Re: [PATCH bpf-next v1 11/14] resolve_btfids: Process KF_ARENA_* flags in resolve_btfids

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

On 6/16/26 1:36 PM, Emil Tsalapatis wrote:
> On Tue Jun 16, 2026 at 3:51 PM EDT, Emil Tsalapatis wrote:
>> On Mon Jun 1, 2026 at 6:18 PM EDT, Ihor Solodrai wrote:
>>> For kfuncs flagged KF_ARENA_RET, KF_ARENA_ARG1 or KF_ARENA_ARG2,
>>> address_space(1) attribute (type tag with kflag=1) needs to be emitted
>>> to BTF for the return type or arg type respectively.
>>>
>>> So far this has been done by pahole [1].
>>>
>>> Implement the emission of the arena attributes in resolve_btfids: for
>>> flagged kfuncs create a new function prototype with updated BTF types,
>>> adding tags as necessary.
>>>
>>> Similar to decl tags emission, the transformation is idempotent: if
>>> the types are already tagged with address_space(1), it's a noop.
>>>
>>> [1] https://lore.kernel.org/dwarves/20250228194654.1022535-1-ihor.solodrai@linux.dev/
>>>
>>> Signed-off-by: Ihor Solodrai <ihor.solodrai@linux.dev>
>>> ---
>>>  tools/bpf/resolve_btfids/main.c | 131 ++++++++++++++++++++++++++++++++
>>>  1 file changed, 131 insertions(+)
>>>
>>> diff --git a/tools/bpf/resolve_btfids/main.c b/tools/bpf/resolve_btfids/main.c
>>> index f276200b1a68..aafbcfec755b 100644
>>> --- a/tools/bpf/resolve_btfids/main.c
>>> +++ b/tools/bpf/resolve_btfids/main.c
>>> @@ -162,6 +162,9 @@ struct object {
>>>  };
>>>  
>>>  #define KF_FASTCALL (1 << 12)
>>> +#define KF_ARENA_RET (1 << 13)
>>> +#define KF_ARENA_ARG1 (1 << 14)
>>> +#define KF_ARENA_ARG2 (1 << 15)
>>>  #define KF_IMPLICIT_ARGS (1 << 16)
>>>  #define KF_IMPL_SUFFIX "_impl"
>>>  
>>> @@ -1294,6 +1297,128 @@ static int ensure_decl_tag(struct btf2btf_context *ctx, const char *tag_name,
>>>  	return push_decl_tag_id(ctx, new_id);
>>>  }
>>>  
>>> +static bool is_arena_type_attr(struct btf *btf, u32 id)
>>> +{
>>> +	const struct btf_type *t = btf__type_by_id(btf, id);
>>> +	const char *name;
>>> +
>>> +	if (!t || !btf_is_type_tag(t) || !btf_kflag(t))
>>> +		return false;
>>> +	name = btf__name_by_offset(btf, t->name_off);
>>> +	return name && strcmp(name, "address_space(1)") == 0;
>>
>> The type is just called "arena" - address_space(1) is a different
>> attribute that is consumed by the compiler and is not visible in the BTF.
> 
> Nevermind, this is PEBKAC - please disregard. This is _completely_ different
> scenario, none of the above applies. Same for the documentation patch.

Yeah, I was about to write a few paragraphs explaining
"address_space(1)" thing in BTF.

FWIW you can learn more about generic attributes in BTF from a series
that was landed a while ago:
https://lore.kernel.org/bpf/20250130201239.1429648-1-ihor.solodrai@linux.dev/

TL;DR It is possible to represent an arbitrary __attribute__ in BTF by
setting the kind_flag on a type tag or decl tag. And this is what we
use to pass through the KF_ARENA_* flags.

> 
>>
>> While we're at it, can we factor the name out into a #define?
>>
>>> +}
>>> +
>>> +static s32 ensure_arena_tagged_ptr(struct btf *btf, u32 ptr_id)
>>> +{
>>> +	const struct btf_type *ptr = btf__type_by_id(btf, ptr_id);
>>> +	s32 tag_id;
>>> +
>>> +	if (!ptr || !btf_is_ptr(ptr))
>>> +		return -EINVAL;
>>> +
>>> +	if (is_arena_type_attr(btf, ptr->type))
>>> +		return ptr_id;
>>
>> Another thing: Clang (very very) recently got support for adding
>> btf_type_tag for typedef'ed types. Do we nned to expand the check
>> to run in a loop like so:
>>
>> while (btf_is_modifier()) {
>> 	if type_tag()
>> 		test();
>>
>> 	follow the chain
>> }
> 
> Ditto, completely different scenarios.
> 
>>
>> to find tags hidden within typedefs?
>>
>>> +
>>> +	tag_id = btf__add_type_attr(btf, "address_space(1)", ptr->type);
>>> +	if (tag_id < 0)
>>> +		return tag_id;
>>
>> Same here wrt name.
>>
>>> +
>>> +	return btf__add_ptr(btf, tag_id);
>>> +}
>>> +
>>> +/*
>>> + * Build a FUNC_PROTO for @kfunc with each arena-flagged return/parameter
>>> + * pointer tagged with address_space(1). Pointers already tagged are kept as is.
>>> + *
>>> + * If nothing needs tagging, the original proto id is returned unchanged.
>>> + * Otherwise a new FUNC_PROTO is created and its id returned. The original
>>> + * proto may be shared with sibling FUNCs, so it must not be modified in place.
>>> + */
>>> +static s32 ensure_arena_tagged_proto(struct btf *btf, struct kfunc *kfunc)
>>> +{
>>> +	const struct btf_type *func = btf__type_by_id(btf, kfunc->btf_id);
>>> +	u32 proto_id = func->type;
>>> +	const struct btf_type *proto = btf__type_by_id(btf, proto_id);
>>> +	const struct btf_param *params = btf_params(proto);
>>> +	u32 nr_params = btf_vlen(proto);
>>> +	s32 arg0_type_id = nr_params > 0 ? (s32)params[0].type : -1;
>>> +	s32 arg1_type_id = nr_params > 1 ? (s32)params[1].type : -1;
>>> +	s32 ret_type_id = proto->type;
>>> +	s32 new_proto_id, id;
>>> +	bool changed = false;
>>> +	int err;
>>> +
>>> +	if (kfunc->flags & KF_ARENA_RET) {
>>> +		id = ensure_arena_tagged_ptr(btf, ret_type_id);
>>> +		if (id < 0)
>>> +			return id;
>>> +		changed |= id != ret_type_id;
>>> +		ret_type_id = id;
>>> +	}
>>> +
>>> +	if (nr_params > 0 && (kfunc->flags & KF_ARENA_ARG1)) {
>>> +		id = ensure_arena_tagged_ptr(btf, arg0_type_id);
>>> +		if (id < 0)
>>> +			return id;
>>> +		changed |= id != arg0_type_id;
>>> +		arg0_type_id = id;
>>> +	}
>>> +
>>> +	if (nr_params > 1 && (kfunc->flags & KF_ARENA_ARG2)) {
>>> +		id = ensure_arena_tagged_ptr(btf, arg1_type_id);
>>> +		if (id < 0)
>>> +			return id;
>>> +		changed |= id != arg1_type_id;
>>> +		arg1_type_id = id;
>>> +	}
>>> +
>>> +	if (!changed)
>>> +		return proto_id;
>>> +
>>> +	new_proto_id = btf__add_func_proto(btf, ret_type_id);
>>> +	if (new_proto_id < 0)
>>> +		return new_proto_id;
>>> +
>>> +	for (int i = 0; i < nr_params; i++) {
>>> +		s32 param_type_id;
>>> +		const char *name;
>>> +
>>> +		proto = btf__type_by_id(btf, proto_id);
>>> +		params = btf_params(proto);
>>> +		name = btf__name_by_offset(btf, params[i].name_off);
>>> +
>>> +		if (i == 0)
>> Nit: switch() {}?
>>> +			param_type_id = arg0_type_id;
>>> +		else if (i == 1)
>>> +			param_type_id = arg1_type_id;
>>> +		else
>>> +			param_type_id = params[i].type;
>>> +
>>> +		err = btf__add_func_param(btf, name ?: "", param_type_id);
>>> +		if (err < 0)
>>> +			return err;
>>> +	}
>>> +
>>> +	pr_debug("resolve_btfids: added arena-tagged proto for kfunc %s: %d\n", kfunc->name, new_proto_id);
>>> +
>>> +	return new_proto_id;
>>> +}
>>> +
>>> +static int process_kfunc_with_arena_flags(struct btf2btf_context *ctx, struct kfunc *kfunc)
>>> +{
>>> +	struct btf_type *t;
>>> +	s32 proto_id;
>>> +
>>> +	proto_id = ensure_arena_tagged_proto(ctx->btf, kfunc);
>>> +	if (proto_id < 0)
>>> +		return proto_id;
>>> +
>>> +	t = (struct btf_type *)btf__type_by_id(ctx->btf, kfunc->btf_id);
>>> +	t->type = proto_id;
>>> +
>>> +	return 0;
>>> +}
>>> +
>>>  static int btf2btf(struct object *obj)
>>>  {
>>>  	struct btf2btf_context ctx = {};
>>> @@ -1321,6 +1446,12 @@ static int btf2btf(struct object *obj)
>>>  			if (err)
>>>  				goto out;
>>>  		}
>>> +
>>> +		if (kfunc->flags & (KF_ARENA_RET | KF_ARENA_ARG1 | KF_ARENA_ARG2)) {
>>> +			err = process_kfunc_with_arena_flags(&ctx, kfunc);
>>> +			if (err)
>>> +				goto out;
>>> +		}
>>>  	}
>>>  
>>>  	err = 0;
>