Re: [PATCH] powerpc, pkey: make protection key 0 less special

[Ram Pai <linuxram@us.ibm.com>]

On Mon, Mar 26, 2018 at 04:31:41PM -0700, Ram Pai wrote:
> Applications need the ability to associate an address-range with some
> key and latter revert to its initial default key. Pkey-0 comes close to
> providing this function but falls short, because the current
> implementation disallows applications to explicitly associate pkey-0 to
> the address range.
> 
> Lets make pkey-0 less special and treat it almost like any other key.
> Thus it can be explicitly associated with any address range, and can be
> freed. This gives the application more flexibility and power.  The
> ability to free pkey-0 must be used responsibily, since pkey-0 is
> associated with almost all address-range by default.
> 
> Even with this change pkey-0 continues to be slightly more special
> from the following point of view.
> (a) it is implicitly allocated.
> (b) it is the default key assigned to any address-range.
> 
> Tested on powerpc.

This patch is not entirely correct.
> 
> cc: Thomas Gleixner <tglx@linutronix.de>
> cc: Dave Hansen <dave.hansen@intel.com>
> cc: Michael Ellermen <mpe@ellerman.id.au>
> cc: Ingo Molnar <mingo@kernel.org>
> cc: Andrew Morton <akpm@linux-foundation.org>
> Signed-off-by: Ram Pai <linuxram@us.ibm.com>
> ---
>  arch/powerpc/include/asm/pkeys.h | 24 ++++++++++++++++++++----
>  1 file changed, 20 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/pkeys.h b/arch/powerpc/include/asm/pkeys.h
> index 0409c80..9c7d3bd 100644
> --- a/arch/powerpc/include/asm/pkeys.h
> +++ b/arch/powerpc/include/asm/pkeys.h
> @@ -101,10 +101,18 @@ static inline u16 pte_to_pkey_bits(u64 pteflags)
> 
>  static inline bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
>  {
> -	/* A reserved key is never considered as 'explicitly allocated' */
> -	return ((pkey < arch_max_pkey()) &&
> -		!__mm_pkey_is_reserved(pkey) &&
> -		__mm_pkey_is_allocated(mm, pkey));
> +	/* pkey 0 is allocated by default. */
> +	if (!pkey)
> +		return true;

This is wrong. pkey-0 should not be treated any special here. Will fix
this and send a new patch. Sorry for the noise.

RP