linux-kernel - Re: fs/exec.c: use BUILD_BUG_ON for VM_STACK_FLAGS & VM_STACK_INCOMPLETE

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Date:	Wed, 15 Jun 2011 21:52:07 -0700
From:	Andrew Morton <akpm@...ux-foundation.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Randy Dunlap <randy.dunlap@...cle.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Michal Hocko <mhocko@...e.cz>,
	Richard Weinberger <richard@....at>
Subject: Re: fs/exec.c: use BUILD_BUG_ON for VM_STACK_FLAGS &
 VM_STACK_INCOMPLETE_SETUP

On Wed, 15 Jun 2011 21:45:00 -0700 Linus Torvalds <torvalds@...ux-foundation.org> wrote:

> On Wed, Jun 15, 2011 at 9:35 PM, Randy Dunlap <randy.dunlap@...cle.com> wrote:
> >
> > fwiw, I just reported that this causes a build error in UML on x86_64
> > in mmotm-2011-06-15:
> 
> Gaah. That sounds really familiar. Have we done this same mistake once
> before already, or is it just deja-vu for some other reason.
> 
> On x86-64 UML, we have
> 
>   #define VM_STACK_DEFAULT_FLAGS vm_stack_flags
> 
> so VM_STACK_FLAGS ends up not being a constant, even though it really
> looks like one.
> 
> UML is _really_ confused in this area, btw. It seems to re-define that
> thing if TIF_IA32 is defined. That's some crazy stuff. Doesn't the
> compiler warn about it?
> 
> I guess I should just revert that commit, though. Even if it does seem
> to be the case that UML is just being crazy. Andrew?
> 

Sure, it was just a tiny optimization.

We can bring it back if Richard feels like decrazyifying UML ;)



From: Michal Hocko <mhocko@...e.cz>

a8bef8ff ("mm: migration: avoid race between shift_arg_pages() and
rmap_walk() during migration by not migrating temporary stacks")
introduced a BUG_ON() to ensure that VM_STACK_FLAGS and
VM_STACK_INCOMPLETE_SETUP do not overlap.  The check is a compile time
one, so BUILD_BUG_ON is more appropriate.

Signed-off-by: Michal Hocko <mhocko@...e.cz>
Cc: Mel Gorman <mel@....ul.ie>
Signed-off-by: Andrew Morton <akpm@...ux-foundation.org>
---

 fs/exec.c |    2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff -puN fs/exec.c~fs-execc-use-build_bug_on-for-vm_stack_flags-vm_stack_incomplete_setup fs/exec.c
--- a/fs/exec.c~fs-execc-use-build_bug_on-for-vm_stack_flags-vm_stack_incomplete_setup
+++ a/fs/exec.c
@@ -277,7 +277,7 @@ static int __bprm_mm_init(struct linux_b
 	 * use STACK_TOP because that can depend on attributes which aren't
 	 * configured yet.
 	 */
-	BUG_ON(VM_STACK_FLAGS & VM_STACK_INCOMPLETE_SETUP);
+	BUILD_BUG_ON(VM_STACK_FLAGS & VM_STACK_INCOMPLETE_SETUP);
 	vma->vm_end = STACK_TOP_MAX;
 	vma->vm_start = vma->vm_end - PAGE_SIZE;
 	vma->vm_flags = VM_STACK_FLAGS | VM_STACK_INCOMPLETE_SETUP;
_

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