linux-kernel - Re: [PATCH 1/2] dm crypt: replaced #if defined with IS

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Message-ID: <6bf5f23d-ff89-7e03-a226-d23959f34abb@pengutronix.de>
Date:   Tue, 2 Feb 2021 19:19:39 +0100
From:   Ahmad Fatoum <a.fatoum@...gutronix.de>
To:     Mike Snitzer <snitzer@...hat.com>
Cc:     Arnd Bergmann <arnd@...db.de>,
        Dmitry Baryshkov <dbaryshkov@...il.com>,
        linux-kernel@...r.kernel.org, dm-devel@...hat.com,
        kernel@...gutronix.de, Alasdair Kergon <agk@...hat.com>
Subject: Re: [PATCH 1/2] dm crypt: replaced #if defined with IS_ENABLED

Hello Mike,

On 02.02.21 19:10, Mike Snitzer wrote:
> On Fri, Jan 22 2021 at  3:43am -0500,
> Ahmad Fatoum <a.fatoum@...gutronix.de> wrote:
> 
>> IS_ENABLED(CONFIG_ENCRYPTED_KEYS) is true whether the option is built-in
>> or a module, so use it instead of #if defined checking for each
>> separately.
>>
>> The other #if was to avoid a static function defined, but unused
>> warning. As we now always build the callsite when the function
>> is defined, we can remove that first #if guard.
>>
>> Suggested-by: Arnd Bergmann <arnd@...db.de>
>> Signed-off-by: Ahmad Fatoum <a.fatoum@...gutronix.de>
>> ---
>> Cc: Dmitry Baryshkov <dbaryshkov@...il.com>
>> ---
>>  drivers/md/dm-crypt.c | 7 ++-----
>>  1 file changed, 2 insertions(+), 5 deletions(-)
>>
>> diff --git a/drivers/md/dm-crypt.c b/drivers/md/dm-crypt.c
>> index 8c874710f0bc..7eeb9248eda5 100644
>> --- a/drivers/md/dm-crypt.c
>> +++ b/drivers/md/dm-crypt.c
>> @@ -2436,7 +2436,6 @@ static int set_key_user(struct crypt_config *cc, struct key *key)
>>  	return 0;
>>  }
>>  
>> -#if defined(CONFIG_ENCRYPTED_KEYS) || defined(CONFIG_ENCRYPTED_KEYS_MODULE)
>>  static int set_key_encrypted(struct crypt_config *cc, struct key *key)
>>  {
>>  	const struct encrypted_key_payload *ekp;
>> @@ -2452,7 +2451,6 @@ static int set_key_encrypted(struct crypt_config *cc, struct key *key)
>>  
>>  	return 0;
>>  }
>> -#endif /* CONFIG_ENCRYPTED_KEYS */
>>  
>>  static int crypt_set_keyring_key(struct crypt_config *cc, const char *key_string)
>>  {
>> @@ -2482,11 +2480,10 @@ static int crypt_set_keyring_key(struct crypt_config *cc, const char *key_string
>>  	} else if (!strncmp(key_string, "user:", key_desc - key_string + 1)) {
>>  		type = &key_type_user;
>>  		set_key = set_key_user;
>> -#if defined(CONFIG_ENCRYPTED_KEYS) || defined(CONFIG_ENCRYPTED_KEYS_MODULE)
>> -	} else if (!strncmp(key_string, "encrypted:", key_desc - key_string + 1)) {
>> +	} else if (IS_ENABLED(CONFIG_ENCRYPTED_KEYS) &&
>> +		   !strncmp(key_string, "encrypted:", key_desc - key_string + 1)) {
>>  		type = &key_type_encrypted;
>>  		set_key = set_key_encrypted;
>> -#endif
>>  	} else {
>>  		return -EINVAL;
>>  	}
>> -- 
>> 2.30.0
>>
> 
> I could be mistaken but the point of the previous way used to enable
> this code was to not compile the code at all.  How you have it, the
> branch just isn't taken but the compiled code is left to bloat dm-crypt.
> 
> Why not leave this as is and follow same pattern in your next patch?

It's safe to assume the compiler will constant-fold the resulting if (0) away.
The kernel coding style documentation got a section on that:
https://www.kernel.org/doc/html/v5.11-rc6/process/coding-style.html#conditional-compilation

Cheers,
Ahmad

> 
> Mike
> 
> 
> 

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