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 v2 4/9] dwarf_laoder: Handle locations with DW_OP_fbreg
Date: Mon, 9 Mar 2026 08:32:36 -0700 [thread overview]
Message-ID: <20260309153236.1918945-1-yonghong.song@linux.dev> (raw)
In-Reply-To: <20260309153215.1917033-1-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 | 17 ++++++++++++++---
1 file changed, 14 insertions(+), 3 deletions(-)
diff --git a/dwarf_loader.c b/dwarf_loader.c
index 7da3926..d538607 100644
--- a/dwarf_loader.c
+++ b/dwarf_loader.c
@@ -1196,6 +1196,7 @@ struct func_info {
};
#define PARM_DEFAULT_FAIL -1
+#define PARM_FAIL_CLANG -2
/* For DW_AT_location 'attr':
* - if first location is DW_OP_regXX with expected number, return the register;
@@ -1204,7 +1205,7 @@ 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 cu *cu, struct conf_load *conf)
{
Dwarf_Addr base, start, end;
Dwarf_Op *expr, *entry_ops;
@@ -1240,6 +1241,16 @@ 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 (cu->producer_clang && conf->true_signature)
+ ret = PARM_FAIL_CLANG;
+ break;
/* match DW_OP_entry_value(DW_OP_regXX) at any location */
case DW_OP_entry_value:
case DW_OP_GNU_entry_value:
@@ -1328,11 +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, cu, conf);
if (actual_reg == PARM_DEFAULT_FAIL)
parm->optimized = 1;
- else if (expected_reg >= 0 && expected_reg != actual_reg)
+ else if (actual_reg == PARM_FAIL_CLANG || (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
--
2.47.3
next prev parent reply other threads:[~2026-03-09 15:32 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-03-09 15:32 [PATCH dwarves v2 0/9] pahole: Encode true signatures in kernel BTF Yonghong Song
2026-03-09 15:32 ` [PATCH dwarves v2 1/9] dwarf_loader: Reduce parameter checking with clang DW_AT_calling_convention attr Yonghong Song
2026-03-09 15:32 ` [PATCH dwarves v2 2/9] dwarf_loader: Handle signatures with dead arguments Yonghong Song
2026-03-09 15:32 ` [PATCH dwarves v2 3/9] dwarf_loader: Refactor initial ret -1 to be macro PARM_DEFAULT_FAIL Yonghong Song
2026-03-09 15:32 ` Yonghong Song [this message]
2026-03-09 15:32 ` [PATCH dwarves v2 5/9] dwarf_loader: Change exprlen checking condition in parameter__reg() Yonghong Song
2026-03-09 15:32 ` [PATCH dwarves v2 6/9] dwarf_loader: Detect optimized parameters with locations having constant values Yonghong Song
2026-03-09 15:32 ` [PATCH dwarves v2 7/9] dwarf_loader: Handle expression lists Yonghong Song
2026-03-09 15:32 ` [PATCH dwarves v2 8/9] btf_encoder: Handle optimized parameter properly Yonghong Song
2026-03-09 15:33 ` [PATCH dwarves v2 9/9] tests: Add a few clang true signature tests Yonghong Song
2026-03-09 18:39 ` [PATCH dwarves v2 0/9] pahole: Encode true signatures in kernel BTF Alan Maguire
2026-03-09 19:25 ` Yonghong Song
2026-03-19 16:23 ` Yonghong Song
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