On Wednesday, January 22nd, 2025 at 11:47 AM, Jose E. Marchesi <jose.marchesi@xxxxxxxxxx> wrote:
> [...]
> >
> > Using the contents of the tag to indicate it's meaning (such as
> > "cattrubite:always_inline") will work too. However I don't think it's
> > desirable to have to parse the tag strings within libbpf, even more so
> > in BPF verifier.
>
>
> I expect the verifier will in any case have to distinguish the different
> strings it gets in the tags, for other purposes, right, so this wouldn't
> be introducing anything different?
I only see direct string comparisons in the BTF verification, for example:
if (btf_type_is_type_tag(t)) {
tag_value = __btf_name_by_offset(btf, t->name_off);
if (strcmp(tag_value, "user") == 0)
info->reg_type |= MEM_USER;
if (strcmp(tag_value, "percpu") == 0)
info->reg_type |= MEM_PERCPU;
}
What's different is that this way a syntax is introduced, even if very
simple like "prefix:suffix". And so it potentially has to be parsed by
the tag reader, be it btf_dump or anything else. Testing a kflag is
just a much simpler operation. Maybe if we had N kinds of tags, and
not just two this would make sense?
Also. would this way of encoding be a part of the BTF spec then?
It can be done in principle, I just don't know if it's a good idea.
>
> Also FWIW DWARF doesn't have a kind_flag.
Right, but BTF was designed with different goals, and one of them is to
be compact. kind_flag so far just hasn't been used by the tags, but it
is used in other BTF types.
>
> [...]
