Incorrect FFI binding

[icmccorm]

I've been experimenting with a version of Miri that can execute foreign functions by interpreting the LLVM bytecode that is produced during a crate's build process. We're hoping that our results can assist with the Krabcake project.

Miri reports the following error when executing the test x25519::tests::diffie_hellman_test:

error: LLI interoperation error: LLVM field width mismatch: cannot convert an LLVM field of width 1 to a Rust field of type `i32` which has width 4
   --> src/x25519.rs:65:13
    |
65  |             );
    |             ^ LLVM field width mismatch: cannot convert an LLVM field of width 1 to a Rust field of type `i32` which has width 4
    |
    = note: inside `x25519::SecretKey::diffie_hellman` at src/x25519.rs:65:13: 65:14
note: inside closure
   --> src/x25519.rs:117:17
    |
117 |                 sk1.diffie_hellman(&pk2).as_bytes(),
    |                 ^^^^^^^^^^^^^^^^^^^^^^^^

It seems like this function is declared in Rust to return a 32-bit integer, but the C definition returns an 8-bit boolean value:

bool EverCrypt_Curve25519_ecdh(uint8_t *shared, uint8_t *my_priv, uint8_t *their_pub)
{
...
}