Re: [RFC v7 24/25] powerpc: Deliver SEGV signal on pkey violation

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

On Fri, Aug 18, 2017 at 02:48:31PM +1000, Michael Ellerman wrote:
> Ram Pai <linuxram@us.ibm.com> writes:
> > On Fri, Aug 11, 2017 at 08:26:30PM +1000, Michael Ellerman wrote:
> >> Thiago Jung Bauermann <bauerman@linux.vnet.ibm.com> writes:
> >> 
> >> > Ram Pai <linuxram@us.ibm.com> writes:
> >> >
> >> >> The value of the AMR register at the time of exception
> >> >> is made available in gp_regs[PT_AMR] of the siginfo.
> ...
> >> 
> >> I don't understand why we are putting it in there at all?
> >> 
> >> Is there some special handling of the actual register on signals? I
> >> haven't seen it. In which case the process can get the value of AMR by
> >> reading the register. ??
> >
> > The value of AMR register at the time of the key-exception may not be
> > the same when the signal handler is invoked. 
> 
> Why not?

Assume two threads of a task.

T1:  mprotect_key(foo, PAGE_SIZE, pkey=4);
T1:  set AMR to disable access for pkey 4;
T1:  key fault
    T2: set AMR to enable access to pkey 4;
T1:  fault handler called.
	This fault handler will see the new AMR and not the
	one at the time of the fault.

RP