Re: [PATCH bpf-next] libbpf: add libbpf_load_vmlinux_btf/libbpf_load_module_btf APIs

[Hengqi Chen <hengqi.chen@gmail.com>]

On 7/27/21 6:49 AM, Andrii Nakryiko wrote:
> On Fri, Jul 23, 2021 at 10:13 PM Hengqi Chen <hengqi.chen@gmail.com> wrote:
>>
>> Add libbpf_load_vmlinux_btf/libbpf_load_module_btf APIs.
>> This is part of the libbpf v1.0. [1]
>>
>> [1] https://github.com/libbpf/libbpf/issues/280
> 
> Saying it's part of libbpf 1.0 effort and given a link to Github PR is
> not really a sufficient commit message. Please expand on what you are
> doing in the patch and why.
> 

Will do.

>>
>> Signed-off-by: Hengqi Chen <hengqi.chen@gmail.com>
>> ---
>>  tools/lib/bpf/btf.c      | 24 +++++++++++++++++++++++-
>>  tools/lib/bpf/btf.h      |  2 ++
>>  tools/lib/bpf/libbpf.c   |  8 ++++----
>>  tools/lib/bpf/libbpf.map |  2 ++
>>  4 files changed, 31 insertions(+), 5 deletions(-)
>>
>> diff --git a/tools/lib/bpf/btf.c b/tools/lib/bpf/btf.c
>> index b46760b93bb4..414e1c5635ef 100644
>> --- a/tools/lib/bpf/btf.c
>> +++ b/tools/lib/bpf/btf.c
>> @@ -4021,7 +4021,7 @@ static void btf_dedup_merge_hypot_map(struct btf_dedup *d)
>>                  */
>>                 if (d->hypot_adjust_canon)
>>                         continue;
>> -
>> +
>>                 if (t_kind == BTF_KIND_FWD && c_kind != BTF_KIND_FWD)
>>                         d->map[t_id] = c_id;
>>
>> @@ -4395,6 +4395,11 @@ static int btf_dedup_remap_types(struct btf_dedup *d)
>>   * data out of it to use for target BTF.
>>   */
>>  struct btf *libbpf_find_kernel_btf(void)
>> +{
>> +       return libbpf_load_vmlinux_btf();
>> +}
>> +
>> +struct btf *libbpf_load_vmlinux_btf(void)
>>  {
>>         struct {
>>                 const char *path_fmt;
>> @@ -4440,6 +4445,23 @@ struct btf *libbpf_find_kernel_btf(void)
>>         return libbpf_err_ptr(-ESRCH);
>>  }
>>
>> +struct btf *libbpf_load_module_btf(const char *mod)
> 
> So we probably need to allow user to pre-load and re-use vmlinux BTF
> for efficiency, especially if they have some use-case to load a lot of
> BTFs.
> 

Should the API change to this ?

struct btf *libbpf_load_module_btf(struct btf *base, const char *mod)

It seems better for the use-case you mentioned.

>> +{
>> +       char path[80];
>> +       struct btf *base;
>> +       int err;
>> +
>> +       base = libbpf_load_vmlinux_btf();
>> +       err = libbpf_get_error(base);
>> +       if (err) {
>> +               pr_warn("Error loading vmlinux BTF: %d\n", err);
>> +               return base;
> 
> libbpf_err_ptr() needs to be used here, pr_warn() could have destroyed
> errno already
> 

OK.

>> +       }
>> +
>> +       snprintf(path, sizeof(path), "/sys/kernel/btf/%s", mod);
>> +       return btf__parse_split(path, base);
> 
> so who's freeing base BTF in this case?
> 

Sorry, missed that.
But if we change the signature, then leave this to user.

>> +}
>> +
>>  int btf_type_visit_type_ids(struct btf_type *t, type_id_visit_fn visit, void *ctx)
>>  {
>>         int i, n, err;
> 
> [...]
>