* [PATCH v3] libbpf: fix USDT SIB argument handling causing unrecognized register error
@ 2025-07-29 16:17 Jiawei Zhao
2025-07-29 22:54 ` Yonghong Song
0 siblings, 1 reply; 2+ messages in thread
From: Jiawei Zhao @ 2025-07-29 16:17 UTC (permalink / raw)
To: andrii; +Cc: bpf
On x86-64, USDT arguments can be specified using Scale-Index-Base (SIB)
addressing, e.g. "1@-96(%rbp,%rax,8)". The current USDT implementation
in libbpf cannot parse this format, causing `bpf_program__attach_usdt()`
to fail with -ENOENT (unrecognized register).
This patch fixes this by implementing the necessary changes:
- add correct handling for SIB-addressed arguments in `bpf_usdt_arg`.
- add adaptive support to `__bpf_usdt_arg_type` and
`__bpf_usdt_arg_spec` to represent SIB addressing parameters.
Change since v1(https://lore.kernel.org/lkml/20250729125244.28364-1-Phoenix500526@163.com/):
- refactor the code to make it more readable
- modify the commit message to explain why and how
Change since v2:
- fix the `scale` uninitialized error
Signed-off-by: Jiawei Zhao <Phoenix500526@163.com>
---
tools/lib/bpf/usdt.bpf.h | 33 ++++++++++++++++++++++++++++++++-
tools/lib/bpf/usdt.c | 26 +++++++++++++++++++++++---
2 files changed, 55 insertions(+), 4 deletions(-)
diff --git a/tools/lib/bpf/usdt.bpf.h b/tools/lib/bpf/usdt.bpf.h
index 2a7865c8e3fe..246513088c3a 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,
@@ -149,7 +154,7 @@ int bpf_usdt_arg(struct pt_regs *ctx, __u64 arg_num, long *res)
{
struct __bpf_usdt_spec *spec;
struct __bpf_usdt_arg_spec *arg_spec;
- unsigned long val;
+ unsigned long val, idx;
int err, spec_id;
*res = 0;
@@ -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.
+ */
+ 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..260211e896d5 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 +1286,28 @@ 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_off, 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;
+ arg->scale = scale;
+
+ reg_off = calc_pt_regs_off(reg_name);
+ if (reg_off < 0)
+ return reg_off;
+ arg->reg_off = reg_off;
+
+ 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;
+ } 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)
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH v3] libbpf: fix USDT SIB argument handling causing unrecognized register error
2025-07-29 16:17 [PATCH v3] libbpf: fix USDT SIB argument handling causing unrecognized register error Jiawei Zhao
@ 2025-07-29 22:54 ` Yonghong Song
0 siblings, 0 replies; 2+ messages in thread
From: Yonghong Song @ 2025-07-29 22:54 UTC (permalink / raw)
To: Jiawei Zhao, andrii; +Cc: bpf
On 7/29/25 9:17 AM, Jiawei Zhao wrote:
> On x86-64, USDT arguments can be specified using Scale-Index-Base (SIB)
> addressing, e.g. "1@-96(%rbp,%rax,8)". The current USDT implementation
> in libbpf cannot parse this format, causing `bpf_program__attach_usdt()`
> to fail with -ENOENT (unrecognized register).
>
> This patch fixes this by implementing the necessary changes:
> - add correct handling for SIB-addressed arguments in `bpf_usdt_arg`.
> - add adaptive support to `__bpf_usdt_arg_type` and
> `__bpf_usdt_arg_spec` to represent SIB addressing parameters.
>
> Change since v1(https://lore.kernel.org/lkml/20250729125244.28364-1-Phoenix500526@163.com/):
> - refactor the code to make it more readable
> - modify the commit message to explain why and how
>
> Change since v2:
> - fix the `scale` uninitialized error
>
> Signed-off-by: Jiawei Zhao <Phoenix500526@163.com>
> ---
> tools/lib/bpf/usdt.bpf.h | 33 ++++++++++++++++++++++++++++++++-
> tools/lib/bpf/usdt.c | 26 +++++++++++++++++++++++---
> 2 files changed, 55 insertions(+), 4 deletions(-)
>
> diff --git a/tools/lib/bpf/usdt.bpf.h b/tools/lib/bpf/usdt.bpf.h
> index 2a7865c8e3fe..246513088c3a 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,
> @@ -149,7 +154,7 @@ int bpf_usdt_arg(struct pt_regs *ctx, __u64 arg_num, long *res)
> {
> struct __bpf_usdt_spec *spec;
> struct __bpf_usdt_arg_spec *arg_spec;
> - unsigned long val;
> + unsigned long val, idx;
> int err, spec_id;
>
> *res = 0;
> @@ -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.
> + */
> + 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:
If possible, could you add some tests which actually trigger a usdt pattern
like "-1@-96(%rbp,%rax,8)"?
There are much more usdt patterns e.g. in
https://lore.kernel.org/bpf/b3ce39f0-c52b-4787-980c-973bd4228349@linux.dev/
===
with -O2 and with gcc14 on x86:
stapsdt 0x00000087 NT_STAPSDT (SystemTap probe descriptors)
Provider: test
Name: usdt12
Location: 0x000000000000258f, Base: 0x0000000000000000, Semaphore: 0x0000000000000006
Arguments: -4@$2 -4@$3 -8@$42 -8@$44 -4@$5 -8@$6 8@%rdx 8@%rsi -4@$-9 -2@%cx -2@nums(%rax,%rax) -1@t1+4(%rip)
...
===
But we didn't add those '-2@nums(%rax,%rax)' '-1@t1+4(%rip)' as
they are very rare.
> diff --git a/tools/lib/bpf/usdt.c b/tools/lib/bpf/usdt.c
> index 4e4a52742b01..260211e896d5 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 +1286,28 @@ 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_off, 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;
> + arg->scale = scale;
> +
> + reg_off = calc_pt_regs_off(reg_name);
> + if (reg_off < 0)
> + return reg_off;
> + arg->reg_off = reg_off;
> +
> + 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;
> + } 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;
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2025-07-29 16:17 [PATCH v3] libbpf: fix USDT SIB argument handling causing unrecognized register error Jiawei Zhao
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