Re: Usage of kfuncs in tracepoints

[David <david@davidv.dev>]

On 13/01/2026 13:08, Jiri Olsa wrote:
> On Tue, Jan 13, 2026 at 12:53:18PM +0100, David wrote:
>> On 13/01/2026 09:05, Viktor Malik wrote:
>>> 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.
>>>
>> I thought the change took place because `make vmlinux` took a while,
>> but after a clean build, it does work.
>>
>>>> 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).
>>>
>> This is good to know, thanks. I've removed the /boot/ file from my system.
>>>> 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
>>>
>> Do you happen to know how to generate the kfunc headers with bpftool?
>> Even a new bpftool build from the newest commit,
>> ad5d76e5c6b622e5ed05fecfa68029bae949d408, does not generate headers:
>>
>> ```
>> $ ./bpftool btf dump file ~/git/linux-6.18.2/vmlinux | grep strstr
>> [28376] FUNC 'bpf_strstr' type_id=28354 linkage=static
>> [60023] FUNC 'strstr' type_id=60022 linkage=static
>> $ ./bpftool btf dump file ~/git/linux-6.18.2/vmlinux format c  | grep -c
>> strstr
>> 0
>> ```
>>
>> Could this be related to my host's kernel being older than 6.18?
>>
>> For now, I'll generate my files from `kernel/bpf/helpers.c`, but it'd be
>> great if I could use bpftool.
> hi,
> do you use the latest pahole ?
I was using v1.25, using master tip 
(d1dda58ffac121b10a87d2738f3b931847e29acb),
  which reports as v1.31, made it work.

I needed newer binaries for both bpftool and pahole for the headers to 
be fully generated.

Thanks for the help

>
> jirka