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

Jiri Olsa
2026-03-21T23:10:32Z

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
>
>