Re: [PATCH bpf-next v2] libbpf: Add getter for pointer to data area for internal maps

[Alexei Starovoitov <ast@fb.com>]

On 3/27/20 3:26 PM, Toke Høiland-Jørgensen wrote:
> Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:
> 
>> On Fri, Mar 27, 2020 at 5:58 AM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>>
>>> For internal maps (most notably the maps backing global variables), libbpf
>>> uses an internal mmaped area to store the data after opening the object.
>>> This data is subsequently copied into the kernel map when the object is
>>> loaded.
>>>
>>> This adds a getter for the pointer to that internal data store. This can be
>>> used to modify the data before it is loaded into the kernel, which is
>>> especially relevant for RODATA, which is frozen on load. This same pointer
>>> is already exposed to the auto-generated skeletons, so access to it is
>>> already API; this just adds a way to get at it without pulling in the full
>>> skeleton infrastructure.
>>>
>>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>>> ---
>>> v2:
>>>    - Add per-map getter for data area instead of a global rodata getter for bpf_obj
>>>
>>> tools/lib/bpf/libbpf.c   | 9 +++++++++
>>>   tools/lib/bpf/libbpf.h   | 1 +
>>>   tools/lib/bpf/libbpf.map | 1 +
>>>   3 files changed, 11 insertions(+)
>>>
>>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>>> index 085e41f9b68e..a0055f8908fd 100644
>>> --- a/tools/lib/bpf/libbpf.c
>>> +++ b/tools/lib/bpf/libbpf.c
>>> @@ -6756,6 +6756,15 @@ void *bpf_map__priv(const struct bpf_map *map)
>>>          return map ? map->priv : ERR_PTR(-EINVAL);
>>>   }
>>>
>>> +void *bpf_map__data_area(const struct bpf_map *map, size_t *size)
>>
>> I'm not entirely thrilled about "data_area" name. This is entirely for
>> providing initial value for maps, so maybe something like
>> bpf_map__init_value() or something along those lines?
>>
>> Actually, how about a different API altogether:
>>
>> bpf_map__set_init_value(struct bpf_map *map, void *data, size_t size)?
>>
>> Application will have to prepare data of correct size, which will be
>> copied to libbpf's internal storage. It also doesn't expose any of
>> internal pointer. I don't think extra memcopy is a big deal here.
>> Thoughts?
> 
> Huh, yeah, that's way better. Why didn't I think of that? Think maybe I
> was too focused on doing this the same way the skeleton code is. I'll
> send a v3 :)

Could you please add a selftest as well?
I'm not excited about new features without tests.