Re: [PATCH 2/4] dm crypt: Fix zoned block device support

[Johannes Thumshirn <Johannes.Thumshirn@wdc.com>]

On 16/04/2021 09:30, Damien Le Moal wrote:
> On 2021/04/16 16:13, Johannes Thumshirn wrote:
>> On 16/04/2021 05:05, Damien Le Moal wrote:
>>
>> [...]
>>
>>> +	CRYPT_IV_NO_SECTORS,		/* IV calculation does not use sectors */
>>
>> [...]
>>
>>> -	if (ivmode == NULL)
>>> +	if (ivmode == NULL) {
>>>  		cc->iv_gen_ops = NULL;
>>> -	else if (strcmp(ivmode, "plain") == 0)
>>> +		set_bit(CRYPT_IV_NO_SECTORS, &cc->cipher_flags);
>>> +	} else if (strcmp(ivmode, "plain") == 0)
>>
>> [...]
>>
>>> +		if (!test_bit(CRYPT_IV_NO_SECTORS, &cc->cipher_flags)) {
>>> +			DMWARN("Zone append is not supported with sector-based IV cyphers");
>>> +			ti->zone_append_not_supported = true;
>>> +		}
>>
>> I think this negation is hard to follow, at least I had a hard time
>> reviewing it.
>>
>> Wouldn't it make more sense to use CRYPT_IV_USE_SECTORS, set the bit
>> for algorithms that use sectors as IV (like plain64) and then do a 
>> normal
> 
> There are only 2 IV modes that do not use sectors. null and random. All others
> do. Hence the "NO_SECTORS" choice. That is the exception rather than the norm,
> the flag indicates that.
> 
>>
>> 	if (test_bit(CRYPT_IV_USE_SECTORS, &cc->cipher_flags)) {
>> 		DMWARN("Zone append is not supported with sector-based IV cyphers");
>> 		ti->zone_append_not_supported = true;
>> 	}
>>
>> i.e. without the double negation?
> 
> Yes. I agree, it is more readable. But adds more lines for the same result. I
> could add a small boolean helper to make the "!test_bit(CRYPT_IV_NO_SECTORS,
> &cc->cipher_flags)" easier to understand.
> 

Yes I guessed this was the reason for the choice.
Maybe 

set_bit(CRYPT_IV_USE_SECTORS, &cc->cipher_flags);

if (!strcmp(ivmode, "plain") || !strcmp(ivmode, "random"))
	clear_bit(CRYPT_IV_USE_SECTORS, &cc->cipher_flags);

if (test_bit(CRYPT_IV_USE_SECTORS, &cc->cipher_flags)) {
	DMWARN("Zone append is not supported with sector-based IV cyphers");
	ti->zone_append_not_supported = true;
}


Ultimately it's your and Mikes's call, but I /think/ this makes the code easier
to understand.