linux-kernel - Re: [PATCH 2/6] kexec jump: check code size in control page

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Date:	Fri, 8 Aug 2008 09:52:25 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Huang Ying <ying.huang@...el.com>
Cc:	Pavel Machek <pavel@...e.cz>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	nigel@...el.suspend2.net, "Rafael J. Wysocki" <rjw@...k.pl>,
	Andrew Morton <akpm@...ux-foundation.org>, mingo@...e.hu,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	linux-kernel@...r.kernel.org,
	Kexec Mailing List <kexec@...ts.infradead.org>
Subject: Re: [PATCH 2/6] kexec jump: check code size in control page

On Fri, Aug 08, 2008 at 10:44:37AM +0800, Huang Ying wrote:
> On Thu, 2008-08-07 at 22:31 +0200, Pavel Machek wrote:
> > Hi!
> > 
> > > > PAGE_SIZE/2. This patch adds runtime checking for this.
> > > > 
> > > > Signed-off-by: Huang Ying <ying.huang@...el.com>
> > ...
> > 
> > > >  {
> > > >  	if (nx_enabled)
> > > >  		set_pages_x(image->control_code_page, 1);
> > > > +
> > > > +	BUG_ON((unsigned long)kexec_control_page_code_end - \
> > > > +	       (unsigned long)relocate_kernel >= PAGE_SIZE/2);
> > > > +
> > > 
> > 
> > > Run time check is better than nothing but I think in this case it would
> > > be better if we can catch it at compile time. 
> > > 
> > > One of the methods will be to write a small program of your own and
> > > put in script/ and at build time check for the size and flag error. May
> > > be there are other better ways to do this.
> > 
> > BUILD_BUG_ON()?
> 
> I tried with BUILD_BUG_ON(), and compiling is OK for both of following
> statement:
> 
> BUILD_BUG_ON((unsigned long)kexec_control_page_code_end - \
> 	     (unsigned long)relocate_kernel >= PAGE_SIZE/2);
> 
> BUILD_BUG_ON((unsigned long)kexec_control_page_code_end - \
> 	     (unsigned long)relocate_kernel < PAGE_SIZE/2);
> 
> In general, I think value of kexec_control_page_code_end and
> relocate_kernel is not determined during compiling time. So
> BUILD_BUG_ON() doesn't work.
> 
> Another idea, use ASSERT() command of ld link script as in the following
> patch:
> 
> --- a/arch/x86/kernel/vmlinux_32.lds.S
> +++ b/arch/x86/kernel/vmlinux_32.lds.S
> @@ -209,3 +209,5 @@ SECTIONS
>  
>    DWARF_DEBUG
>  }
> +
> +#include "vmlinux_check_32.lds.S"
> --- /dev/null
> +++ b/arch/x86/kernel/vmlinux_check_32.lds.S
> @@ -0,0 +1,3 @@
> +#include <asm/kexec.h>
> +
> +ASSERT(kexec_control_page_code_end - relocate_kernel >= 2048, "kexec control page code size is too big")
> 
> 

Use of ASSERT() looks good to me. I think creation of extra file for a 
single ASSERT() can be avoided. Instead, we can just put this assert in
vmlinux_32.lds.S itself?

Thansk
Vivek
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