Re: [PATCH dwarves 2/9] dwarf_loader: Handle signatures with dead arguments

[Yonghong Song <yonghong.song@linux.dev>]

On 3/19/26 11:55 AM, Alan Maguire wrote:
> On 05/03/2026 22:55, Yonghong Song wrote:
>> For llvm dwarf, the dead argument may be in the middle of
>> DW_TAG_subprogram. So we introduce skip_idx in order to
>> match expected registers properly.
>>
>> For example:
>>    0x00042897:   DW_TAG_subprogram
>>                    DW_AT_name      ("create_dev")
>>                    DW_AT_calling_convention        (DW_CC_nocall)
>>                    DW_AT_type      (0x0002429a "int")
>>                    ...
>>
>>    0x000428ab:     DW_TAG_formal_parameter
>>                      DW_AT_name    ("name")
>>                      DW_AT_type    (0x000242ed "char *")
>>                      ...
>>
>>    0x000428b5:     DW_TAG_formal_parameter
>>                      DW_AT_location        (indexed (0x3f) loclist = 0x000027f8:
>>                         [0xffffffff87681370, 0xffffffff8768137a): DW_OP_reg5 RDI
>>                         [0xffffffff8768137a, 0xffffffff87681392): DW_OP_reg3 RBX
>>                         [0xffffffff87681392, 0xffffffff876813ae): DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value)
>>                      DW_AT_name    ("dev")
>>                      DW_AT_type    (0x00026859 "dev_t")
>>                      ...
>>
>> With skip_idx, we can identify that the second original argument
>> 'dev' becomes the first one after optimization.
>>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>>   dwarf_loader.c | 27 +++++++++++++++++++++++----
>>   1 file changed, 23 insertions(+), 4 deletions(-)
>>
>> diff --git a/dwarf_loader.c b/dwarf_loader.c
>> index 610b69e..1ced5e2 100644
>> --- a/dwarf_loader.c
>> +++ b/dwarf_loader.c
>> @@ -1192,6 +1192,7 @@ static ptrdiff_t __dwarf_getlocations(Dwarf_Attribute *attr,
>>   
>>   struct func_info {
>>   	bool signature_changed;
>> +	int skip_idx;
>>   };
>>   
>>   /* For DW_AT_location 'attr':
>> @@ -1264,13 +1265,28 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
>>   	if (parm != NULL) {
>>   		bool has_const_value;
>>   		Dwarf_Attribute attr;
>> +		int reg_idx;
>>   
>>   		tag__init(&parm->tag, cu, die);
>>   		parm->name = attr_string(die, DW_AT_name, conf);
>>   		parm->idx = param_idx;
>> -		if (param_idx >= cu->nr_register_params || param_idx < 0)
>> +		if (param_idx < 0)
>>   			return parm;
>> -		if (cu->producer_clang && !info->signature_changed)
>> +		if (!cu->producer_clang && param_idx >= cu->nr_register_params)
>> +			return parm;
>> +		if (cu->producer_clang) {
>> +			if (!info->signature_changed)
>> +				return parm;
>> +			/* if true_signature is not enabled, mark parameter as
>> +			 * unexpected_reg since there is a skipped parameter before.
>> +			 */
>> +			if (!conf->true_signature && info->skip_idx) {
>> +				parm->unexpected_reg = 1;
>> +				return parm;
>> +			}
>> +		}
>> +		reg_idx = param_idx - info->skip_idx;
>> +		if (reg_idx >= cu->nr_register_params)
>>   			return parm;
>>   		/* Parameters which use DW_AT_abstract_origin to point at
>>   		 * the original parameter definition (with no name in the DIE)
>> @@ -1309,7 +1325,7 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
>>   		parm->has_loc = dwarf_attr(die, DW_AT_location, &attr) != NULL;
>>   
>>   		if (parm->has_loc) {
>> -			int expected_reg = cu->register_params[param_idx];
>> +			int expected_reg = cu->register_params[reg_idx];
>>   			int actual_reg = parameter__reg(&attr, expected_reg);
>>   
>>   			if (actual_reg < 0)
>> @@ -1322,8 +1338,11 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
>>   				 * contents.
>>   				 */
>>   				parm->unexpected_reg = 1;
>> -		} else if (has_const_value) {
>> +		} else if (!cu->producer_clang && has_const_value) {
>> +			parm->optimized = 1;
>> +		} else if (cu->producer_clang) {
>>   			parm->optimized = 1;
>> +			info->skip_idx++;
>>   		}
>>   	}
>>   
> In [1]  (dwarf_loader/btf_encoder: Detect reordered parameters)
>
> we detect reordered/missing parameters for gcc by comparing abstract origin to concrete representation
> and mark any such functions by setting the reordered_parm bitfield in the encoder func state. In this
> approach reordered encompasses both missing and actual reordering; in practice it's always the former, but
> the DWARF representation was confusing because it had the actually-used parameters (with location info)
> followed by the unused ones (without location info). Before the above commit we were treating these
> function signatures incorrectly as valid leading to wrong functions signatures.
>
> All of this is to say: should we mark with reordered_parm instead? The reason I suggest this is that
> btf_encoder__save_func() has handling for skipping functions without locations with reordered_parm set;
> maybe we could find a way to unify that across clang/gcc cases?

For v2, I focused on clang side. I have not studied such unification yet. I think it is possible
since ultimately both clang and gcc tries to skip some parameters.

>
> Now in the clang case, I guess reordered is a poor description; for gcc it really means "reordered as
> compared to abstract origin". If we could find a better way to describe/encompass both cases that would
> be ideal I think. The terminology used in the gcc true signature code isn't great today.

Agree that we want *common* names to represent both gcc and clang for skipped parameters.
BTW, please review v2 instead:
    https://lore.kernel.org/bpf/20260309153215.1917033-1-yonghong.song@linux.dev/

Thanks!

>
> [1] https://git.kernel.org/pub/scm/devel/pahole/pahole.git/commit/?id=109e5e4554f663e5f12fb634bc54cceb710aec61