Re: C aggregate passing (Rust kernel policy)

Ventura Jack
2025-02-24T23:04:25Z

On Mon, Feb 24, 2025 at 3:03 PM Benno Lossin <benno.lossin@proton.me> wrote:
>
> On 24.02.25 17:57, Ventura Jack wrote:
> > One example I tested against MIRI:
> >
> > use std::cell::UnsafeCell;
> >
> > fn main() {
> >
> > let val: UnsafeCell<i32> = UnsafeCell::new(42);
> > let x: & UnsafeCell<i32> = &val;
> > let y: & UnsafeCell<i32> = &val;
> >
> > unsafe {
> >
> > // UB.
> > //let pz: & i32 = & *val.get();
> >
> > // UB.
> > //let pz: &mut i32 = &mut *val.get();
> >
> > // Okay.
> > //let pz: *const i32 = &raw const *val.get();
> >
> > // Okay.
> > let pz: *mut i32 = &raw mut *val.get();
> >
> > let px: *mut i32 = x.get();
> > let py: *mut i32 = y.get();
> >
> > *px = 0;
> > *py += 42;
> > *px += 24;
> >
> > println!("x, y, z: {}, {}, {}", *px, *py, *pz);
> > }
> > }
> >
> > It makes sense that the Rust "raw pointers" `*const i32` and `*mut
> > i32` are fine here, and that taking Rust "references" `& i32` and
> > `&mut i32` causes UB, since Rust "references" have aliasing rules that
> > must be followed.
>
> So it depends on what exactly you do, since if you just uncomment one of
> the "UB" lines, the variable never gets used and thus no actual UB
> happens. But if you were to do this:

I did actually test it against MIRI with only one line commented in at
a time, and the UB lines did give UB according to MIRI, I did not
explain that. It feels a lot like juggling with very sharp knives, but
I already knew that, because the Rust community generally does a great
job of warning people against unsafe. MIRI is very good, but it cannot
catch everything, so it cannot be relied upon in general. And MIRI
shares some of the advantages and disadvantages of sanitizers for C.

Best, VJ.