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Date: Fri, 10 Jul 2015 15:31:31 +0200
From: Radim =?utf-8?B?S3LEjW3DocWZ?= <rkrcmar@redhat.com>
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Subject: Re: [Qemu-devel] [PATCH] crypt: fix build with nettle >= 3.0.0
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To: Peter Maydell <peter.maydell@linaro.org>
Cc: QEMU Developers <qemu-devel@nongnu.org>

2015-07-10 13:56+0100, Peter Maydell:
> On 10 July 2015 at 13:33, Radim Kr=C4=8Dm=C3=A1=C5=99 <rkrcmar@redhat.c=
om> wrote:
>> @@ -83,8 +87,8 @@ QCryptoCipher *qcrypto_cipher_new(QCryptoCipherAlgor=
ithm alg,
>> -        ctx->alg_encrypt =3D (nettle_crypt_func *)des_encrypt;
>> -        ctx->alg_decrypt =3D (nettle_crypt_func *)des_decrypt;
>> +        ctx->alg_encrypt =3D (nettle_cipher_func *)des_encrypt;
>> +        ctx->alg_decrypt =3D (nettle_cipher_func *)des_decrypt;
>> @@ -98,8 +102,8 @@ QCryptoCipher *qcrypto_cipher_new(QCryptoCipherAlgo=
rithm alg,
>> -        ctx->alg_encrypt =3D (nettle_crypt_func *)aes_encrypt;
>> -        ctx->alg_decrypt =3D (nettle_crypt_func *)aes_decrypt;
>> +        ctx->alg_encrypt =3D (nettle_cipher_func *)aes_encrypt;
>> +        ctx->alg_decrypt =3D (nettle_cipher_func *)aes_decrypt;
>=20
> 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, [...])

We could take make struct QCryptoCipherNettle into an aes/des union
(it's already tagged by QCryptoCipher->mode) to make coding a bit safer,
but C types can't guarantee everything.  Also, cbc_encrypt() takes
nettle_cipher_func, so we need to directly cast at some point.