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

Yonghong Song
2026-03-20T19:09:38Z

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 | 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