> On 11 Apr 2019, at 17:44, Jakub Kicinski <jakub.kicinski@xxxxxxxxxxxxx> wrote:
>
> On Thu, 11 Apr 2019 07:13:03 +0100, Jiong Wang wrote:
>>>> @@ -1150,17 +1150,17 @@ static int mark_reg_read(struct bpf_verifier_env *env,
>>>> parent->var_off.value, parent->off);
>>>> return -EFAULT;
>>>> }
>>>> - if (parent->live & REG_LIVE_READ)
>>>> + if ((parent->live & REG_LIVE_READ) == flags)
>>>> /* The parentage chain never changes and
>>>> - * this parent was already marked as LIVE_READ.
>>>> + * this parent was already marked with all read bits.
>>>
>>> Do we have to propagate all read bits? Read64 is strictly stronger
>>> than read32, as long as read64 is set on the parent we should be good?
>>
>> We should be good, but I doubt there is value to differentiate on this in this
>> kind of HOT function.
>
> The entire if clause is an optimization. I'm saying you can maintain it
> as more aggressive.
>
>>>> @@ -6196,12 +6286,19 @@ static int propagate_liveness_reg(struct bpf_verifier_env *env,
>>>> struct bpf_reg_state *reg,
>>>> struct bpf_reg_state *parent_reg)
>>>> {
>>>> + u8 parent_bits = parent_reg->live & REG_LIVE_READ;
>>>> + u8 bits = reg->live & REG_LIVE_READ;
>>>> + u8 bits_diff = parent_bits ^ bits;
>>>> + u8 bits_prop = bits_diff & bits;
>>>> int err;
>>>>
>>>> - if (parent_reg->live & REG_LIVE_READ || !(reg->live & REG_LIVE_READ))
>>>> + /* "reg" and "parent_reg" has the same read bits, or the bit doesn't
>>>> + * belong to "reg".
>>>> + */
>>>> + if (!bits_diff || !bits_prop)
>>>
>>> bits_prop is a subset of bits_diff, no? !bits_prop is always true
>>> if !bits_diff is true, no need to check both.
>>
>> Bits_prop is a subset of bits_diff WHEN it comes from “reg", we don’t want to
>> do the propagation when the diff comes from “parent_reg”, so, we need to check
>> both.
>
> Not sure what you're saying, in this patch:
>
> u8 bits_prop = bits_diff & bits;
>
> Maybe you're talking about some patch down the line..
Ack, indeed, !bits_prop is always true if !bits_diff is true, will remove the
redundant check.
Thanks,
Regards,
Jiong
