Re: Usage of kfuncs in tracepoints

[Viktor Malik <vmalik@redhat.com>]

On 1/12/26 20:08, David wrote:
> On 12/01/2026 19:03, Alan Maguire wrote:
> 
>> I think you need to add "__ksym __weak";" here i.e.
> 
> This change let me load the program, however, libbpf cannot find a 
> kernel image at
> /sys/kernel/btf/vmlinux, because /sys/kernel/btf is not populated on my 
> system.
> 
> My kernel _is_ built with CONFIG_DEBUG_INFO_BTF=y, is there something 
> else I need to do
>   to get this path populated?

Did you try rebuilding from scratch (with `make clean`) after enabling
CONFIG_DEBUG_INFO_BTF? If the sources were already built without debug
info, they will not be automatically rebuilt just by adding the config
option.

> 
> Because this path is missing, libbpf reports:
> 
> ```
> kernel BTF is missing at '/sys/kernel/btf/vmlinux', was 
> CONFIG_DEBUG_INFO_BTF enabled?
> ```
> 
> But I see from strace that it tries a few fallback paths.
> In the meantime, I copied my kernel into /boot/vmlinux-6.18.2 so libbpf 
> can find it, but
> now the loader says
> 
> ```
> calling kernel function is not supported without CONFIG_DEBUG_INFO_BTF
> ```

While libbpf may try other fallback paths to find BTF, using kfuncs
requires the kernel to find that kfunc in BTF and kernel will only use
the system BTF (the one from /sys/kernel/vmlinux/btf).

> Can I not use `bpf_strstr` on a tracepoint? To validate, I tried a 
> `raw_tp` but
> had the same result.

There shouldn't be any issue using bpf_strstr from tracepoints (or any
other program type).

Viktor