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

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Date:	Fri, 08 Aug 2008 10:44:37 +0800
From:	Huang Ying <ying.huang@...el.com>
To:	Pavel Machek <pavel@...e.cz>
Cc:	Vivek Goyal <vgoyal@...hat.com>,
	"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 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")


It works for me. What do you think about that?

Best Regards,
Huang Ying


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