Re: [Qemu-devel] [PATCH] crypt: fix build with nettle >= 3.0.0

["Radim Krčmář" <rkrcmar@redhat.com>]

2015-07-10 14:38+0100, Peter Maydell:
> On 10 July 2015 at 14:31, Radim Krčmář <rkrcmar@redhat.com> wrote:
>> 2015-07-10 13:56+0100, Peter Maydell:
>>> On 10 July 2015 at 13:33, Radim Krčmář <rkrcmar@redhat.com> wrote:
>>>> @@ -83,8 +87,8 @@ QCryptoCipher *qcrypto_cipher_new(QCryptoCipherAlgorithm alg,
>>>> -        ctx->alg_encrypt = (nettle_crypt_func *)des_encrypt;
>>>> -        ctx->alg_decrypt = (nettle_crypt_func *)des_decrypt;
>>>> +        ctx->alg_encrypt = (nettle_cipher_func *)des_encrypt;
>>>> +        ctx->alg_decrypt = (nettle_cipher_func *)des_decrypt;
>>>> @@ -98,8 +102,8 @@ QCryptoCipher *qcrypto_cipher_new(QCryptoCipherAlgorithm alg,
>>>> -        ctx->alg_encrypt = (nettle_crypt_func *)aes_encrypt;
>>>> -        ctx->alg_decrypt = (nettle_crypt_func *)aes_decrypt;
>>>> +        ctx->alg_encrypt = (nettle_cipher_func *)aes_encrypt;
>>>> +        ctx->alg_decrypt = (nettle_cipher_func *)aes_decrypt;
>>>
>>> Why do we need the casts here at all?  If the functions
>>> we're passing around don't have the right signature
>>> anyway we're in big trouble and casting them is
>>> just going to hide the problem until runtime...
>>
>> Yes.
>>
>> We pass 'ctx' as a 'void *' in the code, but these functions accept
>> specialized structures, which makes them incompatible:
>>
>>   void nettle_cipher_func(const void *ctx, size_t length, [...])
>>
>>   void aes_decrypt(const struct aes_ctx *ctx, size_t length, [...])
>>   void des_decrypt(const struct des_ctx *ctx, size_t length, [...])
> 
> But aren't both the typedef and the aes/des_decrypt functions
> provided by the nettle library? Why is the library providing
> functions whose prototypes don't match its own typedef?

They are.  Authors needed to sacrifice something to fit into the type
system and I think they valued safety when using just a single cipher
above safety when mixing them ... (The decision was probably biased by
existing unabstracted code, if I were to guess how the library started.)