kernel virtual memory access (from app) does not generatesegfault

[Nicolas Pitre]

On Wed, 21 Apr 2010, Russell King - ARM Linux wrote:

> On Wed, Apr 21, 2010 at 10:44:47PM +0100, Russell King - ARM Linux wrote:
> > 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.
> 
> Actually, no, you're creating the confusion; this sub-thread is about
> the behaviour on ARMv7, as a completely separate subject from ARM926.

It is well possible that I missed the subject transition.

The only person who provided a test program is Sasha Sirotkin who said:

On Tue, 20 Apr 2010, Sasha Sirotkin wrote:

> P.S. My kernel is 2.6.32.7 and the CPU is ARM926EJ-S rev 5 (v5l)

Message-id: <4BCD7076.9030802 at browserseal.com>

Only later did Dave P. Martin mention having made similar observations 
on an ARMv7.


Nicolas