Subject: Re: [PATCH 15/19] libbpf: Add support to link multi func tracing program

From: Jiri Olsa <[email-masked]>
Date: Mon, 7 Jun 2021 20:28:48 +0200

On Sun, Jun 06, 2021 at 10:49:16PM -0700, Yonghong Song wrote:
> 
> 
> On 6/5/21 4:10 AM, Jiri Olsa wrote:
> > Adding support to link multi func tracing program
> > through link_create interface.
> > 
> > Adding special types for multi func programs:
> > 
> >    fentry.multi
> >    fexit.multi
> > 
> > so you can define multi func programs like:
> > 
> >    SEC("fentry.multi/bpf_fentry_test*")
> >    int BPF_PROG(test1, unsigned long ip, __u64 a, __u64 b, __u64 c, __u64 d, __u64 e, __u64 f)
> > 
> > that defines test1 to be attached to bpf_fentry_test* functions,
> > and able to attach ip and 6 arguments.
> > 
> > If functions are not specified the program needs to be attached
> > manually.
> > 
> > Adding new btf id related fields to bpf_link_create_opts and
> > bpf_link_create to use them.
> > 
> > Signed-off-by: Jiri Olsa <[email-masked]>
> > ---
> >   tools/lib/bpf/bpf.c    | 11 ++++++-
> >   tools/lib/bpf/bpf.h    |  4 ++-
> >   tools/lib/bpf/libbpf.c | 72 ++++++++++++++++++++++++++++++++++++++++++
> >   3 files changed, 85 insertions(+), 2 deletions(-)
> > 
> > diff --git a/tools/lib/bpf/bpf.c b/tools/lib/bpf/bpf.c
> > index 86dcac44f32f..da892737b522 100644
> > --- a/tools/lib/bpf/bpf.c
> > +++ b/tools/lib/bpf/bpf.c
> > @@ -674,7 +674,8 @@ int bpf_link_create(int prog_fd, int target_fd,
> >   		    enum bpf_attach_type attach_type,
> >   		    const struct bpf_link_create_opts *opts)
> >   {
> > -	__u32 target_btf_id, iter_info_len;
> > +	__u32 target_btf_id, iter_info_len, multi_btf_ids_cnt;
> > +	__s32 *multi_btf_ids;
> >   	union bpf_attr attr;
> >   	int fd;
> [...]
> > @@ -9584,6 +9597,9 @@ static int libbpf_find_attach_btf_id(struct bpf_program *prog, int *btf_obj_fd,
> >   	if (!name)
> >   		return -EINVAL;
> > +	if (prog->prog_flags & BPF_F_MULTI_FUNC)
> > +		return 0;
> > +
> >   	for (i = 0; i < ARRAY_SIZE(section_defs); i++) {
> >   		if (!section_defs[i].is_attach_btf)
> >   			continue;
> > @@ -10537,6 +10553,62 @@ static struct bpf_link *bpf_program__attach_btf_id(struct bpf_program *prog)
> >   	return (struct bpf_link *)link;
> >   }
> > +static struct bpf_link *bpf_program__attach_multi(struct bpf_program *prog)
> > +{
> > +	char *pattern = prog->sec_name + prog->sec_def->len;
> > +	DECLARE_LIBBPF_OPTS(bpf_link_create_opts, opts);
> > +	enum bpf_attach_type attach_type;
> > +	int prog_fd, link_fd, cnt, err;
> > +	struct bpf_link *link = NULL;
> > +	__s32 *ids = NULL;
> > +
> > +	prog_fd = bpf_program__fd(prog);
> > +	if (prog_fd < 0) {
> > +		pr_warn("prog '%s': can't attach before loaded\n", prog->name);
> > +		return ERR_PTR(-EINVAL);
> > +	}
> > +
> > +	err = bpf_object__load_vmlinux_btf(prog->obj, true);
> > +	if (err)
> > +		return ERR_PTR(err);
> > +
> > +	cnt = btf__find_by_pattern_kind(prog->obj->btf_vmlinux, pattern,
> > +					BTF_KIND_FUNC, &ids);
> > +	if (cnt <= 0)
> > +		return ERR_PTR(-EINVAL);
> 
> In kernel, looks like we support cnt = 0, here we error out.
> Should we also error out in the kernel if cnt == 0?

hum, I'm not what you mean.. what kernel code are you referring to?

thanks,
jirka