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

[Peter Maydell]

On 10 July 2015 at 14:38, Peter Maydell <address@hidden> wrote:
> On 10 July 2015 at 14:31, Radim Krčmář <address@hidden> wrote:
>> 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?

I had a suspicion that this was undefined behaviour,
and I was right:

http://stackoverflow.com/questions/559581/casting-a-function-pointer-to-another-type/14044244#14044244

If you have a function that takes "const struct foo *f"
but the library code is calling it using a function pointer
whose type says it's "const void *", then you can't just cast
the function pointer before handing it to the library.
You need to provide the obvious wrapper:

void aes_decrypt_wrapper(const void *ctx, size_t length, ...)
{
    aes_decrypt(ctx, length, ...);
}

-- PMM