On Thu, Dec 21, 2023 at 09:35:28AM +0100, Jiri Olsa wrote:
> 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@xxxxxxxxx>
> > > ---
> > > 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
I managed to add that logic. But I did some spot checks and it looks
like SET8 lists are not quite limited to kfuncs. For example, in
net/mptcp/bpf.c:
BTF_SET8_START(bpf_mptcp_fmodret_ids)
BTF_ID_FLAGS(func, update_socket_protocol)
BTF_SET8_END(bpf_mptcp_fmodret_ids)
static const struct btf_kfunc_id_set bpf_mptcp_fmodret_set = {
.owner = THIS_MODULE,
.set = &bpf_mptcp_fmodret_ids,
};
And in net/socket.c:
__bpf_hook_start();
__weak noinline int update_socket_protocol(int family, int type, int protocol)
{
return protocol;
}
__bpf_hook_end();
IOW, update_socket_protocol() is a hook, not a 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
So maybe special annotation is still needed. WDYT?
[..]
Thanks,
Daniel
