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

["Toke Høiland-Jørgensen" <toke@redhat.com>]

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 :)

-Toke