netdev - Re: [PATCH net-next v3 1/3] rust: core abstractions for network PHY drivers

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Date: Sat, 14 Oct 2023 13:59:58 +0200
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: benno.lossin@...ton.me, boqun.feng@...il.com, tmgross@...ch.edu, 
	netdev@...r.kernel.org, rust-for-linux@...r.kernel.org, andrew@...n.ch, 
	greg@...ah.com
Subject: Re: [PATCH net-next v3 1/3] rust: core abstractions for network PHY drivers

On Fri, Oct 13, 2023 at 11:53 AM FUJITA Tomonori
<fujita.tomonori@...il.com> wrote:
>
> I'm not sure the general rules in Rust can be applied to linux kernel.

Benno and others already replied nicely to this, but I wanted to point
out that this happens with C compilers just the same. It is not a
"Rust thing" and what matters is what compilers do here, in practice.

For instance, you can try to compile this with GCC under -O2, and you
will get a program that returns a 2:

    int main(void) {
        _Bool b;
        char c = 42;
        memcpy(&b, &c, 1);
        if (b)
            return 43;
        return 44;
    }

Similarly, one for Rust where LLVM simply generates `ud2`:

    #[repr(u32)]
    pub enum E {
        A = 0,
        B = 1,
    }

    pub fn main() {
        let e = unsafe { core::mem::transmute::<u32, E>(5) };
        std::process::exit(match e {
            E::A => 42,
            E::B => 43,
        });
    }

The `e` variable is what we can get from C without an unsafe block if
you use `--rustified-enum`, i.e. the case in your abstractions.

The critical bit here is that, in C, it is not UB to have value 5 in
its enum, so we cannot rely on that.

Cheers,
Miguel