Re: [PATCH v3 1/2] libbpf: Add setter for initial value for internal maps

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

Andrii Nakryiko <andrii.nakryiko@gmail.com> writes:

> On Sat, Mar 28, 2020 at 11:29 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 function to set a new value for that data, which can be used to
>> before it is loaded into the kernel. This is especially relevant for RODATA
>> maps, since those are frozen on load.
>>
>> Signed-off-by: Toke Høiland-Jørgensen <toke@redhat.com>
>> ---
>> v3:
>>   - Add a setter for the initial value instead of a getter for the pointer to it
>>   - Add selftest
>> v2:
>>   - Add per-map getter for data area instead of a global rodata getter for bpf_obj
>>
>>  tools/lib/bpf/libbpf.c   | 11 +++++++++++
>>  tools/lib/bpf/libbpf.h   |  2 ++
>>  tools/lib/bpf/libbpf.map |  1 +
>>  3 files changed, 14 insertions(+)
>>
>> diff --git a/tools/lib/bpf/libbpf.c b/tools/lib/bpf/libbpf.c
>> index 085e41f9b68e..f9953a8ffcfa 100644
>> --- a/tools/lib/bpf/libbpf.c
>> +++ b/tools/lib/bpf/libbpf.c
>> @@ -6756,6 +6756,17 @@ void *bpf_map__priv(const struct bpf_map *map)
>>         return map ? map->priv : ERR_PTR(-EINVAL);
>>  }
>>
>> +int bpf_map__set_initial_value(struct bpf_map *map,
>> +                              void *data, size_t size)
>
> nit: const void *

ACK.

>> +{
>> +       if (!map->mmaped || map->libbpf_type == LIBBPF_MAP_KCONFIG ||
>> +           size != map->def.value_size)
>> +               return -EINVAL;
>> +
>
> How about also checking that bpf_map wasn't yet created? Checking
> map->fd >= 0 should be enough.


Yup, makes sense.

>> +       memcpy(map->mmaped, data, size);
>> +       return 0;
>> +}
>> +
>>  bool bpf_map__is_offload_neutral(const struct bpf_map *map)
>>  {
>>         return map->def.type == BPF_MAP_TYPE_PERF_EVENT_ARRAY;
>> diff --git a/tools/lib/bpf/libbpf.h b/tools/lib/bpf/libbpf.h
>> index d38d7a629417..ee30ed487221 100644
>> --- a/tools/lib/bpf/libbpf.h
>> +++ b/tools/lib/bpf/libbpf.h
>> @@ -407,6 +407,8 @@ typedef void (*bpf_map_clear_priv_t)(struct bpf_map *, void *);
>>  LIBBPF_API int bpf_map__set_priv(struct bpf_map *map, void *priv,
>>                                  bpf_map_clear_priv_t clear_priv);
>>  LIBBPF_API void *bpf_map__priv(const struct bpf_map *map);
>> +LIBBPF_API int bpf_map__set_initial_value(struct bpf_map *map,
>> +                                         void *data, size_t size);
>>  LIBBPF_API int bpf_map__reuse_fd(struct bpf_map *map, int fd);
>>  LIBBPF_API int bpf_map__resize(struct bpf_map *map, __u32 max_entries);
>>  LIBBPF_API bool bpf_map__is_offload_neutral(const struct bpf_map *map);
>> diff --git a/tools/lib/bpf/libbpf.map b/tools/lib/bpf/libbpf.map
>> index 5129283c0284..f46873b9fe5e 100644
>> --- a/tools/lib/bpf/libbpf.map
>> +++ b/tools/lib/bpf/libbpf.map
>> @@ -243,5 +243,6 @@ LIBBPF_0.0.8 {
>>                 bpf_link__pin;
>>                 bpf_link__pin_path;
>>                 bpf_link__unpin;
>> +               bpf_map__set_initial_value;
>>                 bpf_program__set_attach_target;
>>  } LIBBPF_0.0.7;
>> --
>> 2.26.0
>>