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

[Jiri Olsa <olsajiri@gmail.com>]

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)

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
> 
>