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Message-ID: <20250823065005.1187181-2-phoenix500526@163.com>
Date: Sat, 23 Aug 2025 06:50:03 +0000
From: Jiawei Zhao <phoenix500526@....com>
To: ast@...nel.org
Cc: daniel@...earbox.net,
andrii@...nel.org,
yonghong.song@...ux.dev,
bpf@...r.kernel.org,
linux-kselftest@...r.kernel.org,
linux-kernel@...r.kernel.org
Subject: [PATCH bpf-next v14 1/2] libbpf: fix USDT SIB argument handling causing unrecognized register error
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.
Signed-off-by: Jiawei Zhao <phoenix500526@....com>
---
tools/lib/bpf/usdt.bpf.h | 44 +++++++++++++++++++++++++++++--
tools/lib/bpf/usdt.c | 57 ++++++++++++++++++++++++++++++++++++----
2 files changed, 94 insertions(+), 7 deletions(-)
diff --git a/tools/lib/bpf/usdt.bpf.h b/tools/lib/bpf/usdt.bpf.h
index 2a7865c8e3fe..aaa584a4f4f9 100644
--- a/tools/lib/bpf/usdt.bpf.h
+++ b/tools/lib/bpf/usdt.bpf.h
@@ -34,13 +34,32 @@ enum __bpf_usdt_arg_type {
BPF_USDT_ARG_CONST,
BPF_USDT_ARG_REG,
BPF_USDT_ARG_REG_DEREF,
+ BPF_USDT_ARG_SIB,
};
+/*
+ * This struct layout is designed specifically to be backwards/forward
+ * compatible between libbpf versions for ARG_CONST, ARG_REG, and
+ * ARG_REG_DEREF modes. ARG_SIB requires libbpf v1.7+.
+ */
struct __bpf_usdt_arg_spec {
/* u64 scalar interpreted depending on arg_type, see below */
__u64 val_off;
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
/* arg location case, see bpf_usdt_arg() for details */
- enum __bpf_usdt_arg_type arg_type;
+ enum __bpf_usdt_arg_type arg_type: 8;
+ /* index register offset within struct pt_regs */
+ __u16 idx_reg_off: 12,
+ /* scale factor for index register (1, 2, 4, or 8) */
+ __u16 scale: 4;
+ /* reserved for future use, keeps reg_off offset stable */
+ __u8 __reserved: 8;
+#else
+ __u8 __reserved: 8;
+ __u16 idx_reg_off: 12;
+ __u16 scale: 4;
+ enum __bpf_usdt_arg_type arg_type: 8;
+#endif
/* offset of referenced register within struct pt_regs */
short reg_off;
/* whether arg should be interpreted as signed value */
@@ -149,7 +168,7 @@ 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 +221,27 @@ 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). We first
+ * fetch the base register contents and the index register
+ * contents from pt_regs. Then we calculate the final address
+ * as base + (index * scale) + offset, and do a user-space
+ * probe read to fetch the argument value.
+ */
+ 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 3373b9d45ac4..d90e0b694134 100644
--- a/tools/lib/bpf/usdt.c
+++ b/tools/lib/bpf/usdt.c
@@ -200,12 +200,23 @@ 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 */
struct usdt_arg_spec {
__u64 val_off;
- enum usdt_arg_type arg_type;
+#if __BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__
+ enum usdt_arg_type arg_type: 8;
+ __u16 idx_reg_off: 12;
+ __u16 scale: 4;
+ __u8 __reserved: 8; /* keep reg_off offset stable */
+#else
+ __u8 __reserved: 8; /* keep reg_off offset stable */
+ __u16 idx_reg_off: 12;
+ __u16 scale: 4;
+ enum usdt_arg_type arg_type: 8;
+#endif
short reg_off;
bool arg_signed;
char arg_bitshift;
@@ -1283,11 +1294,46 @@ 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;
- long off;
+ char reg_name[16] = {0}, idx_reg_name[16] = {0};
+ int len, reg_off, idx_reg_off, scale = 1;
+ long off = 0;
+
+ if (sscanf(arg_str, " %d @ %ld ( %%%15[^,] , %%%15[^,] , %d ) %n",
+ arg_sz, &off, reg_name, idx_reg_name, &scale, &len) == 5 ||
+ sscanf(arg_str, " %d @ ( %%%15[^,] , %%%15[^,] , %d ) %n",
+ arg_sz, reg_name, idx_reg_name, &scale, &len) == 4 ||
+ sscanf(arg_str, " %d @ %ld ( %%%15[^,] , %%%15[^)] ) %n",
+ arg_sz, &off, reg_name, idx_reg_name, &len) == 4 ||
+ sscanf(arg_str, " %d @ ( %%%15[^,] , %%%15[^)] ) %n",
+ arg_sz, reg_name, idx_reg_name, &len) == 3
+ ) {
+ /*
+ * Scale Index Base case:
+ * 1@-96(%rbp,%rax,8)
+ * 1@(%rbp,%rax,8)
+ * 1@-96(%rbp,%rax)
+ * 1@(%rbp,%rax)
+ */
+ arg->arg_type = USDT_ARG_SIB;
+ arg->val_off = off;
- if (sscanf(arg_str, " %d @ %ld ( %%%15[^)] ) %n", arg_sz, &off, reg_name, &len) == 3) {
+ 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;
+ /* validate scale factor and set fields directly */
+ if (scale != 1 && scale != 2 && scale != 4 && scale != 8) {
+ pr_warn("usdt: invalid SIB scale %d, expected 1, 2, 4, 8\n", scale);
+ return -EINVAL;
+ }
+ 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;
@@ -1306,6 +1352,7 @@ static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec
} else if (sscanf(arg_str, " %d @ %%%15s %n", arg_sz, reg_name, &len) == 2) {
/* Register read case, e.g., -4@...x */
arg->arg_type = USDT_ARG_REG;
+ /* register read has no memory offset */
arg->val_off = 0;
reg_off = calc_pt_regs_off(reg_name);
--
2.43.0
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