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 <aneesh.ku...@linux.vnet.ibm.com>
> ---
> 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;
> }
>
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