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

[Yonghong Song <yonghong.song@linux.dev>]

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