Use of the PC value in interrupt/exception handlers

[Adam Lackorzynski]

On Fri Aug 02, 2013 at 19:15:51 +0900, Piyus Kedia wrote:
> In addition to previous mail we found that L4-microkernel checks the PC
> value on stack against some kernel PC in page fault handler by looking at
> the source code. Does anybody knows if there is more such cases?

Which check did you find?

There are so-called continuations which let the user-context resume at a
different code location. You could check for those and constructs such
as regs()->ip(X) setters and xx->ip = yy; statements. Now I'm not sure
this is even relevant to you since those are exchanges of values and no
check against some fixed address.


> On Fri, Aug 2, 2013 at 7:10 PM, Piyus Kedia <piyuskedia at gmail.com> 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.


Adam
-- 
Adam                 adam at os.inf.tu-dresden.de
  Lackorzynski         http://os.inf.tu-dresden.de/~adam/