On Sat, Aug 02, 2025 at 08:48:02AM +0000, 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.
Signed-off-by: Jiawei Zhao phoenix500526@163.com
tools/lib/bpf/usdt.bpf.h | 33 +++++++++++++++++++++++++++++- tools/lib/bpf/usdt.c | 43 ++++++++++++++++++++++++++++++++++------ 2 files changed, 69 insertions(+), 7 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..1f8b9e1c9819 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 +1286,39 @@ 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, e.g., 1@-96(%rbp,%rax,8)* 1@(%rbp,%rax,8)* 1@-96(%rbp,%rax)* 1@(%rbp,%rax)*/
hi, I'm getting following error from the test:
subtest_multispec_usdt:PASS:skel_open 0 nsec libbpf: usdt: unrecognized arg #10 spec '-2@nums(%rax,%rax) -1@$-127' libbpf: prog 'usdt12': failed to auto-attach: -EINVAL subtest_multispec_usdt:FAIL:skel_attach unexpected error: -22 (errno 22) #480/2 usdt/multispec:FAIL
arguments look like: Arguments: -4@$3 -4@$4 -8@$42 -8@$45 -4@$5 -8@$6 8@%rdx 8@%rsi -4@$-9 -2@%cx -2@nums(%rax,%rax) -1@$-127
not sure why there's variable name in the arg10 definition
gcc (GCC) 15.1.1 20250521 (Red Hat 15.1.1-2) clang version 20.1.8 (Fedora 20.1.8-3.fc42)
thanks, jirka
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;
- if (sscanf(arg_str, " %d @ %ld ( %%%15[^)] ) %n", arg_sz, &off, reg_name, &len) == 3) {
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) {@@ -1298,7 +1329,7 @@ static int parse_usdt_arg(const char *arg_str, int arg_num, struct usdt_arg_spec } else if (sscanf(arg_str, " %d @ ( %%%15[^)] ) %n", arg_sz, reg_name, &len) == 2) { /* Memory dereference case without offset, e.g., 8@(%rsp) */ arg->arg_type = USDT_ARG_REG_DEREF;
arg->val_off = 0;
arg->val_off = off;@@ -1306,7 +1337,7 @@ 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@%eax */ arg->arg_type = USDT_ARG_REG;
arg->val_off = 0;
arg->val_off = off;reg_off = calc_pt_regs_off(reg_name); if (reg_off < 0) -- 2.43.0