Re: [PATCH bpf-next v4 2/3] libbpf: Add support for dynamic program attach target

["Eelco Chaudron" <echaudro@redhat.com>]

On 18 Feb 2020, at 22:24, Andrii Nakryiko wrote:

> On Tue, Feb 18, 2020 at 8:34 AM Jakub Sitnicki <jakub@cloudflare.com> 
> wrote:
>>
>> Hey Eelco,
>>
>> On Mon, Feb 17, 2020 at 12:43 PM GMT, Eelco Chaudron wrote:
>>> Currently when you want to attach a trace program to a bpf program
>>> the section name needs to match the tracepoint/function semantics.
>>>
>>> However the addition of the bpf_program__set_attach_target() API
>>> allows you to specify the tracepoint/function dynamically.
>>>
>>> The call flow would look something like this:
>>>
>>>   xdp_fd = bpf_prog_get_fd_by_id(id);
>>>   trace_obj = bpf_object__open_file("func.o", NULL);
>>>   prog = bpf_object__find_program_by_title(trace_obj,
>>>                                            "fentry/myfunc");
>>>   bpf_program__set_expected_attach_type(prog, BPF_TRACE_FENTRY);
>>>   bpf_program__set_attach_target(prog, xdp_fd,
>>>                                  "xdpfilt_blk_all");
>>>   bpf_object__load(trace_obj)
>>>
>>> Acked-by: Toke Høiland-Jørgensen <toke@redhat.com>
>>> Signed-off-by: Eelco Chaudron <echaudro@redhat.com>
>>> ---
>>>  tools/lib/bpf/libbpf.c   |   34 ++++++++++++++++++++++++++++++----
>>>  tools/lib/bpf/libbpf.h   |    4 ++++
>>>  tools/lib/bpf/libbpf.map |    2 ++
>>>  3 files changed, 36 insertions(+), 4 deletions(-)
>>>
>>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>>> index 514b1a524abb..0c25d78fb5d8 100644
>>> --- a/tools/lib/bpf/libbpf.c
>>> +++ b/tools/lib/bpf/libbpf.c
>>
>> [...]
>>
>>> @@ -8132,6 +8133,31 @@ void bpf_program__bpil_offs_to_addr(struct 
>>> bpf_prog_info_linear *info_linear)
>>>       }
>>>  }
>>>
>>> +int bpf_program__set_attach_target(struct bpf_program *prog,
>>> +                                int attach_prog_fd,
>>> +                                const char *attach_func_name)
>>> +{
>>> +     int btf_id;
>>> +
>>> +     if (!prog || attach_prog_fd < 0 || !attach_func_name)
>>> +             return -EINVAL;
>>> +
>>> +     if (attach_prog_fd)
>>> +             btf_id = libbpf_find_prog_btf_id(attach_func_name,
>>> +                                              attach_prog_fd);
>>> +     else
>>> +             btf_id = __find_vmlinux_btf_id(prog->obj->btf_vmlinux,
>>> +                                            attach_func_name,
>>> +                                            
>>> prog->expected_attach_type);
>>> +
>>> +     if (btf_id <= 0)
>>> +             return btf_id;
>>
>> Looks like we can get 0 as return value on both error and success
>> (below)?  Is that intentional?
>
> Neither libbpf_find_prog_btf_id nor __find_vmlinux_btf_id are going to
> return 0 on failure. But I do agree that if (btf_id < 0) check would
> be better here.

Is see in theory btf__find_by_name_kind() could return 0:

	if (kind == BTF_KIND_UNKN || !strcmp(type_name, "void"))
   		return 0;

But for our case, this will not happen and is invalid, so what about 
just to make sure its future proof?:

   if (btf_id <= 0)
         return btf_id ? btf_id : -ENOENT;


> With that minor nit:
>
> Acked-by: Andrii Nakryiko <andriin@fb.com>
>
>>
>>> +
>>> +     prog->attach_btf_id = btf_id;
>>> +     prog->attach_prog_fd = attach_prog_fd;
>>> +     return 0;
>>> +}
>>> +
>>>  int parse_cpu_mask_str(const char *s, bool **mask, int *mask_sz)
>>>  {
>>>       int err = 0, n, len, start, end = -1;
>>
>> [...]