Usage of kfuncs in tracepoints

[David <david@davidv.dev>]

Hi

I'm trying to use `bpf_strstr` in a program that's running on kernel 6.18.2,
but I'm getting the following error on load:

 > failed to find BTF for extern 'bpf_strstr' [53] section: -2

A minimal reproducer for this is:

```
extern int bpf_strstr(const char *s1__ign, const char *s2__ign);
SEC("tracepoint/syscalls/sys_enter_sendto")
int trace_sendto_entry(struct trace_event_raw_sys_enter *ctx)
{
       char buf[128];
       int pos = bpf_strstr(buf, "A");
}
```

My kernel was initially built with CONFIG_DEBUG_INFO_BTF=n, rebuilding with
CONFIG_DEBUG_INFO_BTF=y did not change the error.

I've only tried this with a stripped kernel image, which is 4MiB larger than
the image with no BTF info.

Running `bpftool btf dump` on the stripped iamge does show the kfunc:

```
$ bpftool btf dump file ~/git/linux-6.18.2/vmlinux | grep strstr
[26877] FUNC 'bpf_strstr' type_id=26855 linkage=static
[60337] FUNC 'strstr' type_id=60336 linkage=static
```

I'm running this program in a virtual machine with a custom init; running
through strace shows a failed load of `/proc/version_signature`, but I 
assume a
fallback to `uname`.

```
faccessat2(AT_FDCWD, "/proc/version_signature", R_OK, AT_EACCESS) = -1 
ENOENT (No such file or directory)
uname({sysname="Linux", nodename="(none)", ...}) = 0
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) 
= 0x7f3fe6630000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) 
= 0x7f3fe662f000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) 
= 0x7f3fe662e000
munmap(0x7f3fe662e000, 4096) = 0
munmap(0x7f3fe6630000, 8192 <unfinished ...>
nanosleep({tv_sec=0, tv_nsec=10000000} <unfinished ...>
futex(0x7f3fe6a44858, FUTEX_WAKE_PRIVATE, 1) = 1
munmap(0x7f3fe6839000, 16384 <unfinished ...>
sendto(3, "\1", 1, MSG_NOSIGNAL, NULL, 0 <unfinished ...>
futex(0x7f3fe6a43b70, FUTEX_WAIT_PRIVATE, 2, NULL <unfinished ...>
sendto(3, "@", 1, MSG_NOSIGNAL, NULL, 0) = 1
sendto(3, "libbpf: failed to find BTF for e"..., 64, MSG_NOSIGNAL, NULL, 
0) = 64
close(3)
```

Maybe there's another dependency I'm not aware of for kfuncs?

I'm not sure what I'm doing wrong, can you point me in the right direction?


David