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From: "shahab.vahedi at gmail dot com" <sourceware-bugzilla@sourceware.org>
To: gdb-prs@sourceware.org
Subject: [Bug tdep/27015] New: ARC: "eret" value is collected from the wrong
 data in register cache
Date: Fri, 04 Dec 2020 15:04:10 +0000
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https://sourceware.org/bugzilla/show_bug.cgi?id=3D27015

            Bug ID: 27015
           Summary: ARC: "eret" value is collected from the wrong data in
                    register cache
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: critical
          Priority: P2
         Component: tdep
          Assignee: unassigned at sourceware dot org
          Reporter: shahab.vahedi at gmail dot com
  Target Milestone: ---

Versions affected: HEAD and 10.1

--------------------------------

In collect_register() function of arc-linux-tdep.c, the "eret"
(exception return) register value is not being reported correctly.

Background:
When asked for the "pc" value, we have to update the "eret" register
with GDB's STOP_PC.  The "eret" instructs the kernel code where to
jump back when an instruction has stopped due to a breakpoint.  This
is how collect_register() is doing so:

--------------8<--------------
  if (regnum =3D=3D gdbarch_pc_regnum (gdbarch))
    regnum =3D ARC_ERET_REGNUM;
  regcache->raw_collect (regnum, buf + arc_linux_core_reg_offsets[regnum]);
-------------->8--------------

Root cause:
Although this is using the correct offset (ERET register's), it is also
changing the REGNUM itself.  Therefore, raw_collect (regnum, ...) is
not reading from "pc" anymore.

Consequence:
This bug affects the "native ARC gdb" badly and causes kernel code to jump
to addresses after the breakpoint and not executing the "breakpoint"ed
instructions at all.  That "native ARC gdb" feature is not upstream yet and
is in review at the time of writing [1].

[1] Add native GNU/Linux support for ARC in GDB
https://sourceware.org/pipermail/gdb-patches/2020-November/173203.html

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