Re: [PATCH bpf-next v4 3/8] resolve_btfids: Introduce enum btf_id_kind

[Ihor Solodrai <ihor.solodrai@linux.dev>]

On 12/18/25 11:58 AM, Andrii Nakryiko wrote:
> On Thu, Dec 18, 2025 at 9:47 AM Ihor Solodrai <ihor.solodrai@linux.dev> wrote:
>>
>> On 12/17/25 4:47 PM, bot+bpf-ci@kernel.org wrote:
>>>> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
>>>> index b4caae1170dd..da8c7d127632 100644
>>>> --- a/tools/bpf/resolve_btfids/main.c
>>>> +++ b/tools/bpf/resolve_btfids/main.c
>>>
>>> [ ... ]
>>>
>>>> @@ -260,22 +279,36 @@ static char *get_id(const char *prefix_end)
>>>>      return id;
>>>>  }
>>>>
>>>> -static struct btf_id *add_set(struct object *obj, char *name, bool is_set8)
>>>> +static struct btf_id *add_set(struct object *obj, char *name, enum btf_id_kind kind)
>>>>  {
>>>> +    int len = strlen(name);
>>>> +    int prefixlen;
>>>> +    char *id;
>>>> +
>>>>      /*
>>>>       * __BTF_ID__set__name
>>>>       * name =    ^
>>>>       * id   =         ^
>>>>       */
>>>> -    char *id = name + (is_set8 ? sizeof(BTF_SET8 "__") : sizeof(BTF_SET "__")) - 1;
>>>> -    int len = strlen(name);
>>>> +    switch (kind) {
>>>> +    case BTF_ID_KIND_SET:
>>>> +            prefixlen = sizeof(BTF_SET "__") - 1;
>>>> +            break;
>>>> +    case BTF_ID_KIND_SET8:
>>>> +            prefixlen = sizeof(BTF_SET8 "__") - 1;
>>>> +            break;
>>>> +    default:
>>>> +            pr_err("Unexpected kind %d passed to %s() for symbol %s\n", kind, __func__, name);
>>>> +            return NULL;
>>>> +    }
>>>> +
>>>> +    id = name + prefixlen - 1;
>>>
>>> Is this off by one? The original code computed:
>>>
>>>     id = name + sizeof(BTF_SET "__") - 1
>>>
>>> For BTF_SET ("set"), this gives: name + sizeof("set__") - 1 = name + 6 - 1 = name + 5
>>>
>>> The new code computes:
>>>
>>>     prefixlen = sizeof(BTF_SET "__") - 1 = 6 - 1 = 5
>>>     id = name + prefixlen - 1 = name + 5 - 1 = name + 4
>>>
>>> The -1 appears to be applied twice now, once in the prefixlen assignment and
>>> again when computing id. Should this line be:
>>>
>>>     id = name + prefixlen;
>>
>> Yes, this is an off-by-one bug.
>>
>> Good catch, Opus 4.5. Sonnet 4.5 missed this in v3.
>>
>> This was "harmless", since the names stored in the trees don't affect
>> the output.  The comparison between the names still works, as they all
>> simply got a "_" prefix.  But this only makes the bug sneaky, which is
>> very bad.
>>
>> The suggested fix is meh though, IMO a better one is:
> 
> the bug is not in determining prefix length, but in using it to find
> where id starts in the string, so the fix should be
> 
> id = name + prefixlen;
> 
> prefixlen is calculated correctly, IMO

Aaahh, because the null byte is counted by sizeof, right... I missed that.

> 
>>
>> [...]
>>