[PATCH dwarves v3 7/9] dwarf_loader: Handle expression lists

Yonghong Song
2026-03-20T19:10:00Z

The corresponding type is checked for the parameter.
If the parameter size is less or equal to size of long,
the argument should match the corresponding ABI register.
For example:

0x0aba0808: DW_TAG_subprogram
DW_AT_name ("addrconf_ifdown")
DW_AT_calling_convention (DW_CC_nocall)
DW_AT_type (0x0ab7d8e9 "int")
...

0x0aba082b: DW_TAG_formal_parameter
DW_AT_location (indexed (0x32b) loclist = 0x016eabcd:
[0xffffffff83f6fef9, 0xffffffff83f6ff98): DW_OP_reg5 RDI
[0xffffffff83f6ff98, 0xffffffff83f70080): DW_OP_reg12 R12
[0xffffffff83f70080, 0xffffffff83f70111): DW_OP_breg7 RSP+112
[0xffffffff83f70111, 0xffffffff83f7014f): DW_OP_reg12 R12
[0xffffffff83f7014f, 0xffffffff83f7123c): DW_OP_breg7 RSP+112
[0xffffffff83f7123c, 0xffffffff83f7128c): DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
[0xffffffff83f7128c, 0xffffffff83f712a9): DW_OP_reg12 R12
[0xffffffff83f712a9, 0xffffffff83f712cd): DW_OP_breg7 RSP+112
[0xffffffff83f712cd, 0xffffffff83f712d2): DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
[0xffffffff83f712d2, 0xffffffff83f713dd): DW_OP_breg7 RSP+112)
DW_AT_name ("dev")
DW_AT_type (0x0ab7cb7d "net_device *")
...

0x0aba0836: DW_TAG_formal_parameter
DW_AT_location (indexed (0x32c) loclist = 0x016eac39:
[0xffffffff83f6fef9, 0xffffffff83f6ff15): DW_OP_breg4 RSI+0, DW_OP_constu 0xffffffff, DW_OP_and, DW_OP_convert (0x0ab7b571) "DW_ATE_unsigned_1", DW_OP_convert (0x0ab7b576) "DW_ATE_unsigned_8", DW_OP_stack_value
[0xffffffff83f6ff15, 0xffffffff83f7127c): DW_OP_breg7 RSP+36, DW_OP_deref_size 0x4, DW_OP_convert (0x0ab7b571) "DW_ATE_unsigned_1", DW_OP_convert (0x0ab7b576) "DW_ATE_unsigned_8", DW_OP_stack_value
[0xffffffff83f7128c, 0xffffffff83f713dd): DW_OP_breg7 RSP+36, DW_OP_deref_size 0x4, DW_OP_convert (0x0ab7b571) "DW_ATE_unsigned_1", DW_OP_convert (0x0ab7b576) "DW_ATE_unsigned_8", DW_OP_stack_value)
DW_AT_name ("unregister")
DW_AT_type (0x0ab7c933 "bool")
...

The parameter 'unregister' is the second argument which matches ABI register RSI.
So the function "addrconf_ifdown" signature is valid.

If the parameter size is '2 x size_of_long', more handling is necessary, e.g., below:

0x0a01e174: DW_TAG_subprogram
DW_AT_name ("check_zeroed_sockptr")
DW_AT_calling_convention (DW_CC_nocall)
DW_AT_type (0x09fead35 "int")
...

0x0a01e187: DW_TAG_formal_parameter
DW_AT_location (indexed (0x5b6) loclist = 0x0157f03f:
[0xffffffff83c941c0, 0xffffffff83c941c4): DW_OP_reg5 RDI, DW_OP_piece 0x8, DW_OP_reg4 RSI, DW_OP_piece 0x1
[0xffffffff83c941c4, 0xffffffff83c941cc): DW_OP_piece 0x8, DW_OP_reg4 RSI, DW_OP_piece 0x1
[0xffffffff83c941e1, 0xffffffff83c941e4): DW_OP_piece 0x8, DW_OP_reg4 RSI, DW_OP_piece 0x1)
DW_AT_name ("src")
DW_AT_type (0x09ff832d "sockptr_t")
...

0x0a01e193: DW_TAG_formal_parameter
DW_AT_const_value (64)
DW_AT_name ("offset")
DW_AT_type (0x09fee984 "size_t")
...

0x0a01e19e: DW_TAG_formal_parameter
DW_AT_location (indexed (0x5b7) loclist = 0x0157f06b:
[0xffffffff83c941c0, 0xffffffff83c941d1): DW_OP_reg1 RDX
[0xffffffff83c941d1, 0xffffffff83c941e1): DW_OP_entry_value(DW_OP_reg1 RDX), DW_OP_stack_value
[0xffffffff83c941e1, 0xffffffff83c941e9): DW_OP_reg1 RDX)
DW_AT_name ("size")
DW_AT_type (0x09fee984 "size_t")
...

The first parameter 'src' will take two ABI registers. This patch correctly detects such a pattern
to construct the true signature.

However, it is possible that only one 'size_of_long' is used from '2 x size_of_long'. For example

0x019520c6: DW_TAG_subprogram
DW_AT_name ("map_create")
DW_AT_calling_convention (DW_CC_nocall)
DW_AT_type (0x01934b29 "int")
...

0x01952111: DW_TAG_formal_parameter
DW_AT_location (indexed (0x31b) loclist = 0x0034fa0f:
[0xffffffff81892345, 0xffffffff8189237c): DW_OP_reg5 RDI
[0xffffffff8189237c, 0xffffffff818923bd): DW_OP_reg3 RBX
[0xffffffff818923bd, 0xffffffff818923d4): DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
[0xffffffff818923d4, 0xffffffff81892dcb): DW_OP_reg3 RBX
[0xffffffff81892df3, 0xffffffff81892e01): DW_OP_entry_value(DW_OP_reg5 RDI), DW_OP_stack_value
[0xffffffff81892e01, 0xffffffff818932a9): DW_OP_reg3 RBX)
DW_AT_name ("attr")
DW_AT_type (0x01934d17 "bpf_attr *")
...

0x0195211d: DW_TAG_formal_parameter
DW_AT_location (indexed (0x31a) loclist = 0x0034f9dc:
[0xffffffff81892345, 0xffffffff81892357): DW_OP_piece 0x8, DW_OP_reg4 RSI, DW_OP_piece 0x1
[0xffffffff81892357, 0xffffffff81892f02): DW_OP_piece 0x8, DW_OP_breg7 RSP+20, DW_OP_deref_size 0x4, DW_OP_stack_value, DW_OP_piece 0x1
[0xffffffff81892f07, 0xffffffff818932a9): DW_OP_piece 0x8, DW_OP_breg7 RSP+20, DW_OP_deref_size 0x4, DW_OP_stack_value, DW_OP_piece 0x1)
DW_AT_name ("uattr")
DW_AT_type (0x019512ab "bpfptr_t")
...

For parameter 'uattr', only second half of parameter is used. For such cases,
the name and the type is changed in pahole and eventually going to vmlinux btf.
[55697] FUNC_PROTO '(anon)' ret_type_id=106780 vlen=2
'attr' type_id=455
'uattr__is_kernel' type_id=82014
[82014] TYPEDEF 'bool' type_id=67434
[113251] FUNC 'map_create' type_id=55697 linkage=static
You can see the new parameter name is 'uattr__is_kernel' and the type is 'bool'.

This makes thing easier for users to get the true signature.

Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
dwarf_loader.c | 246 +++++++++++++++++++++++++++++++++++++++++++++++--
dwarves.h | 1 +
2 files changed, 240 insertions(+), 7 deletions(-)

diff --git a/dwarf_loader.c b/dwarf_loader.c
index e10e5d8..7d23c7d 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -1100,6 +1100,16 @@ static void arch__set_register_params(const GElf_Ehdr *ehdr, struct cu *cu)
}
}

+static bool arch__agg_use_two_regs(const GElf_Ehdr *ehdr)
+{
+ switch (ehdr->e_machine) {
+ case EM_S390:
+ return false;
+ default:
+ return true;
+ }
+}
+
static struct template_type_param *template_type_param__new(Dwarf_Die *die, struct cu *cu, struct conf_load *conf)
{
struct template_type_param *ttparm = tag__alloc(cu, sizeof(*ttparm));
@@ -1199,8 +1209,98 @@ struct func_info {
#define PARM_UNEXPECTED -2
#define PARM_OPTIMIZED_OUT -3
#define PARM_CONTINUE -4
+#define PARM_TWO_ADDR_LEN -5
+#define PARM_TO_BE_IMPROVED -6
+
+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;

-static int parameter__multi_exprs(Dwarf_Op *expr, int loc_num) {
+ 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;
+}
+
+/* Traverse the parameter type until finding the member type which has expected
+ * struct type offset.
+*/
+static Dwarf_Die *get_member_with_offset(Dwarf_Die *die, int offset, Dwarf_Die *member_die) {
+ Dwarf_Attribute attr;
+ if (dwarf_attr(die, DW_AT_type, &attr) == NULL)
+ return NULL;
+
+ Dwarf_Die type_die;
+ if (dwarf_formref_die(&attr, &type_die) == NULL)
+ return NULL;
+
+ uint64_t bsize = attr_numeric(&type_die, DW_AT_byte_size);
+ if (bsize == 0)
+ return get_member_with_offset(&type_die, offset, member_die);
+
+ if (dwarf_tag(&type_die) != DW_TAG_structure_type)
+ return NULL;
+
+ if (!dwarf_haschildren(&type_die) || dwarf_child(&type_die, member_die) != 0)
+ return NULL;
+ do {
+ if (dwarf_tag(member_die) != DW_TAG_member)
+ continue;
+
+ int off = attr_numeric(member_die, DW_AT_data_bit_offset);
+ if (off == offset * 8)
+ return member_die;
+ } while (dwarf_siblingof(member_die, member_die) == 0);
+
+ return NULL;
+}
+
+/* For two address length case, lower_half and upper_half represents the parameter.
+ * The lower_half and upper_half accumulates field information across possible multiple
+ * location lists.
+ */
+static int parameter__multi_exprs(Dwarf_Op *expr, int loc_num, struct cu *cu, size_t exprlen,
+ Dwarf_Die *die, int expected_reg, int byte_size,
+ unsigned long *lower_half, unsigned long *upper_half, int *ret) {
switch (expr[0].atom) {
case DW_OP_lit0 ... DW_OP_lit31:
case DW_OP_constu:
@@ -1210,9 +1310,119 @@ static int parameter__multi_exprs(Dwarf_Op *expr, int loc_num) {
return PARM_OPTIMIZED_OUT;
}

+ if (byte_size <= cu->addr_size || !cu->agg_use_two_regs) {
+ switch (expr[0].atom) {
+ case DW_OP_reg0 ... DW_OP_reg31:
+ if (loc_num != 0)
+ break;
+ *ret = expr[0].atom;
+ if (*ret == expected_reg)
+ return *ret;
+ break;
+ case DW_OP_breg0 ... DW_OP_breg31:
+ if (loc_num != 0)
+ break;
+ bool has_op_stack_value = false;
+ for (int i = 1; i < exprlen; i++) {
+ if (expr[i].atom == DW_OP_stack_value) {
+ has_op_stack_value = true;
+ break;
+ }
+ }
+ if (!has_op_stack_value)
+ break;
+ /* The existence of DW_OP_stack_value means that
+ * DW_OP_bregX register is used as value.
+ */
+ *ret = expr[0].atom - DW_OP_breg0 + DW_OP_reg0;
+ if (*ret == expected_reg)
+ return *ret;
+ }
+ } else {
+ /* cu->addr * 2 */
+ int off = 0;
+ for (int i = 0; i < exprlen; i++) {
+ if (expr[i].atom == DW_OP_piece) {
+ int num = expr[i].number;
+ if (i == 0) {
+ off = num;
+ continue;
+ }
+ if (off < cu->addr_size) (*lower_half) |= (1 << off);
+ else (*upper_half) |= (1 << (off - cu->addr_size));
+ off += num;
+ } else if (expr[i].atom >= DW_OP_reg0 && expr[i].atom <= DW_OP_reg31) {
+ if (off < cu->addr_size)
+ *ret = expr[i].atom;
+ else if (*ret < 0)
+ *ret = expr[i].atom;
+ }
+ /* FIXME: not handling DW_OP_bregX yet since we do not have
+ * a use case for it yet for linux kernel.
+ */
+ }
+ }
+
return PARM_CONTINUE;
}

+/* The lower_half and upper_half, computed in parameter__multi_exprs(), are handled here.
+ */
+static int parameter__handle_two_addr_len(int expected_reg, unsigned long lower_half, unsigned long upper_half,
+ int ret, Dwarf_Die *die, struct conf_load *conf, struct cu *cu,
+ struct parameter *parm) {
+ if (!lower_half && !upper_half)
+ return ret;
+
+ if (ret != expected_reg)
+ return ret;
+
+ if (!conf->true_signature)
+ return PARM_DEFAULT_FAIL;
+
+ /* Both halfs are used based on dwarf */
+ if (lower_half && upper_half)
+ return PARM_TWO_ADDR_LEN;
+
+ /* FIXME: parm->name may be NULL due to abstract origin. We do not want to
+ * update abstract origin as the type in abstract origin may be used
+ * in some other places. We could remove abstract origin in this parameter
+ * and add name and type in parameter itself. Right now, for current bpf-next
+ * repo, we do not have instances below where parm->name is NULL for x86_64 arch.
+ */
+ if (!parm->name)
+ return PARM_TO_BE_IMPROVED;
+
+ /* FIXME: Only support single field now so we can have a good parameter name and
+ * type for it.
+ */
+ if (__builtin_popcountll(lower_half) >= 2 || __builtin_popcountll(upper_half) >= 2)
+ return PARM_TO_BE_IMPROVED;
+
+ int field_offset;
+ if (__builtin_popcountll(lower_half) == 1)
+ field_offset = __builtin_ctzll(lower_half);
+ else
+ field_offset = cu->addr_size + __builtin_ctzll(upper_half);
+
+ /* FIXME: Only struct type is supported. */
+ Dwarf_Die member_die;
+ if (!get_member_with_offset(die, field_offset, &member_die))
+ return PARM_TO_BE_IMPROVED;
+
+ const char *member_name = attr_string(&member_die, DW_AT_name, conf);
+ int len = sizeof(parm->name) + strlen(member_name) + 3;
+ char *new_name = malloc(len);
+ sprintf(new_name, "%s__%s", parm->name, member_name);
+ parm->name = new_name;
+
+ struct tag *tag = &parm->tag;
+ struct dwarf_tag *dtag = tag__dwarf(tag);
+ dwarf_tag__set_attr_type(dtag, type, &member_die, DW_AT_type);
+
+ return ret;
+}
+
/* 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
@@ -1221,15 +1431,18 @@ static int parameter__multi_exprs(Dwarf_Op *expr, int loc_num) {
* - otherwise if no register was found for locations, return PARM_DEFAULT_FAIL.
*/
static int parameter__reg(Dwarf_Attribute *attr, int expected_reg, struct conf_load *conf,
- struct func_info *info)
+ struct func_info *info, struct cu *cu, Dwarf_Die *die,
+ struct parameter *parm)
{
Dwarf_Addr base, start, end;
Dwarf_Op *expr, *entry_ops;
Dwarf_Attribute entry_attr;
size_t exprlen, entry_len;
ptrdiff_t offset = 0;
+ int byte_size = 0;
int loc_num = -1;
int ret = PARM_DEFAULT_FAIL;
+ unsigned long lower_half = 0, upper_half = 0;

/* use libdw__lock as dwarf_getlocation(s) has concurrency issues
* when libdw is not compiled with experimental --enable-thread-safety
@@ -1249,8 +1462,17 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg, struct conf_l
if (!info->signature_changed || !conf->true_signature)
continue;

+ if (!byte_size)
+ byte_size = get_type_byte_size(die, cu);
+ /* This should not happen. */
+ if (!byte_size) {
+ ret = PARM_UNEXPECTED;
+ goto out;
+ }
+
int res;
- res = parameter__multi_exprs(expr, loc_num);
+ res = parameter__multi_exprs(expr, loc_num, cu, exprlen, die, expected_reg,
+ byte_size, &lower_half, &upper_half, &ret);
if (res == PARM_CONTINUE)
continue;
ret = res;
@@ -1299,6 +1521,10 @@ static int parameter__reg(Dwarf_Attribute *attr, int expected_reg, struct conf_l
break;
}
}
+
+ ret = parameter__handle_two_addr_len(expected_reg, lower_half, upper_half,
+ ret, die, conf, cu, parm);
+
out:
pthread_mutex_unlock(&libdw__lock);
return ret;
@@ -1333,8 +1559,6 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
}
}
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
@@ -1372,15 +1596,22 @@ 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) {
+ if (reg_idx >= cu->nr_register_params)
+ return parm;
+
int expected_reg = cu->register_params[reg_idx];
- int actual_reg = parameter__reg(&attr, expected_reg, conf, info);
+ int actual_reg = parameter__reg(&attr, expected_reg, conf, info, cu, die, parm);

if (actual_reg == PARM_DEFAULT_FAIL) {
parm->optimized = 1;
} 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)) {
+ } else if (actual_reg == PARM_TWO_ADDR_LEN) {
+ /* account for parameter with two registers */
+ info->skip_idx--;
+ } else if (actual_reg == PARM_UNEXPECTED || actual_reg == PARM_TO_BE_IMPROVED ||
+ (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
@@ -3414,6 +3645,7 @@ static int cu__set_common(struct cu *cu, struct conf_load *conf,

cu->little_endian = ehdr.e_ident[EI_DATA] == ELFDATA2LSB;
cu->nr_register_params = arch__nr_register_params(&ehdr);
+ cu->agg_use_two_regs = arch__agg_use_two_regs(&ehdr);
arch__set_register_params(&ehdr, cu);
return 0;
}
diff --git a/dwarves.h b/dwarves.h
index 4cabab0..28792b5 100644
--- a/dwarves.h
+++ b/dwarves.h
@@ -303,6 +303,7 @@ struct cu {
uint8_t uses_global_strings:1;
uint8_t little_endian:1;
uint8_t producer_clang:1;
+ uint8_t agg_use_two_regs:1; /* An aggregate like {long a; long b;} */
uint8_t nr_register_params;
int register_params[ARCH_MAX_REGISTER_PARAMS];
int functions_saved;
--
2.52.0