kernel virtual memory access (from app) does not generatesegfault

[Dave.Martin@arm.com (Dave P. Martin)]

Hi there, 

> -----Original Message-----
> From: Nicolas Pitre [mailto:nico at fluxnic.net] 
> Sent: 21 April 2010 23:59
> To: Russell King - ARM Linux
> Cc: linux-arm-kernel at lists.infradead.org; anfei; Jamie 
> Lokier; Dave P Martin; Ben Dooks
> Subject: Re: kernel virtual memory access (from app) does not 
> generatesegfault
> 
> 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@browserseal.com>
> 
> Only later did Dave P. Martin mention having made similar 
> observations on an ARMv7.

To clarify:

	* I haven't tested this on 926 myself
	* On armv7, I have observed the problem only on *old* kernels
(<2.6.32; which lack any of the patches under discussion)
	* Using 2.6.34-rc1 (from rmk's versatile branch) on armv7, I get the
expected SEGV when userspace tries to execute >= TASK_SIZE

so...
	* Sasha's problem is caused by a problem in the current kernel on
926.
	* My problem relates to v7 and has already been fixed (but isn't
fixed in the Ubuntu kernels yet)

The test case was

int main(void)
{
	((void (*)(void))0xc0000000)();
	return 0;
}

Hope this makes things clearer.

Cheers
---Dave