Subject: Re: [PATCH 03/65] powerpc/mm/subpage: Clear RWX bit to indicate no access

From: Balbir Singh <[email-masked]>
Date: Thu, 31 Mar 2016 12:42:35 +1100

On 27/03/16 19:23, Aneesh Kumar K.V wrote:
> Subpage protection used to depend on _PAGE_USER bit to implement no
> access mode. This patch switch that to use _PAGE_RWX. We clear READ,
> Write and Execute access from pte instead of clearing _PAGE_USER now.
> This was done so that we can switch to _PAGE_PRIVILEGED in later patch.
> subpage_protection() returns pte bits that need to be cleared.
Could you please clarify what bit needs to be cleared. I think the underlying
assumption is that when this routine is called access cannot be _PAGE_RWX
> Instead of updating the interface to handle no-access in a separate way,
> it appears simple to clear RWX acecss to indicate no access.
>
> We still don't insert hash pte for no access implied by !_PAGE_RWX.
> Hence we should not get PROT_FAULT with change.
> Signed-off-by: Aneesh Kumar K.V <[email-masked]>
> ---
>  arch/powerpc/mm/hash_utils_64.c | 11 ++++++++---
>  1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/arch/powerpc/mm/hash_utils_64.c b/arch/powerpc/mm/hash_utils_64.c
> index ea23403b3fc0..ec37f4b0a8ff 100644
> --- a/arch/powerpc/mm/hash_utils_64.c
> +++ b/arch/powerpc/mm/hash_utils_64.c
> @@ -917,7 +917,7 @@ void demote_segment_4k(struct mm_struct *mm, unsigned long addr)
>   * Userspace sets the subpage permissions using the subpage_prot system call.
>   *
>   * Result is 0: full permissions, _PAGE_RW: read-only,
> - * _PAGE_USER or _PAGE_USER|_PAGE_RW: no access.
> + * _PAGE_RWX: no access.
>   */
>  static int subpage_protection(struct mm_struct *mm, unsigned long ea)
>  {
> @@ -943,8 +943,13 @@ static int subpage_protection(struct mm_struct *mm, unsigned long ea)
>  	/* extract 2-bit bitfield for this 4k subpage */
>  	spp >>= 30 - 2 * ((ea >> 12) & 0xf);
>  
> -	/* turn 0,1,2,3 into combination of _PAGE_USER and _PAGE_RW */
> -	spp = ((spp & 2) ? _PAGE_USER : 0) | ((spp & 1) ? _PAGE_RW : 0);
> +	/*
> +	 * 0 -> full premission
> +	 * 1 -> Read only
> +	 * 2 -> no access.
> +	 * We return the flag that need to be cleared.
> +	 */
> +	spp = ((spp & 2) ? _PAGE_RWX : 0) | ((spp & 1) ? _PAGE_WRITE : 0);
>  	return spp;
>  }
>