linux-kernel - Re: [PATCH v5 4/4] rust: Add warn

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Message-Id: <20250410.230138.20974725432146108.fujita.tomonori@gmail.com>
Date: Thu, 10 Apr 2025 23:01:38 +0900 (JST)
From: FUJITA Tomonori <fujita.tomonori@...il.com>
To: gregkh@...uxfoundation.org
Cc: fujita.tomonori@...il.com, linux-kernel@...r.kernel.org,
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Subject: Re: [PATCH v5 4/4] rust: Add warn_on macro

On Wed, 9 Apr 2025 09:38:53 +0200
Greg KH <gregkh@...uxfoundation.org> wrote:

> On Wed, Apr 09, 2025 at 03:58:01PM +0900, FUJITA Tomonori wrote:
>> Add warn_on macro, uses the BUG/WARN feature (lib/bug.c) via assembly
>> for x86_64/arm64/riscv.
>> 
>> The current Rust code simply wraps BUG() macro but doesn't provide the
>> proper debug information. The BUG/WARN feature can only be used from
>> assembly.
>> 
>> This uses the assembly code exported by the C side via ARCH_WARN_ASM
>> macro. To avoid duplicating the assembly code, this approach follows
>> the same strategy as the static branch code: it generates the assembly
>> code for Rust using the C preprocessor at compile time.
>> 
>> Similarly, ARCH_WARN_REACHABLE is also used at compile time to
>> generate the assembly code; objtool's reachable anotation code. It's
>> used for only architectures that use objtool.
>> 
>> For now, Loongarch and arm32 just use a wrapper for WARN macro.
>> 
>> UML doesn't use the assembly BUG/WARN feature; just wrapping generic
>> BUG/WARN functions implemented in C works.
>> 
>> Signed-off-by: FUJITA Tomonori <fujita.tomonori@...il.com>
> 
> I don't object to this change, but I would STRONGLY recommend never
> using this in any driver if at all possible.  Only use this if the
> system is in such a state that the only way out is to reboot the kernel,
> which is what both WARN() and BUG() will do.

Totally agree.

As stated in the commit log, BUG() is already used in multiple
locations within the Rust abstractions.

I'm not trying to introduce BUG() or WARN() as new features. This
patchset aims to improve the information provided by BUG() and WARN().