Re: [PATCH bpf-next v3 04/13] bpftool: Normalize legacy arena type tags before C dumps

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

On 10/30/26 5:16 PM, bot+bpf-ci@kernel.org wrote:
>> bpftool: Normalize legacy arena type tags before C dumps
> 
> TL;DR: The legacy arena tag fixup skips split BTF, so module C dumps on older
> kernels may still print "address_space(1)" for vmlinux arena kfuncs and fail
> to compile; commit message doesn't say so.
> 
>> diff --git a/tools/bpf/bpftool/btf.c b/tools/bpf/bpftool/btf.c
>> --- a/tools/bpf/bpftool/btf.c
>> +++ b/tools/bpf/bpftool/btf.c
> 
> [ ... ]
> 
>> @@ -1092,13 +1093,47 @@ static struct sort_datum *sort_btf_c(const struct btf *btf)
>>  	return datums;
>>  }
>>  
>> -static int dump_btf_c(const struct btf *btf,
>> +/*
>> + * Older kernel BTF encodes kflagged arena type tags as "address_space(1)",
>> + * which btf_dump prints as is. Rewrite them to the complete specifier.
>> + */
>> +static int btf_c_fixup_legacy_arena_tags(struct btf *btf)
>> +{
>> +	__u32 i, cnt = btf__type_cnt(btf);
>> +	int off;
>> +
>> +	if (btf__base_btf(btf) ||
>> +	    btf__find_by_name_kind(btf, LEGACY_ARENA_TAG, BTF_KIND_TYPE_TAG) < 0)
>> +		return 0;
> 
> Can this early return for split BTF leave module C dumps broken on older
> kernels?

Yes, it will. That's intentional: the fixup targets only vmlinux BTF, which is
always the base.

I don't know of any users relying on btf C dump for modules, but if
there are any they are advised to re-compile their modules with
updated resolve_btfids to use a new format.

> 
> [...]