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Message-ID: <5651317b.4d09.198a754d8bf.Coremail.phoenix500526@163.com>
Date: Thu, 14 Aug 2025 14:46:52 +0800 (CST)
From: 赵佳炜 <phoenix500526@....com>
To: "Andrii Nakryiko" <andrii.nakryiko@...il.com>
Cc: ast@...nel.org, 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: Re:Re: [PATCH bpf-next v8 1/2] libbpf: fix USDT SIB argument
handling causing unrecognized register error
>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?
That's a good idea. I'll modify it in the new patch.
At 2025-08-14 07:52:47, "Andrii Nakryiko" <andrii.nakryiko@...il.com> wrote:
>On Wed, Aug 6, 2025 at 7:35 PM Jiawei Zhao <phoenix500526@....com> 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@....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;
>
>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 = 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);
>
>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__ == __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 ||
>
>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) == 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)
>
>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 = 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) {
>> /* Memory dereference case, e.g., -4@-20(%rbp) */
>> arg->arg_type = USDT_ARG_REG_DEREF;
>> arg->val_off = 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) == 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;
>> reg_off = 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) == 2) {
>> /* Register read case, e.g., -4@...x */
>> arg->arg_type = USDT_ARG_REG;
>> - arg->val_off = 0;
>> + arg->val_off = off;
>
>why this change? it makes it seem like val_off might not be zero, for
>no good reason...
>
>>
>> reg_off = calc_pt_regs_off(reg_name);
>> if (reg_off < 0)
>> --
>> 2.43.0
>>
>>
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