linux-kernel - Re: [PATCH,v2] Only add '-fwrapv' to gcc CFLAGS for gcc 4.2 and later

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Date:	Sat, 18 Jul 2009 00:18:46 +0200
From:	Sam Ravnborg <sam@...nborg.org>
To:	Frans Pop <elendil@...net.nl>
Cc:	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	akpm@...ux-foundation.org, torvalds@...ux-foundation.org,
	linux-kbuild@...r.kernel.org, barryn@...ox.com,
	bugme-daemon@...zilla.kernel.org
Subject: Re: [PATCH,v2] Only add '-fwrapv' to gcc CFLAGS for gcc 4.2 and
	later

On Fri, Jul 10, 2009 at 10:05:47PM +0200, Frans Pop wrote:
> On Friday 10 July 2009, Frans Pop wrote:
> > The discussion on issue looks to have died, but it has bitten Debian
> > stable ("Lenny") [1] as it causes init to die on s390 after a kernel
> > update.
> >
> > Here's a possible patch. The exact gcc version to check for is still a
> > bit open I guess. For the s390 issue I've confirmed that 4.2.4 is OK,
> > but for safety and because of Andrew's comment above I've set the test
> > for 4.3 in the patch.
> 
> Here's an updated patch as I found the gcc version check was incorrect 
> (0430 should have been 0403; sorry).
> 
> I've now changed the check to allow -fwrapv for gcc 4.2 as that has been 
> shown to work and because of the consideration mentioned in my previous 
> mail.
> 
> ---
> From: Frans Pop <elendil@...net.nl>
> Subject: Only add '-fwrapv' to gcc CFLAGS for gcc 4.2 and later
> 
> This flag has been shown to cause init to segfault for kernels
> compiled with gcc-4.1. gcc version 4.2.4 has been shown to be OK.
> 
> This fixes http://bugzilla.kernel.org/show_bug.cgi?id=13012.
> 
> Reported-by: Barry K. Nathan <barryn@...ox.com>
> Signed-off-by: Frans Pop <elendil@...net.nl>
> 
> diff --git a/Makefile b/Makefile
> index 0aeec59..2519fde 100644
> --- a/Makefile
> +++ b/Makefile
> @@ -565,7 +565,8 @@ KBUILD_CFLAGS += $(call 
> cc-option,-Wdeclaration-after-statement,)
>  KBUILD_CFLAGS += $(call cc-option,-Wno-pointer-sign,)
>  
>  # disable invalid "can't wrap" optimizations for signed / pointers
> -KBUILD_CFLAGS	+= $(call cc-option,-fwrapv)
> +KBUILD_CFLAGS  += $(shell if [ $(call cc-version) -ge 0402 ]; then \
> +		    echo $(call cc-option,-fwrapv); fi ;)

This would be simpler if you use:
# cc-ifversion
# Usage:  EXTRA_CFLAGS += $(call cc-ifversion, -lt, 0402, -O1)
cc-ifversion = $(shell [ $(call cc-version, $(CC)) $(1) $(2) ] && echo $(3))

We have only one user at the moment so I understand why you missed it.

	Sam
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