ARM: 6836/1: kprobes/fix emulation of LDR/STR instruction when Rn == PC

ARM: 6836/1: kprobes/fix emulation of LDR/STR instruction when Rn == PC

The Rn value from the emulation is unconditionally written back;
this is fine as long as Rn != PC because in that case, even if the
instruction isn't a write back instruction, it will only result in the
same value being written back.

In case Rn == PC, then the emulated instruction doesn't have the
actual PC value in Rn but an adjusted value; when this is written
back, it will result in the PC being incorrectly updated.

An altenative solution would be to check bits 24 and 22 to see whether
the instruction actually is a write back instruction or not. I think
it's enough to check whether Rn != PC,  because:
- it's looks cheaper than the alternative
- to my understaning it's not permitted to update the PC with a write
back instruction, so we don't lose any ability to emulate legal
instructions.
- in case of writing back for non write back instructions where Rn != PC, it doesn't matter because the values are the same.

Regarding the second point above, it would possibly be prudent to add
some checking to prep_emulate_ldr_str(), so that instructions with
both write back and Rn == PC would be rejected.

Signed-off-by: Viktor Rosendahl <viktor.rosendahl@nokia.com>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>