Re: [PATCH bpf-next v1] bpftool: Add support for custom BTF path in prog load/loadall

["Jiayuan Chen" <jiayuan.chen@linux.dev>]

2025/5/14 05:19, "Alexei Starovoitov" <alexei.starovoitov@gmail.com> wrote:

> 
> On Mon, May 12, 2025 at 8:59 PM Jiayuan Chen <jiayuan.chen@linux.dev> wrote:
> 
> > 
> > This patch exposes the btf_custom_path feature to bpftool, allowing users
> > 
> >  to specify a custom BTF file when loading BPF programs using prog load or
> > 
> >  prog loadall commands. This feature is already supported by libbpf, and
> > 
> >  this patch makes it accessible through the bpftool command-line interface.
> > 
> >  Signed-off-by: Jiayuan Chen <jiayuan.chen@linux.dev>
> > 
> >  ---
> > 
> >  tools/bpf/bpftool/prog.c | 11 ++++++++++-
> > 
> >  1 file changed, 10 insertions(+), 1 deletion(-)
> > 
> >  diff --git a/tools/bpf/bpftool/prog.c b/tools/bpf/bpftool/prog.c
> > 
> >  index f010295350be..63f84e765b34 100644
> > 
> >  --- a/tools/bpf/bpftool/prog.c
> > 
> >  +++ b/tools/bpf/bpftool/prog.c
> > 
> >  @@ -1681,8 +1681,17 @@ static int load_with_options(int argc, char **argv, bool first_prog_only)
> > 
> >  } else if (is_prefix(*argv, "autoattach")) {
> > 
> >  auto_attach = true;
> > 
> >  NEXT_ARG();
> > 
> >  + } else if (is_prefix(*argv, "custom_btf")) {
> > 
> >  + NEXT_ARG();
> > 
> >  +
> > 
> >  + if (!REQ_ARGS(1))
> > 
> >  + goto err_free_reuse_maps;
> > 
> >  +
> > 
> >  + open_opts.btf_custom_path = GET_ARG();
> > 
> 
> I don't see a use case yet.
> 
> What exactly is the scenario where it's useful ?
>

This patch just exposes the btf_custom_path feature of libbpf to bpftool.
The argument 'btf_custom_path' in libbpf is used for those kernes that
don't have CONFIG_DEBUG_INFO_BTF enabled but still want to perform CO-RE
relocations. Specifically for older kernels, separate BTF files are already
provided: https://github.com/aquasecurity/btfhub-archive/.
If we want load prog using bpftool on those systems, we have to hack
btf__load_vmlinux_btf() before or write custom loader with libbpf and specify
'btf_custom_path'.

I also found a the similar topic:
https://lore.kernel.org/bpf/20220215225856.671072-1-mauricio@kinvolk.io/

Additionally, pwru supports "--kernel-btf" which serves the same purpose as
this patch.

Therefore, using an external BTF file is a common practice.