kernel virtual memory access (from app) does not generatesegfault

[Russell King - ARM Linux]

On Wed, Apr 21, 2010 at 05:24:51PM -0400, Nicolas Pitre wrote:
> On Wed, 21 Apr 2010, Russell King - ARM Linux wrote:
> 
> > On Wed, Apr 21, 2010 at 12:17:41PM +0100, Dave P. Martin wrote:
> > >  
> > > 
> > > > -----Original Message-----
> > > > From: Russell King - ARM Linux [mailto:linux at arm.linux.org.uk] 
> > > > Sent: 20 April 2010 23:41
> > > > To: Jamie Lokier
> > > > Cc: Ben Dooks; anfei; Dave P Martin; 
> > > > linux-arm-kernel at lists.infradead.org
> > > > Subject: Re: kernel virtual memory access (from app) does not 
> > > > generatesegfault
> > > 
> > > [...]
> > > 
> > > > The difference between instruction faults and data faults is 
> > > > that we always interpret instruction faults on pre-ARMv6 CPUs 
> > > > as a 'translation fault' rather than a permission fault since 
> > > > they can't tell us what the problem was.
> > > 
> > > Note that my observations were on an armv7 kernel.  Should we still hit the
> > > same bit of code in this case, or have I misdiagnosed the problem?
> > 
> > If it was ARMv7, we should be reading the IFSR, which should be telling
> > us that there's a permission fault trying to read instructions from
> > 0xc0000000.
> > 
> > If changing do_translation_fault() on a recent kernel fixes your problem,
> > something's going wrong.  Any chance you could add some debugging to
> > do_PrefetchAbort() so that when you see your test program running
> > (eg, if (strcmp(current->comm, "progname") == 0) { ... }) you could
> > dump out the values of ifsr and addr please?
> 
> If I remember right, the original bug report mentioned ARM926.

So here we go again with confusion raining.

Someone please tell me _definitively_ _what_ is being seen on _what_ CPU,
and separate the two issues into two different threads.  I'm going to
ignore any further comments on this issue until that's done.  Life is
too short to try to work this out on my own.