Re: [PATCH bpf-next v2 7/9] bpf: Prepare verifier logs for upcoming kfunc stack arguments

Yonghong Song
2026-04-22T23:09:38Z

On 4/22/26 2:58 PM, Alexei Starovoitov wrote:
> On Wed Apr 22, 2026 at 8:37 AM PDT, Yonghong Song wrote:
>>
>> On 4/22/26 7:57 AM, Yonghong Song wrote:
>>>
>>> On 4/21/26 11:25 PM, bot+bpf-ci@kernel.org wrote:
>>>>> diff --git a/include/linux/bpf_verifier.h
>>>>> b/include/linux/bpf_verifier.h
>>>>> index b148f816f25b..d5b4303315dd 100644
>>>>> --- a/include/linux/bpf_verifier.h
>>>>> +++ b/include/linux/bpf_verifier.h
>>>>> @@ -913,6 +913,7 @@ struct bpf_verifier_env {
>>>>>        * e.g., in reg_type_str() to generate reg_type string
>>>>>        */
>>>>>       char tmp_str_buf[TMP_STR_BUF_LEN];
>>>>> +    char tmp_arg_name[32];
>>>>>       struct bpf_insn insn_buf[INSN_BUF_SIZE];
>>>>>       struct bpf_insn epilogue_buf[INSN_BUF_SIZE];
>>>>>       struct bpf_scc_callchain callchain_buf;
>>>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>>>> index c777a294302a..e7f5f0bb499b 100644
>>>>> --- a/kernel/bpf/verifier.c
>>>>> +++ b/kernel/bpf/verifier.c
>>>>> @@ -261,6 +261,30 @@ struct bpf_kfunc_meta {
>>>>>
>>>>>   struct btf *btf_vmlinux;
>>>>>
>>>>> +typedef struct argno {
>>>>> +    int argno;
>>>>> +} argno_t;
>>>>> +
>>>>> +static argno_t argno_from_reg(u32 regno)
>>>>> +{
>>>>> +    return (argno_t){ .argno = regno };
>>>>> +}
>>>>> +
>>>>> +static argno_t argno_from_arg(u32 arg)
>>>>> +{
>>>>> +    return (argno_t){ .argno = -arg };
>>>>> +}
>>>>> +
>>>>> +static int reg_from_argno(argno_t a)
>>>>> +{
>>>>> +    return a.argno >= 0 ? a.argno : -1;
>>>>> +}
>>>>> +
>>>>> +static int arg_from_argno(argno_t a)
>>>>> +{
>>>>> +    return a.argno < 0 ? -a.argno : -1;
>>>>> +}
>>>> [ ... ]
>>>>
>>>>> @@ -7026,8 +7068,8 @@ static int check_helper_mem_access(struct
>>>>> bpf_verifier_env *env, struct bpf_reg_
>>>>>    */
>>>>>   static int check_mem_size_reg(struct bpf_verifier_env *env,
>>>>>                     struct bpf_reg_state *mem_reg,
>>>>> -                  struct bpf_reg_state *size_reg, u32 mem_regno,
>>>>> -                  u32 size_regno, enum bpf_access_type access_type,
>>>>> +                  struct bpf_reg_state *size_reg, argno_t mem_argno,
>>>>> +                  argno_t size_argno, enum bpf_access_type
>>>>> access_type,
>>>>>                     bool zero_size_allowed,
>>>>>                     struct bpf_call_arg_meta *meta)
>>>>>   {
>>>> [ ... ]
>>>>
>>>>> @@ -7052,31 +7094,31 @@ static int check_mem_size_reg(struct
>>>>> bpf_verifier_env *env,
>>>>>           meta = NULL;
>>>>>
>>>>>       if (size_reg->smin_value < 0) {
>>>>> -        verbose(env, "R%d min value is negative, either use
>>>>> unsigned or 'var &= const'\n",
>>>>> -            size_regno);
>>>>> +        verbose(env, "%s min value is negative, either use unsigned
>>>>> or 'var &= const'\n",
>>>>> +            reg_arg_name(env, size_argno));
>>>>>           return -EACCES;
>>>>>       }
>>>>>
>>>>>       if (size_reg->umin_value == 0 && !zero_size_allowed) {
>>>>> -        verbose(env, "R%d invalid zero-sized read: u64=[%lld,%lld]\n",
>>>>> -            size_regno, size_reg->umin_value, size_reg->umax_value);
>>>>> +        verbose(env, "%s invalid zero-sized read: u64=[%lld,%lld]\n",
>>>>> +            reg_arg_name(env, size_argno), size_reg->umin_value,
>>>>> size_reg->umax_value);
>>>>>           return -EACCES;
>>>>>       }
>>>>>
>>>>>       if (size_reg->umax_value >= BPF_MAX_VAR_SIZ) {
>>>>> -        verbose(env, "R%d unbounded memory access, use 'var &=
>>>>> const' or 'if (var < const)'\n",
>>>>> -            size_regno);
>>>>> +        verbose(env, "%s unbounded memory access, use 'var &=
>>>>> const' or 'if (var < const)'\n",
>>>>> +            reg_arg_name(env, size_argno));
>>>>>           return -EACCES;
>>>>>       }
>>>>> -    err = check_helper_mem_access(env, mem_reg, mem_regno,
>>>>> size_reg->umax_value,
>>>>> +    err = check_helper_mem_access(env, mem_reg, mem_argno,
>>>>> size_reg->umax_value,
>>>>>                         access_type, zero_size_allowed, meta);
>>>>>       if (!err)
>>>>> -        err = mark_chain_precision(env, size_regno);
>>>>> +        err = mark_chain_precision(env, reg_from_argno(size_argno));
>>>>>       return err;
>>>>>   }
>>>>                                       ^^^^^^^
>>>>
>>>> Does mark_chain_precision(env, reg_from_argno(size_argno)) properly
>>>> handle
>>>> kfunc size arguments?
>>>>
>>>> Before this patch, check_mem_size_reg() was called with size_regno
>>>> being a
>>>> u32 register number. For kfuncs with KF_ARG_PTR_TO_MEM_SIZE, the
>>>> pre-patch
>>>> call at check_kfunc_args was check_kfunc_mem_size_reg(env, reg,
>>>> size_reg,
>>>> regno, regno+1), passing real register numbers like R2..R5.
>>>>
>>>> After this patch, check_kfunc_args constructs size_argno via
>>>> argno_from_arg(i+2) for the size parameter of a KF_ARG_PTR_TO_MEM_SIZE
>>>> pair. The argno_from_arg() helper creates a negative encoding (-.argno).
>>>> When reg_from_argno(size_argno) is called, it checks a.argno >= 0 and
>>>> returns -1 for negative values.
>>>>
>>>> mark_chain_precision(env, -1) then reaches bpf_mark_chain_precision in
>>>> backtrack.c, where the "if (regno >= 0)" guard causes it to skip marking
>>>> the register, and the subsequent "if (bt_empty(bt)) return 0;" silently
>>>> returns success without doing any precision tracking.
>>>>
>>>> For helpers, this works correctly because check_func_arg calls
>>>> check_mem_size_reg with argno_from_reg(regno), which produces a
>>>> non-negative
>>>> encoding that reg_from_argno() correctly extracts back to the register
>>>> number.
>>>>
>>>> Can you verify whether the loss of precision tracking for kfunc size
>>>> registers might cause programs that previously verified to hit the
>>>> BPF_COMPLEXITY_LIMIT_INSNS threshold? Kfuncs with __sz/__szk size
>>>> arguments are widely used in-tree.
>>> This is indeed a problem.
>>> Function check_mem_size_reg() is the one which does
>>>    mark_chain_precision(env, reg_from_argno(size_argno))
>>>
>>>
>>> There are two callers for check_mem_size_reg():
>>>    - check_kfunc_mem_size_reg() <- check_kfunc_args
>>>      so arg_from_argno() should be used here to get the value
>>>    - check_func_arg() <- check_helper_call()
>>>      in this case, since helper_call is fixed at most 5 registers,
>>>      so argno_from_reg() is used in check_func_arg(), so later
>>>      on it should use reg_from_argno() should be used to get value.
>>>
>>> There are two options to fix:
>>>  1. do proper check like
>>>     int reg_val = reg_from_argno(size_regno);
>>>     if (reg_val >= 0)
>>>       mark_chain_precision(env, reg_val);
>>>     else
>>>       mark_chain_precision(env, arg_from_argno(size_regno));
>>>
>>> 2. for arguments in check_helper_call(), we also use
>>>    arg_from_argno() instead of reg_from_argno().
>>>    This way, we can just do
>>>       mark_chain_precision(env, arg_from_argno(size_regno));
>>>
>>> Not sure which is preferred or there are some other better alternatives.
>> The following should work.
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index e7f5f0bb499b..2ead411b949d 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -7112,8 +7112,12 @@ static int check_mem_size_reg(struct bpf_verifier_env *env,
>> }
>> err = check_helper_mem_access(env, mem_reg, mem_argno, size_reg->umax_value,
>> access_type, zero_size_allowed, meta);
>> - if (!err)
>> - err = mark_chain_precision(env, reg_from_argno(size_argno));
>> + if (!err) {
>> + int regno = reg_from_argno(size_argno);
>> +
>> + regno = regno >= 0 ? regno : arg_from_argno(size_argno);
>> + err = mark_chain_precision(env, regno);
>> + }
> Both options are not great and
> regno = arg >= 0 ? arg : reg_from_argno(argno);
> in reg_arg_name() points to the same issue.
>
> I think it should be:
> static int reg_from_argno(argno_t a)
> {
> if (a.argno >= 0)
> return a.argno;
> if (a.argno >= -MAX_BPF_FUNC_REG_ARGS)
> return -a.argno;
> return -1;
> }
>
> whether arg_from_argno() should do similar logic is something to think about.

I think the following arg_from_argno() should be okay:

static int arg_from_argno(argno_t a)
{
if (a.argno < 0)
return -a.argno;
return -1;
}

In some cases, there is a need to get an arg_idx which will be
used for an array element. e.g.,

process_iter_arg(..., argno, ...)
...
u32 arg_idx = arg_from_argno(argno) - 1;
...
btf_id = btf_check_iter_arg(meta->btf, meta->func_proto, arg_idx);

btf_check_iter_arg(..., arg_idx)
...
arg = &btf_params(func)[arg_idx];
...