netdev - Re: [PATCH 0/5] Rust abstractions for network device drivers

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Message-ID: <CANiq72n91z7heDU5MDk_=jYY8h8VJsfboevmFS0vrD-zQCKq5Q@mail.gmail.com>
Date: Tue, 20 Jun 2023 19:44:27 +0200
From: Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
To: Alice Ryhl <alice@...l.io>, Gary Guo <gary@...yguo.net>
Cc: Jakub Kicinski <kuba@...nel.org>, Andrew Lunn <andrew@...n.ch>, 
	FUJITA Tomonori <fujita.tomonori@...il.com>, netdev@...r.kernel.org, 
	rust-for-linux@...r.kernel.org, aliceryhl@...gle.com
Subject: Re: [PATCH 0/5] Rust abstractions for network device drivers

On Tue, Jun 20, 2023 at 6:55 PM Alice Ryhl <alice@...l.io> wrote:
>
> We could probably have the destructor call some method that the linker
> wont be able to find, then mark the destructor with #[inline] so that
> dead-code elimination removes it if unused.
>
> (At least, in godbolt the inline marker was necessary for the destructor
> to get removed when not used.)

Yeah, and we could use `build_assert!(false);` to ensure we don't ever
call it (by users, or by the the custom destructor methods) -- it
seems to work [1], but I am Cc'ing Gary in case he has some concerns
(IIRC we discussed this in the past; I may be forgetting an issue with
this).

Cheers,
Miguel

[1]

diff --git a/rust/kernel/lib.rs b/rust/kernel/lib.rs
index c20b37e88ab2..7c313c75ff14 100644
--- a/rust/kernel/lib.rs
+++ b/rust/kernel/lib.rs
@@ -261,3 +261,28 @@ fn panic(info: &core::panic::PanicInfo<'_>) -> ! {
     // instead of `!`. See
<https://github.com/rust-lang/rust-bindgen/issues/2094>.
     loop {}
 }
+
+/// Custom destructor example.
+pub struct S;
+
+impl S {
+    /// Custom destructor 1.
+    pub fn close1(self) {
+        pr_info!("close1");
+        core::mem::forget(self); // If commented out, it is a build error.
+    }
+
+    /// Custom destructor 2.
+    pub fn close2(self) {
+        pr_info!("close2");
+        core::mem::forget(self); // If commented out, it is a build error.
+    }
+}
+
+impl Drop for S {
+    #[inline(always)]
+    fn drop(&mut self) {
+        // This should never be called.
+        build_assert!(false);
+    }
+}
diff --git a/samples/rust/rust_minimal.rs b/samples/rust/rust_minimal.rs
index 54de32b78cec..f07064ff6341 100644
--- a/samples/rust/rust_minimal.rs
+++ b/samples/rust/rust_minimal.rs
@@ -3,6 +3,7 @@
 //! Rust minimal sample.

 use kernel::prelude::*;
+use kernel::S;

 module! {
     type: RustMinimal,
@@ -26,6 +27,12 @@ impl kernel::Module for RustMinimal {
         numbers.try_push(108)?;
         numbers.try_push(200)?;

+        let _s = S;
+        _s.close1(); // If commented out, it is a build error.
+
+        let _s = S;
+        _s.close2(); // If commented out, it is a build error.
+
         Ok(RustMinimal { numbers })
     }
 }