Re: Usage of kfuncs in tracepoints

[Jiri Olsa <olsajiri@gmail.com>]

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 ?

jirka