Re: [tip:x86/urgent] mm: Adjust do_pages_stat() so gcc can see copy_from_user() is safe

[KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>]

> Commit-ID:  b925585039cf39275c2e0e57512e5df27fa73aad
> Gitweb:     http://git.kernel.org/tip/b925585039cf39275c2e0e57512e5df27fa73aad
> Author:     H. Peter Anvin <hpa@zytor.com>
> AuthorDate: Tue, 8 Dec 2009 14:01:32 -0800
> Committer:  H. Peter Anvin <hpa@zytor.com>
> CommitDate: Fri, 11 Dec 2009 15:27:47 -0800
> 
> mm: Adjust do_pages_stat() so gcc can see copy_from_user() is safe
> 
> Slightly adjust the logic for determining the size of the
> copy_form_user() in do_pages_stat(); with this change, gcc can see
> that the copying is safe.
> 
> Without this, we get a build error for i386 allyesconfig:
> 
> /home/hpa/kernel/linux-2.6-tip.urgent/arch/x86/include/asm/uaccess_32.h:213:
> error: call to ‘copy_from_user_overflow’ declared with attribute
> error: copy_from_user() buffer size is not provably correct
> 
> Unlike an earlier patch from Arjan, this doesn't introduce new
> variables; merely reshuffles the compare so that gcc can see that an
> overflow cannot happen.
> 
> Signed-off-by: H. Peter Anvin <hpa@zytor.com>
> Cc: Brice Goglin <Brice.Goglin@inria.fr>
> Cc: Arjan van de Ven <arjan@infradead.org>
> Cc: Andrew Morton <akpm@linux-foundation.org>
> Cc: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>
> LKML-Reference: <20090926205406.30d55b08@infradead.org>
> ---
>  mm/migrate.c |    2 +-
>  1 files changed, 1 insertions(+), 1 deletions(-)
> 
> diff --git a/mm/migrate.c b/mm/migrate.c
> index 7dbcb22..0bc640f 100644
> --- a/mm/migrate.c
> +++ b/mm/migrate.c
> @@ -1044,7 +1044,7 @@ static int do_pages_stat(struct mm_struct *mm, unsigned long nr_pages,
>  	int err;
>  
>  	for (i = 0; i < nr_pages; i += chunk_nr) {
> -		if (chunk_nr + i > nr_pages)
> +		if (chunk_nr > nr_pages - i)
>  			chunk_nr = nr_pages - i;

Ah, good. Thank you.
	Reviewed-by: KOSAKI Motohiro <kosaki.motohiro@jp.fujitsu.com>