It should be unsafe to cast an extern function to a function pointer

[matthewjasper]

The following code gives a linker error, which arguably shouldn't happen in safe code.

extern {
    fn doesnt_exist();
}
fn main() {
    println!("{:p}", doesnt_exist as unsafe extern "C" fn());
}

Playground

[hanna-kruppe]

How is this unsound? Linker errors aren't great, but how do they jeopardize memory safety in any way?

[matthewjasper]

Closing as a dupe of #46188 (which shows how this is a soundness issue) or not

[hanna-kruppe]

That issue has nothing to do with linker errors. The only common factor is that it involves an extern declaration.

[hanna-kruppe]

Ok so apparently we agree it's a different issue from #46188, which means my original question still stands: how is this a soundness issue?

[anirudhb]

This seems like a linker problem to me; Rust has no way to fix this without something like a non-executable/null pointer check, which would have performance impacts on casting. For all rustc knows, when the code is linked, it could be referring to a global static, but only the linker knows that.

edit: on systems where NX is supported, tbh this really isn't a soundness issue.

[Centril]

@matthewjasper any thoughts on #55226 (comment)?

[Mark-Simulacrum]

To my knowledge this is essentially unfixable without implementing a linker ourselves or somehow catching the linker errors and re-displaying them in a nicer way, both of which seem impossible or out of scope, so I'm going to close this. I agree with folks that this is not a soundness issue.