[PATCH dwarves v3 4/9] dwarf_laoder: Handle locations with DW_OP_fbreg

Yonghong Song
2026-03-20T19:09:50Z

DW_OP_fbreg means the parameter value will be stored on the
stack. So the corresponding parameter register is not used.
For example:

0x071f7717: DW_TAG_subprogram
DW_AT_name ("jent_health_failure")
DW_AT_calling_convention (DW_CC_nocall)
DW_AT_type (0x071f7626 "unsigned int")
...

0x071f7728: DW_TAG_formal_parameter
DW_AT_location (DW_OP_fbreg -8)
DW_AT_name ("ec")
DW_AT_type (0x071f7ab6 "rand_data *")
...

0x071f7734: NULL

In the above, the parameter 'ec' type is a pointer so it perfectly fits
into a register. But the location uses 'DW_OP_fbreg -8' which prevents
from generating a function with true signatures.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
dwarf_loader.c | 18 +++++++++++++++---
1 file changed, 15 insertions(+), 3 deletions(-)

diff --git a/dwarf_loader.c b/dwarf_loader.c
index 7b3f551..c259417 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -1196,6 +1196,7 @@ struct func_info {
};

#define PARM_DEFAULT_FAIL -1
+#define PARM_UNEXPECTED -2

/* For DW_AT_location 'attr':
* - if first location is DW_OP_regXX with expected number, return the register;
@@ -1204,7 +1205,8 @@ struct func_info {
* list, return the register; otherwise save register for later return
* - otherwise if no register was found for locations, return PARM_DEFAULT_FAIL.
*/
-static int parameter__reg(Dwarf_Attribute *attr, int expected_reg)
+static int parameter__reg(Dwarf_Attribute *attr, int expected_reg, struct conf_load *conf,
+ struct func_info *info)
{
Dwarf_Addr base, start, end;
Dwarf_Op *expr, *entry_ops;
@@ -1240,6 +1242,16 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg)
if (ret == expected_reg)
goto out;
break;
+ case DW_OP_fbreg:
+ /* The locaiton like
+ * DW_AT_location (DW_OP_fbreg +<num>)
+ * indicates that the parameter is on the stack. But it is possible
+ * that the parameter can fit in register(s). So conservatively
+ * mark this parameter not suitable for true signatures.
+ */
+ if (info->signature_changed && conf->true_signature)
+ ret = PARM_UNEXPECTED;
+ break;
/* match DW_OP_entry_value(DW_OP_regXX) at any location */
case DW_OP_entry_value:
case DW_OP_GNU_entry_value:
@@ -1327,11 +1339,11 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,

if (parm->has_loc) {
int expected_reg = cu->register_params[reg_idx];
- int actual_reg = parameter__reg(&attr, expected_reg);
+ int actual_reg = parameter__reg(&attr, expected_reg, conf, info);

if (actual_reg == PARM_DEFAULT_FAIL)
parm->optimized = 1;
- else if (expected_reg >= 0 && expected_reg != actual_reg)
+ else if (actual_reg == PARM_UNEXPECTED || (expected_reg >= 0 && expected_reg != actual_reg))
/* mark parameters that use an unexpected
* register to hold a parameter; these will
* be problematic for users of BTF as they
--
2.52.0