Re: [PATCH v3] x86: treat pkey-0 special

[Thomas Gleixner <tglx@linutronix.de>]

On Wed, 14 Mar 2018, 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.
> 
> This patch clarifies the semantics of pkey-0 and provides the

grep 'This patch' Documentation/process

> corresponding implementation on powerpc.
> 
> Pkey-0 is special with the following semantics.
> (a) it is implicitly allocated and can never be freed. It always exists.
> (b) it is the default key assigned to any address-range.
> (c) it can be explicitly associated with any address-range.
> 
> Tested on x86_64.

I'm curious how the corresponding implementation on powerpc can be tested
on x86_64. Copy and paste is not enough ...

> 
> History:
>     v3 : added clarification of the semantics of pkey0.
>     		-- suggested by Dave Hansen
>     v2 : split the patch into two, one for x86 and one for powerpc
>     		-- suggested by Michael Ellermen

Please put the history below the --- seperator. It's not part of the
changelog. That way the tools can discard it when picking up the patch.

> 
> cc: Dave Hansen <dave.hansen@intel.com>
> cc: Michael Ellermen <mpe@ellerman.id.au>
> cc: Ingo Molnar <mingo@kernel.org>
> Signed-off-by: Ram Pai <linuxram@us.ibm.com>
> ---
>  arch/x86/include/asm/pkeys.h | 5 +++--
>  1 file changed, 3 insertions(+), 2 deletions(-)
> 
> diff --git a/arch/x86/include/asm/pkeys.h b/arch/x86/include/asm/pkeys.h
> index a0ba1ff..6ea7486 100644
> --- a/arch/x86/include/asm/pkeys.h
> +++ b/arch/x86/include/asm/pkeys.h
> @@ -52,7 +52,7 @@ bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
>  	 * from pkey_alloc().  pkey 0 is special, and never
>  	 * returned from pkey_alloc().
>  	 */
> -	if (pkey <= 0)
> +	if (pkey < 0)
>  		return false;
>  	if (pkey >= arch_max_pkey())
>  		return false;
> @@ -92,7 +92,8 @@ int mm_pkey_alloc(struct mm_struct *mm)
>  static inline
>  int mm_pkey_free(struct mm_struct *mm, int pkey)
>  {
> -	if (!mm_pkey_is_allocated(mm, pkey))
> +	/* pkey 0 is special and can never be freed */

This comment is pretty useless. How should anyone figure out whats special
about pkey 0?

> +	if (!pkey || !mm_pkey_is_allocated(mm, pkey))

Why this extra check? mm_pkey_is_allocated(mm, 0) should not return true
ever. If it does, then this wants to be fixed.

Thanks,

	tglx