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

[Yonghong Song <yhs@fb.com>]

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 <jolsa@kernel.org>
> ---
>   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?

> +
> +	link = calloc(1, sizeof(*link));
> +	if (!link) {
> +		err = -ENOMEM;
> +		goto out_err;
> +	}
> +	link->detach = &bpf_link__detach_fd;
> +
> +	opts.multi_btf_ids = ids;
> +	opts.multi_btf_ids_cnt = cnt;
> +
> +	attach_type = bpf_program__get_expected_attach_type(prog);
> +	link_fd = bpf_link_create(prog_fd, 0, attach_type, &opts);
> +	if (link_fd < 0) {
> +		err = -errno;
> +		goto out_err;
> +	}
> +	link->fd = link_fd;
> +	free(ids);
> +	return link;
> +
> +out_err:
> +	free(link);
> +	free(ids);
> +	return ERR_PTR(err);
> +}
> +
[...]