Re: [Qemu-devel] [PATCH 2/5] Fix Thumb-1 BE32 execution and disassembly.

[Julian Brown <julian@codesourcery.com>]

On Fri, 4 Nov 2016 13:30:12 +0000
Peter Maydell <peter.maydell@linaro.org> wrote:

> On 3 November 2016 at 17:30, Julian Brown <julian@codesourcery.com>
> wrote:
> > Thumb-1 code has some issues in BE32 mode (as currently
> > implemented). In short, since bytes are swapped within words at
> > load time for BE32 executables, this also swaps pairs of adjacent
> > Thumb-1 instructions.
> >
> > This patch un-swaps those pairs of instructions again, both for
> > execution, and for disassembly.
> >
> > Signed-off-by: Julian Brown <julian@codesourcery.com>
> > ---
> >  disas/arm.c           | 46
> > +++++++++++++++++++++++++++++++++++-----------
> > include/disas/bfd.h   |  1 + target-arm/arm_ldst.h | 10 +++++++++-
> >  target-arm/cpu.c      |  4 ++++
> >  4 files changed, 49 insertions(+), 12 deletions(-)
> >
> > diff --git a/disas/arm.c b/disas/arm.c
> > index 93c6503..4807ba3 100644
> > --- a/disas/arm.c
> > +++ b/disas/arm.c
> > @@ -3863,10 +3863,11 @@ print_insn_arm (bfd_vma pc, struct
> > disassemble_info *info) int           is_data = false;
> >    unsigned int size = 4;
> >    void         (*printer) (bfd_vma, struct disassemble_info *,
> > long);
> > -  int little;
> > +  int little, is_thumb1_be32 = false;
> >
> >    little = (info->endian == BFD_ENDIAN_LITTLE);
> >    is_thumb |= (pc & 1);
> > +  is_thumb1_be32 = (info->flags & INSN_ARM_THUMB1_BE32) != 0;
> >    pc &= ~(bfd_vma)1;
> >
> >    if (force_thumb)
> > @@ -3915,11 +3916,22 @@ print_insn_arm (bfd_vma pc, struct
> > disassemble_info *info) info->bytes_per_chunk = 2;
> >        size = 2;
> >
> > -      status = info->read_memory_func (pc, (bfd_byte *)b, 2, info);
> > -      if (little)
> > -       given = (b[0]) | (b[1] << 8);
> > -      else
> > -       given = (b[1]) | (b[0] << 8);
> > +      if (is_thumb1_be32) {
> > +          status = info->read_memory_func(pc & ~3, (bfd_byte *)b,
> > 4, info);
> > +          assert(little);
> > +          if ((pc & 2) == 0) {
> > +              given = b[2] | (b[3] << 8);
> > +          } else {
> > +              given = b[0] | (b[1] << 8);
> > +          }
> > +      } else {
> > +          status = info->read_memory_func(pc, (bfd_byte *)b, 2,
> > info);
> > +          if (little) {
> > +              given = (b[0]) | (b[1] << 8);
> > +          } else {  
> 
> > +              given = (b[1]) | (b[0] << 8);
> > +          }
> > +      }  
> 
> Could we do this instead by changing the read_memory_func() so that it
> did the appropriate XORing of addresses ? (Chaining through to
> the original read_memory_func would be a bit irritating as you'd
> need to find a place to stash that function pointer where you
> could get at it again from the new read_memory_func.)

Hmm, not sure. I'll try to think about whether that can be done nicely.

Julian