Re: [PATCH 1/6] x86: fix NX bit handling in change_page_attr

[Jeremy Fitzhardinge <jeremy@goop.org>]

Huang, Ying wrote:
> This patch fixes a bug of change_page_attr/change_page_attr_addr on
> Intel i386/x86_64 CPUs.  After changing page attribute to be
> executable with these functions, the page remains un-executable on
> Intel i386/x86_64 CPU.  Because on Intel i386/x86_64 CPU, only if the
> "NX" bits of all three level page tables are cleared (PAE is enabled),
> the corresponding page is executable (refer to section 4.13.2 of Intel
> 64 and IA-32 Architectures Software Developer's Manual).  So, the bug
> is fixed through clearing the "NX" bit of PMD when splitting the huge
> PMD.
>
> Signed-off-by: Huang Ying <ying.huang@intel.com>
>
> ---
>  arch/x86/mm/pageattr.c |    1 +
>  1 file changed, 1 insertion(+)
>
> --- a/arch/x86/mm/pageattr.c
> +++ b/arch/x86/mm/pageattr.c
> @@ -124,6 +124,7 @@ static int split_large_page(pte_t *kpte,
>  	/*
>  	 * Install the new, split up pagetable:
>  	 */
> +	pgprot_val(ref_prot) &= ~_PAGE_NX;
>   

I don't think its a good idea to treat pgprot_val() as an lvalue - it 
precludes it from being turned into an inline function.  I know there 
are numerous other places which do, but we should avoid making it worse.

    J