[PATCH bpf-next v3 07/11] bpf: Fix a false rejection caused by AND operation

[Xu Kuohai]

On 4/30/2024 6:18 AM, Eduard Zingerman wrote:
> On Mon, 2024-04-29 at 13:58 -0700, Andrii Nakryiko wrote:
> 
> [...]
> 
>>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>>> index 8f0f2e21699e..b69c89bc5cfc 100644
>>> --- a/kernel/bpf/verifier.c
>>> +++ b/kernel/bpf/verifier.c
>>> @@ -13478,6 +13478,28 @@ static void scalar32_min_max_and(struct bpf_reg_state *dst_reg,
>>>                   return;
>>>           }
>>>
>>> +       /* Special case: dst_reg is in range [-1, 0] */
>>> +       if (dst_reg->s32_min_value == -1 && dst_reg->s32_max_value == 0) {
>>> +               var32_off = tnum_union(src_reg->var_off, tnum_const(0));
>>> +               dst_reg->var_off = tnum_with_subreg(dst_reg->var_off, var32_off);
>>> +               dst_reg->u32_min_value = var32_off.value;
>>> +               dst_reg->u32_max_value = min(dst_reg->u32_max_value, umax_val);
>>
>> can you explain the logic behing u32 min/max updates, especially that
>> we use completely different values for min/max and it's not clear why
>> u32_min <= u32_max invariant will always hold. Same below
> 
> I agree with Andrii here.
> It appears that dst_reg.{min,max} fields should be set as
> {min(src.min, 0), max(src.max, 0)} for both signed and unsigned cases.
> Wdyt?
>

Agree, since 0 is the minimum unsigned number, the result range is
equal to [0, src.u32_max].

>>
>>> +               dst_reg->s32_min_value = min_t(s32, src_reg->s32_min_value, 0);
>>> +               dst_reg->s32_max_value = max_t(s32, src_reg->s32_max_value, 0);
>>> +               return;
>>> +       }
>>> +
>>> +       /* Special case: src_reg is in range [-1, 0] */
>>> +       if (src_reg->s32_min_value == -1 && src_reg->s32_max_value == 0) {
>>> +               var32_off = tnum_union(dst_reg->var_off, tnum_const(0));
>>> +               dst_reg->var_off = tnum_with_subreg(dst_reg->var_off, var32_off);
>>> +               dst_reg->u32_min_value = var32_off.value;
>>> +               dst_reg->u32_max_value = min(dst_reg->u32_max_value, umax_val);
>>> +               dst_reg->s32_min_value = min_t(s32, dst_reg->s32_min_value, 0);
>>> +               dst_reg->s32_max_value = max_t(s32, dst_reg->s32_max_value, 0);
>>> +               return;
>>> +       }
>>> +
>>>           /* We get our minimum from the var_off, since that's inherently
>>>            * bitwise.  Our maximum is the minimum of the operands' maxima.
>>>            */
> 
> [...]