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charset="UTF-8" Content-Transfer-Encoding: quoted-printable On Wed, Aug 6, 2025 at 7:35=E2=80=AFPM 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 > --- > 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; I'd really prefer not to increase the size of __bpf_usdt_arg_spec and not change its layout for all existing BPF_USDT_ARG_* modes just to not have to worry about any backwards/forward compatibility issues. Scale can be 1, 2,4, 8, is that right? Instead of using 2 bytes for it, we should be able to use just 2 bits to represent bit shift (0, 1, 2, 3 should be enough). We can carve out at least 3 bytes by making arg_type field into packed single-byte enum (we'd need to be careful with big endian). Then we can add idx_reg_off:12 and idx_scale_shift:4 somewhere between arg_type and reg_off, taking 2 bytes in total. We'll still be left with one byte to spare for the future (and there are tricks we can do with arg_signed and arg_bitshift, but I'd not touch them yet). WDYT? pw-bot: cr > /* 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 =3D 0; > @@ -202,6 +207,32 @@ int bpf_usdt_arg(struct pt_regs *ctx, __u64 arg_num,= long *res) > return err; > #if __BYTE_ORDER__ =3D=3D __ORDER_BIG_ENDIAN__ > val >>=3D arg_spec->arg_bitshift; > +#endif > + break; > + case BPF_USDT_ARG_SIB: > + /* Arg is in memory addressed by SIB (Scale-Index-Base) m= ode > + * (e.g., "-1@-96(%rbp,%rax,8)" in USDT arg spec). Regist= er > + * is identified like with BPF_USDT_ARG_SIB case, the off= set > + * is in arg_spec->val_off, the scale factor is in arg_sp= ec->scale. > + * Firstly, we fetch the base register contents and the i= ndex > + * 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 =3D bpf_probe_read_kernel(&val, sizeof(val), (void *)= ctx + arg_spec->reg_off); > + if (err) > + return err; > + err =3D bpf_probe_read_kernel(&idx, sizeof(idx), (void *)= ctx + arg_spec->idx_reg_off); > + if (err) > + return err; > + err =3D bpf_probe_read_user(&val, sizeof(val), > + (void *)val + idx * arg_spec->scale + arg= _spec->val_off); it might be just how gmail renders it, but please make sure that wrapped argument is aligned with first argument on the previous line > + if (err) > + return err; > +#if __BYTE_ORDER__ =3D=3D __ORDER_BIG_ENDIAN__ > + val >>=3D 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] =3D {0}, idx_reg_name[16] =3D {0}; > + int len, reg_off, idx_reg_off, scale =3D 1; > + long off =3D 0; > + > + if (sscanf(arg_str, " %d @ %ld ( %%%15[^,] , %%%15[^,] , %d ) %n"= , > + arg_sz, &off, reg_name, idx_reg_name, &sc= ale, &len) =3D=3D 5 || see comment above about aligning wrapped argument list > + sscanf(arg_str, " %d @ ( %%%15[^,] , %%%15[^,] , %d ) %n"= , > + arg_sz, reg_name, idx_reg_name, &scale, &= len) =3D=3D 4 || > + sscanf(arg_str, " %d @ %ld ( %%%15[^,] , %%%15[^)] ) %n", > + arg_sz, &off, reg_name, idx_reg_name, &le= n) =3D=3D 4 || > + sscanf(arg_str, " %d @ ( %%%15[^,] , %%%15[^)] ) %n", > + arg_sz, reg_name, idx_reg_name, &len) =3D= =3D 3 > + ) { > + /* Scale Index Base case, e.g., 1@-96(%rbp,%rax,8) > + * 1@(%rbp,%rax,8) > + * 1@-96(%rbp,%rax) > + * 1@(%rbp,%rax) nit: let's list all variants at the same indentation level (and let's use the more standard multi-level comment format) /* * Scale-Index-Base case: * - 1@-96(%rbp,%rax,8) * - 1@(%rbp,%rax,8) * ... */ > + */ > + arg->arg_type =3D USDT_ARG_SIB; > + arg->val_off =3D off; > + arg->scale =3D scale; > + > + reg_off =3D calc_pt_regs_off(reg_name); > + if (reg_off < 0) > + return reg_off; > + arg->reg_off =3D reg_off; > > - if (sscanf(arg_str, " %d @ %ld ( %%%15[^)] ) %n", arg_sz, &off, r= eg_name, &len) =3D=3D 3) { > + idx_reg_off =3D calc_pt_regs_off(idx_reg_name); > + if (idx_reg_off < 0) > + return idx_reg_off; > + arg->idx_reg_off =3D idx_reg_off; > + } else if (sscanf(arg_str, " %d @ %ld ( %%%15[^)] ) %n", > + arg_sz, &off, reg_name, &len) =3D=3D 3) { > /* Memory dereference case, e.g., -4@-20(%rbp) */ > arg->arg_type =3D USDT_ARG_REG_DEREF; > arg->val_off =3D off; > @@ -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) =3D=3D 2) { > /* Memory dereference case without offset, e.g., 8@(%rsp)= */ > arg->arg_type =3D USDT_ARG_REG_DEREF; > - arg->val_off =3D 0; > + arg->val_off =3D off; > reg_off =3D calc_pt_regs_off(reg_name); > if (reg_off < 0) > return reg_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) =3D=3D 2) { > /* Register read case, e.g., -4@%eax */ > arg->arg_type =3D USDT_ARG_REG; > - arg->val_off =3D 0; > + arg->val_off =3D off; why this change? it makes it seem like val_off might not be zero, for no good reason... > > reg_off =3D calc_pt_regs_off(reg_name); > if (reg_off < 0) > -- > 2.43.0 > >