linux-kernel - Re: [PATCH bpf-next v2 3/4] resolve_btfids: introduce enum btf_id

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Message-ID: <ea43e483-b329-4601-a12c-30231c3c17c2@linux.dev>
Date: Wed, 3 Dec 2025 20:35:05 -0800
From: Ihor Solodrai <ihor.solodrai@...ux.dev>
To: Andrii Nakryiko <andrii.nakryiko@...il.com>
Cc: Alexei Starovoitov <ast@...nel.org>,
 Daniel Borkmann <daniel@...earbox.net>, Andrii Nakryiko <andrii@...nel.org>,
 Martin KaFai Lau <martin.lau@...ux.dev>, Eduard Zingerman
 <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 Jiri Olsa <jolsa@...nel.org>, Nathan Chancellor <nathan@...nel.org>,
 Nicolas Schier <nicolas.schier@...ux.dev>,
 Nick Desaulniers <nick.desaulniers+lkml@...il.com>,
 Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>,
 Alan Maguire <alan.maguire@...cle.com>, Donglin Peng
 <dolinux.peng@...il.com>, bpf@...r.kernel.org, dwarves@...r.kernel.org,
 linux-kernel@...r.kernel.org, linux-kbuild@...r.kernel.org
Subject: Re: [PATCH bpf-next v2 3/4] resolve_btfids: introduce enum
 btf_id_kind

On 12/3/25 4:42 PM, Andrii Nakryiko wrote:
> On Tue, Dec 2, 2025 at 11:08 AM Ihor Solodrai <ihor.solodrai@...ux.dev> wrote:
>>
>> On 12/1/25 9:27 AM, Andrii Nakryiko wrote:
>>> On Thu, Nov 27, 2025 at 10:53 AM Ihor Solodrai <ihor.solodrai@...ux.dev> wrote:
>>>>
>>>> Instead of using multiple flags, make struct btf_id tagged with an
>>>> enum value indicating its kind in the context of resolve_btfids.
>>>>
>>>> Signed-off-by: Ihor Solodrai <ihor.solodrai@...ux.dev>
>>>> ---
>>>>  tools/bpf/resolve_btfids/main.c | 62 ++++++++++++++++++++++-----------
>>>>  1 file changed, 42 insertions(+), 20 deletions(-)
>>>
>>> [...]
>>>
>>>>
>>>> -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)
>>>>  {
>>>>         /*
>>>>          * __BTF_ID__set__name
>>>>          * name =    ^
>>>>          * id   =         ^
>>>>          */
>>>> -       char *id = name + (is_set8 ? sizeof(BTF_SET8 "__") : sizeof(BTF_SET "__")) - 1;
>>>> +       int prefixlen = kind == BTF_ID_KIND_SET8 ? sizeof(BTF_SET8 "__") : sizeof(BTF_SET "__");
>>>> +       char *id = name + prefixlen - 1;
>>>>         int len = strlen(name);
>>>> +       struct btf_id *btf_id;
>>>>
>>>>         if (id >= name + len) {
>>>>                 pr_err("FAILED to parse set name: %s\n", name);
>>>>                 return NULL;
>>>>         }
>>>>
>>>> -       return btf_id__add(&obj->sets, id, true);
>>>> +       btf_id = btf_id__add(&obj->sets, id, true);
>>>> +       if (btf_id)
>>>> +               btf_id->kind = kind;
>>>> +
>>>> +       return btf_id;
>>>>  }
>>>>
>>>>  static struct btf_id *add_symbol(struct rb_root *root, char *name, size_t size)
>>>>  {
>>>> +       struct btf_id *btf_id;
>>>>         char *id;
>>>>
>>>>         id = get_id(name + size);
>>>> @@ -288,7 +301,11 @@ static struct btf_id *add_symbol(struct rb_root *root, char *name, size_t size)
>>>>                 return NULL;
>>>>         }
>>>>
>>>> -       return btf_id__add(root, id, false);
>>>> +       btf_id = btf_id__add(root, id, false);
>>>> +       if (btf_id)
>>>> +               btf_id->kind = BTF_ID_KIND_SYM;
>>>
>>> seeing this pattern repeated, wouldn't it make sense to just pass this
>>> kind to btf_id__add() and set it there?
>>
>> I like the idea, because we could get rid the "unique" flag then.
>>
>> But the btf_id__add() does not necessarily create a new struct, and so
>> if we pass the kind in, what do we do with existing objects?
>> Overwrite the kind? If not, do we check for a mismatch?
>>
> 
> no idea, don't know code well enough, but your newly added code seems
> to overwrite the kind always, no?

You're right, I am overwriting here, haven't realized that.

I think I'll go with a mismatch check inside btf_id__add() in v3, that
would indicate a bug.

> 
>>>
>>>> +
>>>> +       return btf_id;
>>>>  }
>>>>
>>>
>>> [...]
>>>
>>>> @@ -643,7 +656,7 @@ static int id_patch(struct object *obj, struct btf_id *id)
>>>>         int i;
>>>>
>>>>         /* For set, set8, id->id may be 0 */
>>>> -       if (!id->id && !id->is_set && !id->is_set8) {
>>>> +       if (!id->id && id->kind == BTF_ID_KIND_SYM) {
>>>
>>> nit: comment says the exception is specifically for SET and SET8, so I
>>> think checking for those two instead of for SYM (implying that only
>>> other possible options are set and set8) would be a bit more
>>> future-proof?
>>
>> ok
>>
>>>
>>>>                 pr_err("WARN: resolve_btfids: unresolved symbol %s\n", id->name);
>>>>                 warnings++;
>>>>         }
>>>
>>> [...]
>>