MbedTLS does not compile for projects with serde on i686-unknown-linux-gnu (32bits) targets

[PierrickGuitter]

When building a project with serde dependency for i686-unknown-linux-gnu target, the following error message appear:

error[E0512]: cannot transmute between types of different sizes, or dependently-sized types --> /home/xxxxx/.cargo/registry/src/index.crates.io-1949cf8c6b5b557f/mbedtls-0.12.5/src/cipher/raw/serde.rs:356:5 | 356 | ::core::mem::transmute(ctx) | ^^^^^^^^^^^^^^^^^^^^^^ | = note: source type: mbedtls_sys::gcm_context (3136 bits) = note: target type: [u8; 400] (3200 bits)

How to reproduce

• Create a new project with only the following dependencies in Cargo.toml:
  mbedtls = "0.13.2" serde = "1.0.219"

• Then simply compile with cargo build --target i686-unknown-linux-gnu

Troubleshoot

This seems to be an alignment error occurring for 32bits architecture.
I pinpointed it to the definition of _SIZE_OF_GCM_CONTEXT:

const _SIZE_OF_GCM_CONTEXT: usize = (_SIZE_OF_CIPHER_CONTEXT + 7) / 8 * 8 + 8 * 16 + 8 * 16 + 8 + 8 + 16 + 16 + 16 + 8; // first summand: cipher_context 8-byte aligned

IMO, there are 2 errors in it:

• As the comment suggests, _SIZE_OF_CIPHER_CONTEXT is 8 byte aligned. To be ported to 32 bits architecture, it should be aligned on size_of::<usize>().
• The very last operand (8) is the size of ::types::raw_types::c_int, which should also be size_of::<usize>().

Therefore, we have the following expression:
const _SIZE_OF_GCM_CONTEXT: usize = (_SIZE_OF_CIPHER_CONTEXT + size_of::<usize>() - 1) / size_of::<usize>() * size_of::<usize>() + 8 * 16 + 8 * 16 + 8 + 8 + 16 + 16 + 16 + size_of::<usize>();

I tested it on default target (64bits) and i686-unknown-linux-gnu, and it worked great.

[jethrogb]

c_int being the size of usize sounds suspicious. That's definitely not the case on 64-bit.

[PierrickGuitter]

Ok, you're right, i overlooked that.
However, does that last operand really embodies the size of c_int ?
It is fixed to 8 bytes, but in the documentation of std::os::raw, c_int is almost always as i32.

In fact, replacing the last operand by size_of::<mbedtls_sys::types::raw_types::c_int>() does not work for 64-bit.