Re: [PATCH dwarves v3 2/9] dwarf_loader: Handle signatures with dead arguments

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

On 3/21/26 4:10 PM, Jiri Olsa wrote:
> On Fri, Mar 20, 2026 at 12:09:27PM -0700, 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.
> hi,
> this change removes for me functions below and some of them (ata_dev_select,
> dmi_save_type, perhaps more) seem to be removed because their last 2 arguments
> are constants IIUC, which does not seem to be what the changelog suggests,
> but I guess it's still correct (it's without +true_signature)

For this patch, without +true_signature, it can only get true signatures
when the dead arguments at the end of argument list. With +true_signature,
it allows dead arguments before non-dead arguments.

>
> jirka
>
>
> ata_dev_select
> __cgroup1_procs_write
> check_buffer_access
> dax_disassociate_entry
> devlink_nl_sb_port_pool_fill
> dmi_format_ids
> dmi_save_type
> dmi_save_uuid
> get_buffer
> get_net_track
> gue_remcsum
> icmpv4_xrlim_allow
> insert_vmap_area_augment
> __intel_pmu_enable_all
> ip6addrlbl_fill
> ip6_dst_store
> kernelmode_fixup_or_oops
> merge_note_headers_elf32
> merge_note_headers_elf64
> mt_destroy_walk
> __mutex_lock
> __netdev_walk_all_lower_dev
> pci_create_device_link
> populate_kprobe_blacklist
> proc_get_long
> rt_mutex_adjust_prio_chain
> rt_set_nexthop
> run_lwt_bpf
> simple_attr_write_xsigned
> __skb_gro_checksum_validate_needed
> tclass_notify
> tg_set_conf
> toggle_bp_slot
> trace_event_get_offsets_aer_event
> trace_event_get_offsets_br_fdb_external_learn_add
> trace_event_get_offsets_br_fdb_update
> trace_event_get_offsets_cache_tag_flush
> trace_event_get_offsets_devlink_health_recover_aborted
> trace_event_get_offsets_iommu_error
> trace_event_get_offsets_prq_report
> virtio_net_hdr_from_skb
> writeback_inodes_wb
> xfs_file_dio_write_aligned
>
>
>> Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
>> ---
>>   dwarf_loader.c | 19 +++++++++++++++++--
>>   1 file changed, 17 insertions(+), 2 deletions(-)
>>
>> diff --git a/dwarf_loader.c b/dwarf_loader.c
>> index 21f8bf1..a6e6587 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':
>> @@ -1264,6 +1265,7 @@ 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);
>> @@ -1273,7 +1275,18 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
>>   		if (!info->signature_changed) {
>>   			if (cu->producer_clang || param_idx >= cu->nr_register_params)
>>   				return parm;
>> +		} else {
>> +			/* 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)
>>   		 * are the result of later DWARF generation during compilation
>> @@ -1311,7 +1324,7 @@ 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)
>> @@ -1324,8 +1337,10 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
>>   				 * contents.
>>   				 */
>>   				parm->unexpected_reg = 1;
>> -		} else if (has_const_value) {
>> +		} else if (has_const_value || info->signature_changed) {
>>   			parm->optimized = 1;
>> +			if (info->signature_changed)
>> +				info->skip_idx++;
>>   		}
>>   	}
>>   
>> -- 
>> 2.52.0
>>
>>