Miri doesn't understand C-variadic functions

[bugadani]

I have a program written in C and Rust. This program has a function that is variadic and written in C. To enable testing my Rust code, I needed to mock this function using the #[feature(c_variadic)] unstable feature.

This means I have a function in my test code defined like the following:

pub unsafe extern "C" fn variadic_fn(
        some_arg: *const c_char,
        mut args: ...
    ) {
...
}

When running Miri on my test code, I'm greeted with the following error:
error: Undefined Behavior: calling a function with argument of type std::ffi::VaListImpl passing data of type *const i8

[RalfJung]

Yeah, Miri probably does not support varargs at all currently. I assume this is another thing that is blocked on rust-lang/rust#56166.

If you could provide a self-contained example demonstrating the problem, that would be helpful as a testcase. :)

[bugadani]

Sure thing.

#![feature(c_variadic)]

use std::os::raw::c_char;

macro_rules! c_str {
    ($lit:expr) => {
        concat!($lit, "\0").as_ptr() as *const c_char
    };
}

#[allow(unused_unsafe)]
pub unsafe extern "C" fn variadic_fn(_: *const c_char, mut _args: ...) {
    todo!()
}

fn main() {
    unsafe {
        variadic_fn(c_str!("c string"), 1, 2, 3);
    }
}

[RalfJung]

FWIW, rust-lang/rust#91342 should help with this.

But actually implementing VaList and the associated functions to retrieve the arguments would probably still require a bunch of work, fiddling with many horrible platform-specific details.

[RalfJung]

To be clear, I don't currently have plans to work on those platform-specific parts -- it will be a lot of work for very little gain, as such Rust-to-Rust calls of c-variadic functions are exceedingly rare, to my knowledge.

An alternative for your usecase might be to use cfg(miri) for mocking, so you can replace the c-variadic calls with regular Rust functions when testing under Miri.

[bugadani]

No worries, no worries, as a workaround I banished 99% of my C code (so I could also remove the C-variadic workaround for my tests), so this isn't important to me any more :) (I guess I should have said this earlier? Sorry 😳)

[saethlin]

Just ran into this in the test suite of kvm-ioctls:

error: Undefined Behavior: incorrect number of arguments: got 2, expected 3
   --> src/ioctls/system.rs:71:28
    |
71  |         let ret = unsafe { open("/dev/kvm\0".as_ptr() as *const c_char, open_flags) };
    |                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ incorrect number of arguments: got 2, expected 3
    |
    = help: this indicates a bug in the program: it performed an invalid operation, and caused Undefined Behavior
    = help: see https://doc.rust-lang.org/nightly/reference/behavior-considered-undefined.html for further information
    = note: inside `ioctls::system::Kvm::open_with_cloexec` at src/ioctls/system.rs:71:28
note: inside `ioctls::system::Kvm::new` at src/ioctls/system.rs:42:18

[RalfJung]

No, that's a different bug -- this bug here is about calling C variadic functions that are defined in Rust, whereas you are calling an FFI function that Miri emulates via a shim. It looks like our implementation of open only supports the 3-argument form. Could you open a new issue for this?

[RalfJung]

Ah, a PR has already been opened. :D
#1970

[saethlin]

Could Miri report this as an unsupported operation, not UB? Would that close this issue?