* [PATCH] libbpf: add SIB addressing mode support for x86-64 USDT arguments
@ 2025-07-29 12:52 Jiawei Zhao
2025-07-30 3:36 ` kernel test robot
0 siblings, 1 reply; 2+ messages in thread
From: Jiawei Zhao @ 2025-07-29 12:52 UTC (permalink / raw)
To: ast; +Cc: linux-kernel
Add support for Scale-Index-Base (SIB) addressing mode in USDT argument
parsing to handle complex memory addressing patterns commonly used in
x86-64 assembly. This enables tracing of USDT probes that reference
arguments using SIB addressing format like "1@-96(%rbp,%rax,8)".
Signed-off-by: Jiawei Zhao <Phoenix500526@163.com>
---
tools/lib/bpf/usdt.bpf.h | 31 +++++++++++++++++++++++++++++++
tools/lib/bpf/usdt.c | 22 +++++++++++++++++++---
2 files changed, 50 insertions(+), 3 deletions(-)
diff --git a/tools/lib/bpf/usdt.bpf.h b/tools/lib/bpf/usdt.bpf.h
index 2a7865c8e3fe..59599159baab 100644
--- a/tools/lib/bpf/usdt.bpf.h
+++ b/tools/lib/bpf/usdt.bpf.h
@@ -34,6 +34,7 @@ enum __bpf_usdt_arg_type {
BPF_USDT_ARG_CONST,
BPF_USDT_ARG_REG,
BPF_USDT_ARG_REG_DEREF,
+ BPF_USDT_ARG_SIB,
};
struct __bpf_usdt_arg_spec {
@@ -43,6 +44,10 @@ struct __bpf_usdt_arg_spec {
enum __bpf_usdt_arg_type arg_type;
/* offset of referenced register within struct pt_regs */
short reg_off;
+ /* offset of index register in pt_regs, only used in SIB mode */
+ short idx_reg_off;
+ /* scale factor for index register, only used in SIB mode */
+ short scale;
/* whether arg should be interpreted as signed value */
bool arg_signed;
/* number of bits that need to be cleared and, optionally,
@@ -202,6 +207,32 @@ int bpf_usdt_arg(struct pt_regs *ctx, __u64 arg_num, long *res)
return err;
#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
val >>= arg_spec->arg_bitshift;
+#endif
+ break;
+ case BPF_USDT_ARG_SIB:
+ /* Arg is in memory addressed by SIB (Scale-Index-Base) mode
+ * (e.g., "-1@-96(%rbp,%rax,8)" in USDT arg spec). Register
+ * is identified like with BPF_USDT_ARG_SIB case, the offset
+ * is in arg_spec->val_off, the scale factor is in arg_spec->scale.
+ * Firstly, we fetch the base register contents and the index
+ * register contents from pt_regs. Secondly, we multiply the
+ * index register contents by the scale factor, then add the
+ * base address and the offset to get the final address.Finally,
+ * we do another user-space probe read to fetch argument value
+ * itself.
+ */
+ unsigned long idx;
+ err = bpf_probe_read_kernel(&val, sizeof(val), (void *)ctx + arg_spec->reg_off);
+ if (err)
+ return err;
+ err = bpf_probe_read_kernel(&idx, sizeof(idx), (void *)ctx + arg_spec->idx_reg_off);
+ if (err)
+ return err;
+ err = bpf_probe_read_user(&val, sizeof(val), (void *)val + idx * arg_spec->scale + arg_spec->val_off);
+ if (err)
+ return err;
+#if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
+ val >>= arg_spec->arg_bitshift;
#endif
break;
default:
diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
index 4e4a52742b01..e432c48f2d2c 100644
--- a/tools/lib/bpf/usdt.c
+++ b/tools/lib/bpf/usdt.c
@@ -200,6 +200,7 @@ enum usdt_arg_type {
USDT_ARG_CONST,
USDT_ARG_REG,
USDT_ARG_REG_DEREF,
+ USDT_ARG_SIB,
};
/* should match exactly struct __bpf_usdt_arg_spec from usdt.bpf.h */
@@ -207,6 +208,8 @@ struct usdt_arg_spec {
__u64 val_off;
enum usdt_arg_type arg_type;
short reg_off;
+ short idx_reg_off;
+ short scale;
bool arg_signed;
char arg_bitshift;
};
@@ -1283,11 +1286,24 @@ static int calc_pt_regs_off(const char *reg_name)
static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec *arg, int *arg_sz)
{
- char reg_name[16];
- int len, reg_off;
+ char reg_name[16], idx_reg_name[16];
+ int len, reg_off, scale;
long off;
- if (sscanf(arg_str, " %d @ %ld ( %%%15[^)] ) %n", arg_sz, &off, reg_name, &len) == 3) {
+ if (sscanf(arg_str, " %d @ %ld ( %%%15[^,] , %%%15[^,] , %d ) %n", arg_sz, &off, reg_name, idx_reg_name, &scale, &len) == 5) {
+ /* Scale Index Base case, e.g., 1@-96(%rbp,%rax,8)*/
+ arg->arg_type = USDT_ARG_SIB;
+ arg->val_off = off;
+ reg_off = calc_pt_regs_off(reg_name);
+ if (reg_off < 0)
+ return reg_off;
+ arg->reg_off = reg_off;
+ int idx_reg_off = calc_pt_regs_off(idx_reg_name);
+ if (idx_reg_off < 0)
+ return idx_reg_off;
+ arg->idx_reg_off = idx_reg_off;
+ arg->scale = scale;
+ }else if (sscanf(arg_str, " %d @ %ld ( %%%15[^)] ) %n", arg_sz, &off, reg_name, &len) == 3) {
/* Memory dereference case, e.g., -4@-20(%rbp) */
arg->arg_type = USDT_ARG_REG_DEREF;
arg->val_off = off;
--
2.39.5 (Apple Git-154)
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2025-07-29 12:52 [PATCH] libbpf: add SIB addressing mode support for x86-64 USDT arguments Jiawei Zhao
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