Subject: Re: [PATCH v3 bpf-next 05/19] bpf: split read liveness into REG_LIVE_READ64 and REG_LIVE_READ32

From: Jiong Wang <[email-masked]>
Date: Sat, 13 Apr 2019 07:39:45 +0100

Jakub Kicinski writes:

> On Fri, 12 Apr 2019 22:59:38 +0100, Jiong Wang wrote:
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index c722015..3c5ca00 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -1135,7 +1135,7 @@ static int check_subprogs(struct bpf_verifier_env *env)
>>   */
>>  static int mark_reg_read(struct bpf_verifier_env *env,
>>  			 const struct bpf_reg_state *state,
>> -			 struct bpf_reg_state *parent)
>> +			 struct bpf_reg_state *parent, u8 flags)
>>  {
>>  	bool writes = parent == state->parent; /* Observe write marks */
>>  	int cnt = 0;
>> @@ -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.
>
> No big deal, but I though said you'd modify this patch here...

Ouch, sorry, I created one internal branch before start the test
changes. Looks like the branch is v10 which listed before v2~v9 that
somehow later I switched v9 for the test changing thought it is the latest
branch.

Regards,
Jiong

>
>>  			 * There is no need to keep walking the chain again and
>> -			 * keep re-marking all parents as LIVE_READ.
>> +			 * keep re-marking all parents with reads bits in flags.
>>  			 * This case happens when the same register is read
>>  			 * multiple times without writes into it in-between.
>>  			 */
>>  			break;
>>  		/* ... then we depend on parent's value */
>> -		parent->live |= REG_LIVE_READ;
>> +		parent->live |= flags;
>>  		state = parent;
>>  		parent = state->parent;
>>  		writes = true;
>
>> @@ -6227,12 +6317,19 @@ 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)
>>  		return 0;
>
> .. and here?