Soundness issues in StaticMut

[ammaraskar]

Hi there, we (Rust group @sslab-gatech) are scanning crates on crates.io for potential soundness bugs. We noticed a few soundness issues with the StaticMut type.

1. There is an aliasing violation with as_mut

   stderr/src/static_mut.rs

   Lines 78 to 80 in 2fd8eb8

   	pub fn as_mut(&self) -> &mut T {
   	unsafe { self.0.get().as_mut().unwrap() }
   	}

2. It implements Sync for all types T, this should be restricted to T: Sync

   stderr/src/static_mut.rs

   Line 62 in 2fd8eb8

   unsafe impl<T> Sync for StaticMut<T> {}

We noticed you pointed out some of these issues in the documentation, maybe it would be better to mark these methods as unsafe or restrict the visibility of StaticMut, otherwise this allows for safety violations from Rust without using unsafe. For example:

#![forbid(unsafe_code)]

use stderr::StaticMut;

// A simple tagged union used to demonstrate problems with aliasing.
#[derive(Debug, Clone, Copy)]
enum RefOrInt { Ref(&'static u64), Int(u128) }

fn main() {
    let ptr = StaticMut::new(RefOrInt::Ref(&42));

    let mutable_ref_one = ptr.as_mut();
    let mutable_ref_two = ptr.as_mut();

    println!("Pointer points to: {:?}", mutable_ref_one);
    if let RefOrInt::Ref(ref addr) = mutable_ref_one {
        *mutable_ref_two = RefOrInt::Int(0xdeadbeef);

        println!("Pointer now points to: {:p}", *addr);
        println!("Dereferencing addr will now segfault: {}", **addr);
    }
}

causes:

Pointer points to: Ref(42)
Pointer now points to: 0xdeadbeef

Terminated with signal 11 (SIGSEGV)

[biluohc]

Thanks for your work, but this library should not be used anymore, because eprintln of the standard library has been stable for too long

[ammaraskar]

Thank you for the quick response, in that case I would recommend yanking all the crate's versions so no one uses it by accident :) https://doc.rust-lang.org/cargo/commands/cargo-yank.html

[biluohc]

Not for the time being, I still have some projects that rely on it. And StaticMut itself is not designed as RwLock, Mutex or Atomic