Re: [PATCH v2 dwarves 2/5] btf_encoder: Add true_signature feature support for "."-suffixed functions

On 26/01/2026 09:52, Matt Bobrowski wrote:
> On Fri, Jan 23, 2026 at 05:26:47PM +0000, Alan Maguire wrote:
>> Currently we collate function information by name and add functions
>> provided there are no inconsistencies across various representations.
>>
>> For true_signature support - where we wish to add the real signature
>> of a function even if it differs from source level - we need to do
>> a few things:
>>
>> 1. For "."-suffixed functions, we need to match from DWARF->ELF;
>>    we can do this via the address associated with the function.
>>    In doing this, we can then be confident that the debug info
>>    for foo.isra.0 is the right info for the function at that
>>    address.
>>
>> 2. When adding saved functions we need to look for such cases
>>    and provided they do not violate other constraints around BTF
>>    representation - unexpected reg usage for function, uncertain
>>    parameter location or ambiguous address - we add them with
>>    their "."-suffixed name.  The latter can be used as a signal
>>    that the function is transformed from the original.
>>
>> Doing this adds 500 functions to BTF.  These are traceable with
>> their "."-suffix names and because we have excluded ambiguous
>> address cases we know exactly which function address they refer
>> to.
>>
>> Signed-off-by: Alan Maguire <[email protected]>
> 
> Some minor nits, but apart from that it looks OK to me.
> 
> Acked-by: Matt Bobrowski <[email protected]>
> 
>> ---
>>  btf_encoder.c | 76 ++++++++++++++++++++++++++++++++++++++++++++++-----
>>  dwarves.h     |  1 +
>>  pahole.c      |  1 +
>>  3 files changed, 71 insertions(+), 7 deletions(-)
>>
>> diff --git a/btf_encoder.c b/btf_encoder.c
>> index 9a567e4..c1002c3 100644
>> --- a/btf_encoder.c
>> +++ b/btf_encoder.c
>> @@ -77,9 +77,16 @@ struct btf_encoder_func_annot {
>>  	int16_t component_idx;
>>  };
>>  
>> +struct elf_function_sym {
>> +	const char *name;
>> +	uint64_t addr;
>> +};
>> +
>>  /* state used to do later encoding of saved functions */
>>  struct btf_encoder_func_state {
>>  	struct elf_function *elf;
>> +	struct elf_function_sym *sym;
>> +	uint64_t addr;
>   		 ^
> 		 This appears to have leaked into wrong commit?
> 		 This member should've been introduced within patch
> 		 5/5.
>

It's used here too though; specifically in btf_encoder__save_func()
below. We need to use addresses to map between a DWARF representation
and the associated ELF function to ensure we're using debug info
for the correct "."-suffixed function. We need to do this because
the late DWARF generated after optimizations are applied uses
the original name ("foo" rather than "foo.isra.0"). 
 		 
>>  	uint32_t type_id_off;
>>  	uint16_t nr_parms;
>>  	uint16_t nr_annots;
>> @@ -94,11 +101,6 @@ struct btf_encoder_func_state {
>>  	struct btf_encoder_func_annot *annots;
>>  };
>>  
>> -struct elf_function_sym {
>> -	const char *name;
>> -	uint64_t addr;
>> -};
>> -
>>  struct elf_function {
>>  	char		*name;
>>  	struct elf_function_sym *syms;
>> @@ -145,7 +147,8 @@ struct btf_encoder {
>>  			  skip_encoding_decl_tag,
>>  			  tag_kfuncs,
>>  			  gen_distilled_base,
>> -			  encode_attributes;
>> +			  encode_attributes,
>> +			  true_signature;
>>  	uint32_t	  array_index_id;
>>  	struct elf_secinfo *secinfo;
>>  	size_t             seccnt;
>> @@ -1270,14 +1273,36 @@ static int32_t btf_encoder__save_func(struct btf_encoder *encoder, struct functi
>>  			goto out;
>>  		}
>>  	}
>> +	if (encoder->true_signature && fn->lexblock.ip.addr) {
>> +		int i;
>> +
>> +		for (i = 0; i < func->sym_cnt; i++) {
>> +			if (fn->lexblock.ip.addr != func->syms[i].addr)
>> +				continue;
>> +			/* Only need to record address for '.'-suffixed
>> +			 * functions, since we only currently need true
>> +			 * signatures for them.
>> +			 */
>> +			if (!strchr(func->syms[i].name, '.'))
>> +				continue;
>> +			state->sym = &func->syms[i];
>> +			break;
>> +		}
>> +	}
>>  	state->inconsistent_proto = ftype->inconsistent_proto;
>>  	state->unexpected_reg = ftype->unexpected_reg;
>>  	state->optimized_parms = ftype->optimized_parms;
>>  	state->uncertain_parm_loc = ftype->uncertain_parm_loc;
>>  	state->reordered_parm = ftype->reordered_parm;
>>  	ftype__for_each_parameter(ftype, param) {
>> -		const char *name = parameter__name(param) ?: "";
>> +		const char *name;
>>  
>> +		/* No location info/optimized + reordered means optimized out. */
>> +		if (ftype->reordered_parm && (!param->has_loc || param->optimized)) {
>> +			state->nr_parms--;
>> +			continue;
>> +		}
>> +		name = parameter__name(param) ?: "";
>>  		str_off = btf__add_str(btf, name);
>>  		if (str_off < 0) {
>>  			err = str_off;
>> @@ -1366,6 +1391,9 @@ static int32_t btf_encoder__add_func(struct btf_encoder *encoder,
>>  
>>  	btf_fnproto_id = btf_encoder__add_func_proto_for_state(encoder, state);
>>  	name = func->name;
>> +	if (encoder->true_signature && state->sym)
>> +		name = state->sym->name;
>> +
>>  	if (btf_fnproto_id >= 0)
>>  		btf_fn_id = btf_encoder__add_ref_type(encoder, BTF_KIND_FUNC, btf_fnproto_id,
>>  						      name, false);
>> @@ -1508,6 +1536,39 @@ static int btf_encoder__add_saved_funcs(struct btf_encoder *encoder, bool skip_e
>>  		while (j < nr_saved_fns && saved_functions_combine(encoder, &saved_fns[i], &saved_fns[j]) == 0)
>>  			j++;
>>  
>> +		/* Add true signatures for case where we have an exact
>> +		 * symbol match by address from DWARF->ELF and have a
>> +		 * "." suffixed name.
>> +		 */
>> +		if (encoder->true_signature) {
>> +			bool true_added = false;
>> +			int k;
>> +
>> +			for (k = i; k < nr_saved_fns; k++) {
>> +				struct btf_encoder_func_state *true_state = &saved_fns[k];
>> +
>> +				if (state->elf != true_state->elf)
>> +					break;
>> +				if (!true_state->sym)
>> +					continue;
>> +				/* Unexpected reg, uncertain parm loc and
>> +				 * ambiguous address mean we cannot trust fentry.
>> +				 */
>> +				if (true_state->unexpected_reg ||
>> +				    true_state->uncertain_parm_loc ||
>> +				    true_state->ambiguous_addr)
>> +					continue;
>> +				err = btf_encoder__add_func(encoder, true_state);
>> +				if (err < 0)
>> +					goto out;
>> +				true_added = true;
>> +				break;
>> +			}
>> +			/* If true symbol was added, skip the below. */
>> +			if (true_added)
>> +				continue;
>> +		}
>> +
> 
> I think this hunk should be factored out into a
> helper. btf_encoder__add_saved_funcs() is starting to get hairy IMO.
> 

Yeah, I think that's reasonable. The combination of loops within loops,
breaks and continues is getting hard to follow. I'll respin with a
btf_encoder__add_true_signature() function replicating this logic.


>>  		/* do not exclude functions with optimized-out parameters; they
>>  		 * may still be _called_ with the right parameter values, they
>>  		 * just do not _use_ them.  Only exclude functions with
>> @@ -2584,6 +2645,7 @@ struct btf_encoder *btf_encoder__new(struct cu *cu, const char *detached_filenam
>>  		encoder->tag_kfuncs	 = conf_load->btf_decl_tag_kfuncs;
>>  		encoder->gen_distilled_base = conf_load->btf_gen_distilled_base;
>>  		encoder->encode_attributes = conf_load->btf_attributes;
>> +		encoder->true_signature = conf_load->true_signature;
>>  		encoder->verbose	 = verbose;
>>  		encoder->has_index_type  = false;
>>  		encoder->need_index_type = false;
>> diff --git a/dwarves.h b/dwarves.h
>> index 78bedf5..d7c6474 100644
>> --- a/dwarves.h
>> +++ b/dwarves.h
>> @@ -101,6 +101,7 @@ struct conf_load {
>>  	bool			btf_decl_tag_kfuncs;
>>  	bool			btf_gen_distilled_base;
>>  	bool			btf_attributes;
>> +	bool			true_signature;
>>  	uint8_t			hashtable_bits;
>>  	uint8_t			max_hashtable_bits;
>>  	uint16_t		kabi_prefix_len;
>> diff --git a/pahole.c b/pahole.c
>> index ef01e58..02a0d19 100644
>> --- a/pahole.c
>> +++ b/pahole.c
>> @@ -1234,6 +1234,7 @@ struct btf_feature {
>>  	BTF_NON_DEFAULT_FEATURE(global_var, encode_btf_global_vars, false),
>>  	BTF_NON_DEFAULT_FEATURE_CHECK(attributes, btf_attributes, false,
>>  				      attributes_check),
>> +	BTF_NON_DEFAULT_FEATURE(true_signature, true_signature, false),
>>  };
>>  
>>  #define BTF_MAX_FEATURE_STR	1024
>> -- 
>> 2.43.5
>>