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Date:   Thu, 23 Jul 2015 14:35:44 +0200
From:   Ralf Baechle <ralf@linux-mips.org>
To:     James Hogan <james.hogan@imgtec.com>
Cc:     linux-mips@linux-mips.org, David Daney <ddaney@caviumnetworks.com>,
        Markos Chandras <Markos.Chandras@imgtec.com>
Subject: Re: [PATCH 1/2] MIPS: Handle page faults of executable but
 unreadable pages correctly.
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On Thu, Jul 23, 2015 at 11:15:15AM +0100, James Hogan wrote:

> On 23/07/15 10:10, Ralf Baechle wrote:
> > Without this we end taking execeptions in an endless loop hanging the
> > thread.
> 
> A little more explanation would be nice. Under what situations does this
> occur? Does this mean any VM_EXEC and !VM_READ page can't actually be
> faulted in without it being treated as an RI violation, or does it only
> affect when read from kernel emulation code?

> > -			if (!(vma->vm_flags & VM_READ)) {
> > +			if (!(vma->vm_flags & VM_READ) &&
> > +			    exception_epc(regs) != address) {
> >  #if 0
> >  				pr_notice("Cpu%d[%s:%d:%0*lx:%ld:%0*lx] RI violation\n",
> >  					  raw_smp_processor_id(),
> > 
> 

The general idea is the change the code to treat loads of an instruction
just like an instruction fetch.  Which is achieved by adding the second
condition "exception_epc(regs) != address" to the if above.
exception_epc(regs) == address means

It would all be easier if Linux was enabling the separate exception codes
for read and execution failure but short of that, a test like above must
provide if a fault was an attempted instruction fetch or happend fetching
data.

  Ralf