Re: [PATCH dwarves] pahole: Inject kfunc decl tags into BTF

[Jiri Olsa <olsajiri@gmail.com>]

On Wed, Dec 20, 2023 at 11:37:01PM -0700, Daniel Xu wrote:
> On Wed, Dec 20, 2023 at 03:19:52PM -0700, Daniel Xu wrote:
> > This commit teaches pahole to parse symbols in .BTF_ids section in
> > vmlinux and discover exported kfuncs. Pahole then takes the list of
> > kfuncs and injects a BTF_KIND_DECL_TAG for each kfunc.
> >
> > This enables downstream users and tools to dynamically discover which
> > kfuncs are available on a system by parsing vmlinux or module BTF, both
> > available in /sys/kernel/btf.
> >
> > Example of encoding:
> >
> >         $ bpftool btf dump file .tmp_vmlinux.btf | rg DECL_TAG | wc -l
> >         388
> >
> >         $ bpftool btf dump file .tmp_vmlinux.btf | rg 68940
> >         [68940] FUNC 'bpf_xdp_get_xfrm_state' type_id=68939 linkage=static
> >         [128124] DECL_TAG 'kfunc' type_id=68940 component_idx=-1
> >
> > Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
> > ---
> >  btf_encoder.c | 202 ++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 202 insertions(+)
> >
> 
> Hmm, looking more, seems like this will pick up non-kfunc functions as
> well. For example, kernel/trace/bpf_trace.c:
> 
> 
>         BTF_SET_START(btf_allowlist_d_path)
>         #ifdef CONFIG_SECURITY
>         BTF_ID(func, security_file_permission)
>         BTF_ID(func, security_inode_getattr)
>         BTF_ID(func, security_file_open)
>         #endif
>         #ifdef CONFIG_SECURITY_PATH
>         BTF_ID(func, security_path_truncate)
>         #endif
>         BTF_ID(func, vfs_truncate)
>         BTF_ID(func, vfs_fallocate)
>         BTF_ID(func, dentry_open)
>         BTF_ID(func, vfs_getattr)
>         BTF_ID(func, filp_close)
>         BTF_SET_END(btf_allowlist_d_path)

you need to pick up only 'BTF_ID(func, ...)' IDs that belongs to SET8 lists,
which are bounded by __BTF_ID__set8__<name> symbols, which also provide size

__BTF_ID__func_* symbol that has address inside the SET8 list is kfunc

> 
> Maybe we need a codemod from:
> 
>         BTF_ID(func, ...
> 
> to:
> 
>         BTF_ID(kfunc, ...

I think it's better to keep just 'func' and not to do anything special for
kfuncs in resolve_btfids logic to keep it simple

also it's going to be already in pahole so if we want to make a fix in future
you need to change pahole, resolve_btfids and possibly also kernel

jirka

> 
> 
> The change to resolve_btfids would be relatively small. Not sure what
> the drawbacks are yet.
> 
> One way or another we'll need to annotate the kfuncs in source.