linux-kernel - Re: [PATCH] lkdtm/bugs: In lkdtm_HUNG_TASK() use BUG(), not BUG

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Message-Id: <e70be9ff-5bf8-45ab-bd31-66d4a75acce5@app.fastmail.com>
Date: Fri, 26 Jan 2024 17:08:11 +0100
From: "Arnd Bergmann" <arnd@...db.de>
To: "Doug Anderson" <dianders@...omium.org>,
 "Kees Cook" <keescook@...omium.org>
Cc: "Greg Kroah-Hartman" <gregkh@...uxfoundation.org>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH] lkdtm/bugs: In lkdtm_HUNG_TASK() use BUG(), not BUG_ON(1)

On Fri, Jan 26, 2024, at 16:28, Douglas Anderson wrote:
> In commit edb6538da3df ("lkdtm/bugs: Adjust lkdtm_HUNG_TASK() to avoid
> tail call optimization") we marked lkdtm_HUNG_TASK() as
> __noreturn. The compiler gets unhappy if it thinks a __noreturn
> function might return, so there's a BUG_ON(1) at the end. Any human
> can see that the function won't return and the compiler can figure
> that out too. Except when it can't.
>
> The MIPS architecture defines HAVE_ARCH_BUG_ON and defines its own
> version of BUG_ON(). The MIPS version of BUG_ON() is not a macro but
> is instead an inline function. Apparently this prevents the compiler
> from realizing that the condition to BUG_ON() is constant and that the
> function will never return.
>
> Let's change the BUG_ON(1) to just BUG(), which it should have been to
> begin with. The only reason I used BUG_ON(1) to begin with was because
> I was used to using WARN_ON(1) when writing test code and WARN() and
> BUG() are oddly inconsistent in this manner. :-/
>
> Fixes: edb6538da3df ("lkdtm/bugs: Adjust lkdtm_HUNG_TASK() to avoid 
> tail call optimization")
> Signed-off-by: Douglas Anderson <dianders@...omium.org>
> ---

Acked-by: Arnd Bergmann <arnd@...db.de>

>  	schedule();
> -	BUG_ON(1);
> +	BUG();
>  }

Yes, this is a common problem that I've fixed in other
places before.

     Arnd