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

[Andrii Nakryiko]

On Thu, Dec 18, 2025 at 9:47 AM Ihor Solodrai <ihor.solodrai@xxxxxxxxx> wrote:
>
> On 12/17/25 4:47 PM, bot+bpf-ci@xxxxxxxxxx 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

>
> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
> index 3e88dc862d87..ffc15c249ed4 100644
> --- a/tools/bpf/resolve_btfids/main.c
> +++ b/tools/bpf/resolve_btfids/main.c
> @@ -295,10 +295,10 @@ static struct btf_id *add_set(struct object *obj, char *name, enum btf_id_kind k
>          */
>         switch (kind) {
>         case BTF_ID_KIND_SET:
> -               prefixlen = sizeof(BTF_SET "__") - 1;
> +               prefixlen = sizeof(BTF_SET "__");
>                 break;
>         case BTF_ID_KIND_SET8:
> -               prefixlen = sizeof(BTF_SET8 "__") - 1;
> +               prefixlen = sizeof(BTF_SET8 "__");
>                 break;
>         default:
>                 pr_err("Unexpected kind %d passed to %s() for symbol %s\n", kind, __func__, name);
>
> >
> >>      if (id >= name + len) {
> >>              pr_err("FAILED to parse set name: %s\n", name);
> >>              return NULL;
> >>      }
> >>
> >> -    return btf_id__add(&obj->sets, id, true);
> >> +    return btf_id__add_unique(&obj->sets, id, kind);
> >>  }
> >
> >
> > ---
> > AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
> > See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md
> >
> > CI run summary: https://github.com/kernel-patches/bpf/actions/runs/20321956014
>