Fwd: Use of the PC value in interrupt/exception handlers

[Piyus Kedia]

Thank you, this explains things very clearly. It helped us a lot.


On Sat, Aug 3, 2013 at 1:58 AM, Konstantin Belousov <kostikbel at gmail.com>wrote:

> On Fri, Aug 02, 2013 at 09:31:33AM -0600, Ian Lepore wrote:
> > On Fri, 2013-08-02 at 19:08 +0900, Piyus Kedia wrote:
> > > Dear all,
> > >
> > > We are working on developing a dynamic binary translator for the
> kernel.
> > > Towards this, we wanted to confirm if the interrupted PC value pushed
> on
> > > stack by an interrupt/exception is used by the interrupt/exception
> > > handlers? For example, is the PC value compared against a fixed
> address to
> > > determine the handler behaviour (like
> > > Linux's page fault handler compares the faulting PC against an
> exception
> > > table, to allow functions like copy_from_user to fault).
> > >
> > > Basically, we are wondering if it is safe to replace the pushed PC
> value on
> > > stack by another value. This would be safe if the PC value is only
> used for
> > > returning from interrupt, or for reading contents at that PC address
> (e.g.,
> > > to decode the instruction at current PC). It would be unsafe if the
> value
> > > of the address itself is meaningful to the handler.
> > >
> > > We found that in FreeBSD segment-not-present exception handler checks
> the
> > > trapped PC value against some fixed kernel PC by looking at the code,
> > > except that it is only used for debugging purposes. It would be nice if
> > > somebody could also confirm this.
> > >
> > > Thanks,
> > > Piyus
> >
> > For the ARM architectures which use Restartable Atomic Sequences (RAS)
> > to implement atomic operations, examining the value of the saved PC and
> > possibly modifying it is how RAS works.  See the PUSHFRAMEINSVC macro in
> > sys/arm/include/asmacros.h.
> >
> > In  a nutshell, the RAS code works by having userland code store the
> > begin/end addresses of a small block of code that must be executed to
> > completion without interruption to be correct.  If an exception or
> > interrupt happens while the PC is in that range, the exception-entry
> > code implemented by PUSHFRAMEINSVC modifies the saved PC so that on
> > return to userland, execution resumes at the beginning of the atomic
> > sequence.
>
> This reminds of me the following MIPS code:
> http://svnweb.freebsd.org/base?view=revision&revision=226517
>