffffffffc0011125 bpf_prog_a4a0eb0651e6af8b_lookup_qp_trie+0x5d (bpf...)
 ffffffff8127bc2b bpf_for_each_array_elem+0x7b ([kernel.kallsyms])
 ffffffffc00110a1 +0x25 ()
 ffffffff8121a89a trace_call_bpf+0xca ([kernel.kallsyms])

Fix it by always using prog instead prog->aux->func[0] to emit ksymbol
event for the main program. After the fix, the output of perf script
will be correct:

 ffffffff81284b96 qp_trie_lookup_elem+0xe6 ([kernel.kallsyms])
 ffffffffc001382d bpf_prog_a4a0eb0651e6af8b_lookup_qp_trie+0x5d (bpf...)
 ffffffff8127bc2b bpf_for_each_array_elem+0x7b ([kernel.kallsyms])
 ffffffffc0013779 bpf_prog_245c55ab25cfcf40_qp_trie_lookup+0x25 (bpf...)
 ffffffff8121a89a trace_call_bpf+0xca ([kernel.kallsyms])

Fixes: 0108a4e9 ("bpf: ensure main program has an extable")
Signed-off-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Yonghong Song <yonghong.song@linux.dev>
Reviewed-by: Krister Johansen <kjlx@templeofstupid.com>
Reviewed-by: Jiri Olsa <jolsa@kernel.org>
Link: https://lore.kernel.org/bpf/20240714065533.1112616-1-houtao@huaweicloud.com