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@163.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20250827053128.1301287-2-phoenix500526@163.com

diff --git a/tools/lib/bpf/usdt.bpf.h b/tools/lib/bpf/usdt.bpf.h
index 2a7865c8e3fe3cbc97e033328b5b93db5a1b9d42..43deb05a51970d98072cdd05dafe36b4f23d2342 100644 (file)
--- 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_bitshift: 4;
+       /* reserved for future use, keeps reg_off offset stable */
+       __u8 __reserved: 8;
+#else
+       __u8 __reserved: 8;
+       __u16 idx_reg_off: 12;
+       __u16 scale_bitshift: 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_bitshift) + 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 3373b9d45ac4482333cdf7a14cf12cb13ed6e540..867bff6b06990e6aba7acd05d9f10acec0d119fb 100644 (file)
--- 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_bitshift: 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_bitshift: 4;
+       enum usdt_arg_type arg_type: 8;
+#endif
        short reg_off;
        bool arg_signed;
        char arg_bitshift;
@@ -1283,11 +1294,51 @@ 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;
+               arg->idx_reg_off = idx_reg_off;
+
+               /* validate scale factor and set fields directly */
+               switch (scale) {
+               case 1: arg->scale_bitshift = 0; break;
+               case 2: arg->scale_bitshift = 1; break;
+               case 4: arg->scale_bitshift = 2; break;
+               case 8: arg->scale_bitshift = 3; break;
+               default:
+                       pr_warn("usdt: invalid SIB scale %d, expected 1, 2, 4, 8\n", scale);
+                       return -EINVAL;
+               }
+       } 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 +1357,7 @@ 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;
+               /* register read has no memory offset */
                arg->val_off = 0;
 
                reg_off = calc_pt_regs_off(reg_name);