Re: [PATCH 1/3] powerpc: don't use __WARN() for WARN_ON()

From: Kees Cook <keescook@chromium.org>
To: Christophe Leroy <christophe.leroy@c-s.fr>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>,
	Paul Mackerras <paulus@samba.org>,
	Michael Ellerman <mpe@ellerman.id.au>,
	segher@kernel.crashing.org,
	Drew Davenport <ddavenport@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	linux-kernel@vger.kernel.org, linuxppc-dev@lists.ozlabs.org
Subject: Re: [PATCH 1/3] powerpc: don't use __WARN() for WARN_ON()
Date: Mon, 19 Aug 2019 09:28:03 -0700	[thread overview]
Message-ID: <201908190917.9C65E23D6A@keescook> (raw)
In-Reply-To: <a6781075192afe0c909ce7d091de7931183a5d93.1566219503.git.christophe.leroy@c-s.fr>

On Mon, Aug 19, 2019 at 01:06:28PM +0000, Christophe Leroy wrote:
> __WARN() used to just call __WARN_TAINT(TAINT_WARN)
> 
> But a call to printk() has been added in the commit identified below
> to print a "---- cut here ----" line.
> 
> This change only applies to warnings using __WARN(), which means
> WARN_ON() where the condition is constant at compile time.
> For WARN_ON() with a non constant condition, the additional line is
> not printed.
> 
> In addition, adding a call to printk() forces GCC to add a stack frame
> and save volatile registers. Powerpc has been using traps to implement
> warnings in order to avoid that.
> 
> So, call __WARN_TAINT(TAINT_WARN) directly instead of using __WARN()
> in order to restore the previous behaviour.
> 
> If one day powerpc wants the decorative "---- cut here ----" line, it
> has to be done in the trap handler, not in the WARN_ON() macro.
> 
> Fixes: 6b15f678fb7d ("include/asm-generic/bug.h: fix "cut here" for WARN_ON for __WARN_TAINT architectures")
> Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>

Ah! Hmpf. Yeah, that wasn't an intended side-effect of this fix.

It seems PPC is not alone in this situation of making this code much
noisier. It looks like there needs to be a way to indicate to the trap
handler that a message was delivered or not. Perhaps we can add another
taint flag?

-kees

> ---
>  arch/powerpc/include/asm/bug.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/arch/powerpc/include/asm/bug.h b/arch/powerpc/include/asm/bug.h
> index fed7e6241349..3928fdaebb71 100644
> --- a/arch/powerpc/include/asm/bug.h
> +++ b/arch/powerpc/include/asm/bug.h
> @@ -99,7 +99,7 @@
>  	int __ret_warn_on = !!(x);				\
>  	if (__builtin_constant_p(__ret_warn_on)) {		\
>  		if (__ret_warn_on)				\
> -			__WARN();				\
> +			__WARN_TAINT(TAINT_WARN);		\
>  	} else {						\
>  		__asm__ __volatile__(				\
>  		"1:	"PPC_TLNEI"	%4,0\n"			\
> -- 
> 2.13.3
> 

-- 
Kees Cook