From: Yonghong Song <yonghong.song@linux.dev>
To: Alan Maguire <alan.maguire@oracle.com>,
Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
dwarves@vger.kernel.org
Cc: Alexei Starovoitov <ast@kernel.org>,
Andrii Nakryiko <andrii@kernel.org>,
bpf@vger.kernel.org, kernel-team@fb.com
Subject: [PATCH dwarves v3 6/9] dwarf_loader: Detect optimized parameters with locations having constant values
Date: Fri, 20 Mar 2026 12:09:48 -0700 [thread overview]
Message-ID: <20260320190948.1972737-1-yonghong.song@linux.dev> (raw)
In-Reply-To: <20260320190917.1970524-1-yonghong.song@linux.dev>
The following is an example:
0x00899a78: DW_TAG_subprogram
DW_AT_low_pc (0xffffffff879be480)
DW_AT_high_pc (0xffffffff879beac4)
DW_AT_frame_base (DW_OP_reg7 RSP)
DW_AT_call_all_calls (true)
DW_AT_name ("split_mem_range")
DW_AT_decl_file ("/home/yhs/work/bpf-next/arch/x86/mm/init.c")
DW_AT_decl_line (401)
DW_AT_prototyped (true)
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_decl_file ("/home/yhs/work/bpf-next/arch/x86/mm/init.c")
DW_AT_decl_line (401)
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_decl_file ("/home/yhs/work/bpf-next/arch/x86/mm/init.c")
DW_AT_decl_line (401)
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_decl_file ("/home/yhs/work/bpf-next/arch/x86/mm/init.c")
DW_AT_decl_line (402)
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_decl_file ("/home/yhs/work/bpf-next/arch/x86/mm/init.c")
DW_AT_decl_line (403)
DW_AT_type (0x008861cf "unsigned long")
The parameter 'nr_range' is a constant and won't consume any ABI register.
Signed-off-by: Yonghong Song <yonghong.song@linux.dev>
---
dwarf_loader.c | 46 ++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 42 insertions(+), 4 deletions(-)
diff --git a/dwarf_loader.c b/dwarf_loader.c
index 117cd49..e10e5d8 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -1197,6 +1197,21 @@ struct func_info {
#define PARM_DEFAULT_FAIL -1
#define PARM_UNEXPECTED -2
+#define PARM_OPTIMIZED_OUT -3
+#define PARM_CONTINUE -4
+
+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;
@@ -1230,8 +1245,17 @@ 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 */
@@ -1252,6 +1276,16 @@ 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:
@@ -1341,9 +1375,12 @@ 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_OPTIMIZED_OUT) {
parm->optimized = 1;
- else if (actual_reg == PARM_UNEXPECTED || (expected_reg >= 0 && expected_reg != actual_reg))
+ 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
@@ -1351,6 +1388,7 @@ static struct parameter *parameter__new(Dwarf_Die *die, struct cu *cu,
* contents.
*/
parm->unexpected_reg = 1;
+ }
} else if (has_const_value || info->signature_changed) {
parm->optimized = 1;
if (info->signature_changed)
--
2.52.0
next prev parent reply other threads:[~2026-03-20 19:09 UTC|newest]
Thread overview: 20+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-03-20 19:09 [PATCH dwarves v3 0/9] pahole: Encode true signatures in kernel BTF Yonghong Song
2026-03-20 19:09 ` [PATCH dwarves v3 1/9] dwarf_loader: Reduce parameter checking with clang DW_AT_calling_convention attr Yonghong Song
2026-03-21 23:10 ` Jiri Olsa
2026-03-22 17:36 ` Yonghong Song
2026-03-23 12:56 ` Alan Maguire
2026-03-23 18:32 ` Yonghong Song
2026-03-20 19:09 ` [PATCH dwarves v3 2/9] dwarf_loader: Handle signatures with dead arguments Yonghong Song
2026-03-21 23:10 ` Jiri Olsa
2026-03-22 18:03 ` Yonghong Song
2026-03-20 19:09 ` [PATCH dwarves v3 3/9] dwarf_loader: Refactor initial ret -1 to be macro PARM_DEFAULT_FAIL Yonghong Song
2026-03-20 19:09 ` [PATCH dwarves v3 4/9] dwarf_laoder: Handle locations with DW_OP_fbreg Yonghong Song
2026-03-20 19:09 ` [PATCH dwarves v3 5/9] dwarf_loader: Change exprlen checking condition in parameter__reg() Yonghong Song
2026-03-20 19:09 ` Yonghong Song [this message]
2026-03-20 19:09 ` [PATCH dwarves v3 7/9] dwarf_loader: Handle expression lists Yonghong Song
2026-03-21 23:10 ` Jiri Olsa
2026-03-22 18:33 ` Yonghong Song
2026-03-20 19:09 ` [PATCH dwarves v3 8/9] btf_encoder: Handle optimized parameter properly Yonghong Song
2026-03-20 19:10 ` [PATCH dwarves v3 9/9] tests: Add a few clang true signature tests Yonghong Song
2026-03-23 15:41 ` Alan Maguire
2026-03-23 19:58 ` Yonghong Song
Reply instructions:
You may reply publicly to this message via plain-text email
using any one of the following methods:
* Save the following mbox file, import it into your mail client,
and reply-to-all from there: mbox
Avoid top-posting and favor interleaved quoting:
https://en.wikipedia.org/wiki/Posting_style#Interleaved_style
* Reply using the --to, --cc, and --in-reply-to
switches of git-send-email(1):
git send-email \
--in-reply-to=20260320190948.1972737-1-yonghong.song@linux.dev \
--to=yonghong.song@linux.dev \
--cc=alan.maguire@oracle.com \
--cc=andrii@kernel.org \
--cc=arnaldo.melo@gmail.com \
--cc=ast@kernel.org \
--cc=bpf@vger.kernel.org \
--cc=dwarves@vger.kernel.org \
--cc=kernel-team@fb.com \
/path/to/YOUR_REPLY
https://kernel.org/pub/software/scm/git/docs/git-send-email.html
* If your mail client supports setting the In-Reply-To header
via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line
before the message body.
This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.