linux-kernel - Re: [PATCH 1/3] powerpc: don't use __WARN() for WARN

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Date:   Mon, 19 Aug 2019 09:28:03 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        segher@...nel.crashing.org,
        Drew Davenport <ddavenport@...omium.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 1/3] powerpc: don't use __WARN() for WARN_ON()

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@....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