Re: [PATCH bpf-next v1 05/14] resolve_btfids: Index BTF ID symbols by address

On 6/3/26 4:45 PM, Andrii Nakryiko wrote:
> On Mon, Jun 1, 2026 at 3:19 PM Ihor Solodrai <[email protected]> wrote:
>>
>> Keep an address-sorted index of parsed .BTF_ids symbols so code that
>> the original BTF_ID symbol name can be recovered from an entry
>> address.
>>
>> Use the index in find_kfunc_flags() to scan BTF_SET8_KFUNCS entries
>> directly and match each entry back to the requested kfunc.
>>
>> Signed-off-by: Ihor Solodrai <[email protected]>
>> ---
>>  tools/bpf/resolve_btfids/main.c | 103 +++++++++++++++++++++++++-------
>>  1 file changed, 80 insertions(+), 23 deletions(-)
>>
>> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
>> index f8a91fa7584f..43512af13148 100644
>> --- a/tools/bpf/resolve_btfids/main.c
>> +++ b/tools/bpf/resolve_btfids/main.c
>> @@ -119,6 +119,11 @@ struct btf_id {
>>         Elf64_Addr       addr[ADDR_CNT];
>>  };
>>
>> +struct addr_sym {
>> +       Elf64_Addr       addr;
>> +       const char      *name;
>> +};
>> +
>>  struct object {
>>         const char *path;
>>         const char *btf_path;
>> @@ -150,6 +155,10 @@ struct object {
>>         int nr_structs;
>>         int nr_unions;
>>         int nr_typedefs;
>> +
>> +       struct addr_sym *addr_syms;
>> +       int nr_addr_syms;
>> +       int max_addr_syms;
> 
> nit: max seems misnamed, it's "capacity", so I'd choose
> "addr_syms_cnt" and "addr_syms_cap" naming (I believe libbpf does that
> relatively consistently)
> 
>>  };
>>
>>  #define KF_IMPLICIT_ARGS (1 << 16)
> 
> [...]
> 
>>         for (next = rb_first(&obj->sets); next; next = rb_next(next)) {
>>                 set_id = rb_entry(next, struct btf_id, rb_node);
>>                 if (set_id->kind != BTF_ID_KIND_SET8 || set_id->addr_cnt != 1)
>>                         continue;
>>
>> -               set_lower_addr = set_id->addr[0];
>> -               set_upper_addr = set_lower_addr + set_id->cnt * sizeof(u64);
>> +               set_addr = set_id->addr[0];
>> +               idx = (set_addr - obj->efile.idlist_addr) / sizeof(u32) + 1;
> 
> where is this +1 coming from? we have some reserved zero entry in
> .BTF_ids section? I'd understand if this was symbols table, where we
> do have zero entry, but I'm not quite following here...

We do a +1 in find_kfunc_flags() three times for slightly different reasons:

  // Here we extract the *set* flags from the header
  idx = (set_addr - obj->efile.idlist_addr) / sizeof(u32) + 1;
  set_flags = elf_data_ptr[idx];

  [...]

  // here we skip the btf_id_set header
  Elf64_Addr addr = set_addr + sizeof(u64) * (i + 1);

  [...]

  // and here we extract the flags from a pair
  idx = (addr - obj->efile.idlist_addr) / sizeof(u32) + 1;
  return elf_data_ptr[idx];

I think the way to make it less confusing is to cast the data pointer
to struct btf_id_set8 before inspecting it, and read the fields.

I'll do that in v2.


> 
> 
> 
>> +               set_flags = elf_data_ptr[idx];
>> +               if (!(set_flags & BTF_SET8_KFUNCS))
>> +                       continue;
>>
>> -               for (u32 i = 0; i < kfunc_id->addr_cnt; i++) {
>> -                       addr = kfunc_id->addr[i];
>> -                       /*
>> -                        * Lower bound is exclusive to skip the 8-byte header of the set.
>> -                        * Upper bound is inclusive to capture the last entry at offset 8*cnt.
>> -                        */
>> -                       if (set_lower_addr < addr && addr <= set_upper_addr) {
>> -                               pr_debug("found kfunc %s in BTF_ID_FLAGS %s\n",
>> -                                        kfunc_id->name, set_id->name);
>> -                               idx = addr - obj->efile.idlist_addr;
>> -                               idx = idx / sizeof(u32) + 1;
>> -                               flags = elf_data_ptr[idx];
>> -
>> -                               return flags;
>> -                       }
>> +               for (u32 i = 0; i < set_id->cnt; i++) {
>> +                       Elf64_Addr addr = set_addr + sizeof(u64) * (i + 1);
>> +                       const char *name = find_name_by_addr(obj, addr);
>> +
>> +                       if (!name || strcmp(name, kfunc_id->name) != 0)
>> +                               continue;
>> +
>> +                       pr_debug("found kfunc %s in BTF_ID_FLAGS %s\n",
>> +                                kfunc_id->name, set_id->name);
>> +
>> +                       idx = (addr - obj->efile.idlist_addr) / sizeof(u32) + 1;
>> +                       return elf_data_ptr[idx];
>>                 }
>>         }
>>
>> @@ -1575,6 +1631,7 @@ int main(int argc, const char **argv)
>>         btf_id__free_all(&obj.typedefs);
>>         btf_id__free_all(&obj.funcs);
>>         btf_id__free_all(&obj.sets);
>> +       free(obj.addr_syms);
>>         if (obj.efile.elf) {
>>                 elf_end(obj.efile.elf);
>>                 close(obj.efile.fd);
>> --
>> 2.54.0
>>