[PATCH dwarves v5 07/11] dwarf_loader: Detect optimized parameters with locations having constant values
Yonghong Song <[email protected]> Sat, 23 May 2026 09:57:50 -0700
| Newsgroups | org.kernel.vger.dwarves,org.kernel.vger.bpf |
|---|---|
| Message-ID | <[email protected]> |
The following is an example:
0x00899a78: DW_TAG_subprogram
DW_AT_calling_convention (DW_CC_nocall)
DW_AT_type (0x008861cb "int")
...
0x00899a8c: DW_TAG_formal_parameter
DW_AT_location (indexed (0x3e) loclist = 0x000b2195:
[0xffffffff879be485, 0xffffffff879be49a): DW_OP_reg5 RDI
[0xffffffff879be49a, 0xffffffff879beac4): DW_OP_breg7 RSP+0)
DW_AT_name ("mr")
DW_AT_type (0x00899c88 "map_range *")
...
0x00899a98: DW_TAG_formal_parameter
DW_AT_location (indexed (0x41) loclist = 0x000b21d4:
[0xffffffff879be480, 0xffffffff879be554): DW_OP_consts +0, DW_OP_stack_value
[0xffffffff879be554, 0xffffffff879be56d): DW_OP_consts +1, DW_OP_stack_value
[0xffffffff879be56d, 0xffffffff879be572): DW_OP_reg2 RCX
[0xffffffff879be572, 0xffffffff879be638): DW_OP_breg7 RSP+8
[0xffffffff879be638, 0xffffffff879be63d): DW_OP_reg0 RAX
[0xffffffff879be63d, 0xffffffff879be6ef): DW_OP_breg7 RSP+8
[0xffffffff879be6ef, 0xffffffff879be6f5): DW_OP_reg14 R14
[0xffffffff879be6f5, 0xffffffff879be6fd): DW_OP_breg7 RSP+8
[0xffffffff879be6fd, 0xffffffff879be879): DW_OP_reg14 R14
[0xffffffff879be879, 0xffffffff879be931): DW_OP_reg12 R12
[0xffffffff879be955, 0xffffffff879be961): DW_OP_reg14 R14
[0xffffffff879be961, 0xffffffff879be966): DW_OP_reg12 R12
[0xffffffff879be966, 0xffffffff879be976): DW_OP_reg14 R14
[0xffffffff879be976, 0xffffffff879be9df): DW_OP_reg12 R12
[0xffffffff879be9df, 0xffffffff879be9e5): DW_OP_reg14 R14
[0xffffffff879be9fc, 0xffffffff879bea24): DW_OP_consts +0, DW_OP_stack_value
[0xffffffff879bea24, 0xffffffff879bea74): DW_OP_breg7 RSP+8
[0xffffffff879bea74, 0xffffffff879beac4): DW_OP_reg14 R14)
DW_AT_name ("nr_range")
DW_AT_type (0x008861cb "int")
...
0x00899aa4: DW_TAG_formal_parameter
DW_AT_location (indexed (0x3f) loclist = 0x000b21a7:
[0xffffffff879be485, 0xffffffff879be4a4): DW_OP_reg4 RSI
[0xffffffff879be4a4, 0xffffffff879be4e6): DW_OP_reg12 R12
[0xffffffff879be4e6, 0xffffffff879beac4): DW_OP_entry_value(DW_OP_reg4 RSI), DW_OP_stack_value)
DW_AT_name ("start")
DW_AT_type (0x008861cf "unsigned long")
...
0x00899ab0: DW_TAG_formal_parameter
DW_AT_location (indexed (0x40) loclist = 0x000b21c2:
[0xffffffff879be485, 0xffffffff879be4a9): DW_OP_reg1 RDX
[0xffffffff879be4a9, 0xffffffff879beac4): DW_OP_breg7 RSP+32)
DW_AT_name ("end")
DW_AT_type (0x008861cf "unsigned long")
...
The parameter 'nr_range' is a constant and won't consume any ABI register.
With this commit, for x86_64, the number of invalid true signature reduced from 96
to 83.
Signed-off-by: Yonghong Song <[email protected]>
---
dwarf_loader.c | 47 +++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 43 insertions(+), 4 deletions(-)
diff --git a/dwarf_loader.c b/dwarf_loader.c
index 4b65e30..97f576a 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -1192,6 +1192,8 @@ static ptrdiff_t __dwarf_getlocations(Dwarf_Attribute *attr,
#define PARM_DEFAULT_FAIL -1
#define PARM_UNEXPECTED -2
+#define PARM_OPTIMIZED_OUT -3
+#define PARM_CONTINUE -4
/* Max 20 register parameters, considering some parameters may be optimized out. */
#define MAX_PRESCAN_PARAMS 20
@@ -1279,6 +1281,20 @@ static int parameter__peek_first_reg(Dwarf_Die *die)
return -1;
}
+static int parameter__multi_exprs(Dwarf_Op *expr, int loc_num)
+{
+ switch (expr[0].atom) {
+ case DW_OP_lit0 ... DW_OP_lit31:
+ case DW_OP_constu:
+ case DW_OP_consts:
+ if (loc_num != 0)
+ break;
+ return PARM_OPTIMIZED_OUT;
+ }
+
+ return PARM_CONTINUE;
+}
+
/* For DW_AT_location 'attr':
* - if first location is DW_OP_regXX with expected number, return the register;
* otherwise save the register for later return
@@ -1311,8 +1327,17 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg, struct conf_l
if (exprlen == 2 && expr[exprlen - 1].atom == DW_OP_stack_value)
exprlen--;
- if (exprlen != 1)
- continue;
+ if (exprlen != 1) {
+ if (!info->signature_changed || !conf->true_signature)
+ continue;
+
+ int res;
+ res = parameter__multi_exprs(expr, loc_num);
+ if (res == PARM_CONTINUE)
+ continue;
+ ret = res;
+ goto out;
+ }
switch (expr->atom) {
/* match DW_OP_regXX at first location */
@@ -1333,6 +1358,16 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg, struct conf_l
if (info->signature_changed && conf->true_signature)
ret = PARM_UNEXPECTED;
break;
+ case DW_OP_lit0 ... DW_OP_lit31:
+ case DW_OP_constu:
+ case DW_OP_consts:
+ if (info->signature_changed && conf->true_signature) {
+ if (loc_num != 0)
+ break;
+ ret = PARM_OPTIMIZED_OUT;
+ goto out;
+ }
+ break;
/* match DW_OP_entry_value(DW_OP_regXX) at any location */
case DW_OP_entry_value:
case DW_OP_GNU_entry_value:
@@ -1417,9 +1452,12 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
int expected_reg = cu->register_params[reg_idx];
int actual_reg = parameter__reg(&attr, expected_reg, conf, info);
- if (actual_reg == PARM_DEFAULT_FAIL)
+ if (actual_reg == PARM_DEFAULT_FAIL) {
parm->optimized = 1;
- else if (actual_reg == PARM_UNEXPECTED || (expected_reg >= 0 && expected_reg != actual_reg))
+ } else if (actual_reg == PARM_OPTIMIZED_OUT) {
+ parm->optimized = 1;
+ info->skip_idx++;
+ } 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
@@ -1427,6 +1465,7 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
* contents.
*/
parm->unexpected_reg = 1;
+ }
} else if (has_const_value && !cu->producer_clang) {
parm->optimized = 1;
} else if (true_sig_enabled) {
--
2.53.0-Meta