linux-kernel - Re: [PATCH v3 5/5] rust: Add warn_on and warn_on

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Message-ID: <Z8gOkeJ8o2pMCwGu@google.com>
Date: Wed, 5 Mar 2025 08:42:57 +0000
From: Alice Ryhl <aliceryhl@...gle.com>
To: FUJITA Tomonori <fujita.tomonori@...il.com>
Cc: linux-kernel@...r.kernel.org, rust-for-linux@...r.kernel.org, 
	x86@...nel.org, linux-riscv@...ts.infradead.org, 
	linux-arm-kernel@...ts.infradead.org, loongarch@...ts.linux.dev, 
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	catalin.marinas@....com, will@...nel.org, chenhuacai@...nel.org, 
	kernel@...0n.name, tangyouling@...ngson.cn, hejinyang@...ngson.cn, 
	yangtiezhu@...ngson.cn, ojeda@...nel.org, alex.gaynor@...il.com, 
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	benno.lossin@...ton.me, a.hindborg@...nel.org, tmgross@...ch.edu
Subject: Re: [PATCH v3 5/5] rust: Add warn_on and warn_on_once

On Thu, Feb 13, 2025 at 10:57:59PM +0900, FUJITA Tomonori wrote:
> Add warn_on and warn_on_once macros. Wrapping the C's WARN_* and BUG_*
> macros doesn't work so this uses the assembly code exported by the C
> side via ARCH_WARN_ASM macro. Like the static branch code, this
> generates the assembly code for rust at compile time by using the C
> preprocessor.
> 
> file()! macro doesn't work for the Rust inline assembly in the same
> way as __FILE__ for the C inline assembly. So the code to handle a
> file name is different from the C assembly code (similar to the
> arm64/loongarch assembly).

Nit: Should be file!() not file()!.

> diff --git a/rust/kernel/.gitignore b/rust/kernel/.gitignore
> index 6ba39a178f30..f1d7f4225332 100644
> --- a/rust/kernel/.gitignore
> +++ b/rust/kernel/.gitignore
> @@ -1,3 +1,5 @@
>  # SPDX-License-Identifier: GPL-2.0
>  
>  /generated_arch_static_branch_asm.rs
> +/generated_arch_warn_asm.rs
> +/generated_arch_reachable_asm.rs
> \ No newline at end of file

There should be a newline.

> +++ b/rust/kernel/bug.rs
> @@ -0,0 +1,100 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +// Copyright (C) 2024 FUJITA Tomonori

2025?

> +#[macro_export]
> +#[doc(hidden)]
> +#[cfg(all(CONFIG_BUG, not(CONFIG_UML)))]
> +macro_rules! warn_flags {
> +    ($flags:expr) => {
> +        const FLAGS: u32 = $crate::bindings::BUGFLAG_WARNING | $flags;
> +        // SAFETY: Just an FFI call.
> +        #[cfg(CONFIG_DEBUG_BUGVERBOSE)]
> +        unsafe {
> +            $crate::asm!(concat!(
> +                "/* {size} */",
> +                ".pushsection .rodata.str1.1, \"aMS\",@progbits, 1\n",
> +                "111:\t .string ", "\"", file!(), "\"\n",
> +                ".popsection\n",
> +                include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_warn_asm.rs")),
> +                include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_reachable_asm.rs")));
> +            line = const line!(),
> +            flags = const FLAGS,
> +            size = const ::core::mem::size_of::<$crate::bindings::bug_entry>(),
> +            );
> +        }
> +        // SAFETY: Just an FFI call.
> +        #[cfg(not(CONFIG_DEBUG_BUGVERBOSE))]
> +        unsafe {
> +            $crate::asm!(
> +            concat!(
> +                "/* {size} */",
> +                include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_warn_asm.rs")),
> +                include!(concat!(env!("OBJTREE"), "/rust/kernel/generated_arch_reachable_asm.rs")));
> +            flags = const FLAGS,
> +            size = const ::core::mem::size_of::<$crate::bindings::bug_entry>(),
> +            );
> +        }

I generally prefer to have the cfgs on the macro rather in its
expansion. That avoids emitting a lot of code that is not actually used.

> +#[doc(hidden)]
> +#[macro_export]
> +macro_rules! bugflag_taint {
> +    ($taint:expr) => {
> +        $taint << 8
> +    };
> +}

This could just be a const fn.

> +/// Report a warning only once.
> +#[macro_export]
> +macro_rules! warn_on_once {
> +    ($cond:expr) => {
> +        if $cond {
> +            $crate::warn_flags!(
> +                $crate::bindings::BUGFLAG_ONCE
> +                    | $crate::bugflag_taint!($crate::bindings::TAINT_WARN)

Or maybe a constant?

const WARN_ON_ONCE_FLAGS: u32 = bindings::BUGFLAG_ONCE | (bindings::TAINT_WARN << 8);

$crate::warn_flags!($crate::bug::WARN_ON_ONCE_FLAGS);

Alice