Re: [PATCH] pkeys: Introduce PKEY_ALLOC_SIGNALINHERIT and change signal semantics

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

On Wed, May 02, 2018 at 09:23:49PM +0000, Andy Lutomirski wrote:
> 
> > If I recall correctly, the POWER maintainer did express a strong desire
> > back then for (what is, I believe) their current semantics, which my
> > PKEY_ALLOC_SIGNALINHERIT patch implements for x86, too.
> 
> Ram, I really really don't like the POWER semantics.  Can you give some
> justification for them?  Does POWER at least have an atomic way for
> userspace to modify just the key it wants to modify or, even better,
> special load and store instructions to use alternate keys?

I wouldn't call it POWER semantics. The way I implemented it on power
lead to the semantics, given that nothing was explicitly stated 
about how the semantics should work within a signal handler.

As far as power ISA is concerned, there are no special load
and store instructions that can somehow circumvent the permissions
of the key associated with the PTE.

Also unlike x86;  on POWER, pkey-permissions are not administered based
on its run context (user context, signal context).
Hence the default behavior tends to make the key permissions remain the
same regardless of the context.


> Does POWER at least have an atomic way for userspace to modify just the 
> key it wants to modify .. ?

No. just like PKRU, on power its a register which has bits corresponding
to each key. Entire register has to be read and written, which means
multiple keys could get modified in the same write.


adding ppc mailing list to the CC.

> 
> --Andy

-- 
Ram Pai