[PATCH dwarves v5 03/11] dwarf_loader: Handle signatures with dead arguments
Yonghong Song <[email protected]> Sat, 23 May 2026 09:57:28 -0700
| Newsgroups | org.kernel.vger.dwarves,org.kernel.vger.bpf |
|---|---|
| Message-ID | <[email protected]> |
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.
The previous patch has the following:
0x0533fd03: DW_TAG_subprogram
DW_AT_name ("acpi_irq_penalty_update")
DW_AT_calling_convention (DW_CC_nocall)
DW_AT_type (0x05334dc7 "int")
...
0x0533fd15: DW_TAG_formal_parameter
DW_AT_name ("str")
DW_AT_type (0x05335918 "char *")
...
0x0533fd1f: DW_TAG_formal_parameter
DW_AT_location (indexed (0x3b) loclist = 0x00eb9d83:
[0xffff80008419f2e0, 0xffff80008419f324): DW_OP_reg1 W1
[0xffff80008419f324, 0xffff80008419f47c): DW_OP_reg19 W19
[0xffff80008419f47c, 0xffff80008419f494): DW_OP_entry_value(DW_OP_reg1 W1), DW_OP_stack_value
[0xffff80008419f494, 0xffff80008419f498): DW_OP_reg19 W19)
DW_AT_name ("used")
DW_AT_type (0x05334dc7 "int")
...
It is also handled properly with parameter 'str' will have W0 register.
With this patch, I checked x86_64 that the number of invalid true signatures is reduced
from 532 to 96. This suggests that majority of optimized functions are caused by
dead arguments.
Signed-off-by: Yonghong Song <[email protected]>
---
dwarf_loader.c | 93 +++++++++++++++++++++++++++++++++++++++++++++++---
1 file changed, 88 insertions(+), 5 deletions(-)
diff --git a/dwarf_loader.c b/dwarf_loader.c
index f0ac699..49993ee 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -1195,10 +1195,62 @@ static ptrdiff_t __dwarf_getlocations(Dwarf_Attribute *attr,
struct func_info {
bool signature_changed;
+ int skip_idx;
int nr_params;
int param_start_regs[MAX_PRESCAN_PARAMS];
};
+static int __get_type_byte_size(Dwarf_Die *die, struct cu *cu)
+{
+ Dwarf_Attribute attr;
+ if (dwarf_attr(die, DW_AT_type, &attr) == NULL)
+ return 0;
+
+ Dwarf_Die type_die;
+ if (dwarf_formref_die(&attr, &type_die) == NULL)
+ return 0;
+
+ /* A type does not have byte_size.
+ * 0x000dac83: DW_TAG_formal_parameter
+ DW_AT_location (indexed (0x385) loclist = 0x00016175:
+ [0xffff800080098cb0, 0xffff800080098cb4): DW_OP_breg8 W8+0
+ [0xffff800080098cb4, 0xffff800080098ff4): DW_OP_breg31 WSP+16, DW_OP_deref
+ [0xffff800080099054, 0xffff80008009908c): DW_OP_breg31 WSP+16, DW_OP_deref)
+ DW_AT_name ("ubuf")
+ DW_AT_decl_file ("/home/yhs/work/bpf-next/arch/arm64/kernel/ptrace.c")
+ DW_AT_decl_line (886)
+ DW_AT_type (0x000d467e "const void *")
+
+ * 0x000d467e: DW_TAG_pointer_type
+ DW_AT_type (0x000c4320 "const void")
+
+ * 0x000c4320: DW_TAG_const_type
+ */
+ if (dwarf_tag(&type_die) == DW_TAG_pointer_type)
+ return cu->addr_size;
+
+ uint64_t bsize = attr_numeric(&type_die, DW_AT_byte_size);
+ if (bsize == 0)
+ return __get_type_byte_size(&type_die, cu);
+
+ return bsize;
+}
+
+static int get_type_byte_size(Dwarf_Die *die, struct cu *cu)
+{
+ int byte_size = 0;
+
+ Dwarf_Attribute attr;
+ if (dwarf_attr(die, DW_AT_abstract_origin, &attr)) {
+ Dwarf_Die origin;
+ if (dwarf_formref_die(&attr, &origin))
+ byte_size = __get_type_byte_size(&origin, cu);
+ } else {
+ byte_size = __get_type_byte_size(die, cu);
+ }
+ return byte_size;
+}
+
/* Get the first DW_OP_X (should be a register) from a parameter's DW_AT_location. */
static int parameter__peek_first_reg(Dwarf_Die *die)
{
@@ -1292,8 +1344,9 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
struct parameter *parm = tag__alloc(cu, sizeof(*parm));
if (parm != NULL) {
- bool has_const_value;
+ bool has_const_value, true_sig_enabled;
Dwarf_Attribute attr;
+ int reg_idx;
tag__init(&parm->tag, cu, die);
parm->name = attr_string(die, DW_AT_name, conf);
@@ -1303,8 +1356,10 @@ 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 (param_idx >= cu->nr_register_params) {
- return parm;
+ } else {
+ 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
@@ -1342,9 +1397,10 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
*/
has_const_value = dwarf_attr(die, DW_AT_const_value, &attr) != NULL;
parm->has_loc = dwarf_attr(die, DW_AT_location, &attr) != NULL;
+ true_sig_enabled = conf->true_signature && info->signature_changed;
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)
@@ -1357,8 +1413,35 @@ 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 && !cu->producer_clang) {
+ parm->optimized = 1;
+ } else if (true_sig_enabled) {
+ int byte_size, num_regs, next_reg_idx;
+
+ if (param_idx + 1 < info->nr_params) {
+ int next_start = info->param_start_regs[param_idx + 1];
+ if (next_start >= 0) {
+ /* check whether we should preserve the argument or not */
+ byte_size = get_type_byte_size(die, cu);
+ /* byte_size 0 should not happen. */
+ if (!byte_size) {
+ parm->unexpected_reg = 1;
+ return parm;
+ }
+
+ num_regs = (byte_size + cu->addr_size - 1) / cu->addr_size;
+ next_reg_idx = reg_idx + num_regs;
+ if (next_reg_idx < cu->nr_register_params &&
+ next_start == cu->register_params[next_reg_idx]) {
+ if (byte_size > cu->addr_size)
+ info->skip_idx--;
+ return parm;
+ }
+ }
+ }
+
parm->optimized = 1;
+ info->skip_idx++;
}
}
--
2.53.0-Meta