[PATCH] Fix ARM sign-extension on PC when using 64-bit BFD

[Sergio Durigan Junior <sergiodj at redhat dot com>]

Hi,

Andrew Haley found a bug on GDB running on ARM when using
--enable-64-bit-bfd.  Basically the issue happens when dealing with "bl"
instructions: GDB does branch destination calculation and (wrongly)
sign-extends the PC.  Here is a piece of his original message explaining
the problem:

>      next_pc = arm_get_next_pc (frame, get_frame_pc (frame));
>
>      /* The Linux kernel offers some user-mode helpers in a high page.  We can
>         not read this page (as of 2.6.23), and even if we could then we couldn't
>         set breakpoints in it, and even if we could then the atomic operations
>         would fail when interrupted.  They are all called as functions and return
>         to the address in LR, so step to there instead.  */
>      if (next_pc > 0xffff0000)
>        next_pc = get_frame_register_unsigned (frame, ARM_LR_REGNUM);
>
>      arm_insert_single_step_breakpoint (gdbarch, aspace, next_pc);
>
>    Unfortunately, branch destination addresses are SIGN EXTENDED to 64
>    bits.  So,
>
>    (top-gdb) p/x next_pc
>    $14 = 0xffffffffb6df2864
>
>    Which triggers the next_pc = get_frame_register_unsigned(), and we
>    cannot step into any branches because the destination PC is wrong.

Anyway, the fix is simple and Andrew himself provided it for us.  It
took a while for me to figure out how to trigger the bug (in order to
write a testcase for it), but I finally made it.

The attached patch fixes the problem (by casting to `unsigned long'
instead of just `long'), and also includes a testcase to reproduce the
issue.

OK to apply?  BTW, I guess this is trivial enough to be included in 7.6
as well, do you agree?

Thanks,

-- 
Sergio

gdb/
2013-04-11  Andrew Haley  <aph@redhat.com>

	* arm-tdep.c (BranchDest): Cast result as "unsigned long",
	instead of "long".

gdb/testsuite
2013-04-11  Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.arch/arm-bl-branch-dest.c: New file.
	* gdb.arch/arm-bl-branch-dest.exp: Likewise.

diff --git a/gdb/arm-tdep.c b/gdb/arm-tdep.c
index 2a11890..16cb488 100644
--- a/gdb/arm-tdep.c
+++ b/gdb/arm-tdep.c
@@ -521,7 +521,7 @@ skip_prologue_function (struct gdbarch *gdbarch, CORE_ADDR pc, int is_thumb)
 #define sbits(obj,st,fn) \
   ((long) (bits(obj,st,fn) | ((long) bit(obj,fn) * ~ submask (fn - st))))
 #define BranchDest(addr,instr) \
-  ((CORE_ADDR) (((long) (addr)) + 8 + (sbits (instr, 0, 23) << 2)))
+  ((CORE_ADDR) (((unsigned long) (addr)) + 8 + (sbits (instr, 0, 23) << 2)))
 
 /* Extract the immediate from instruction movw/movt of encoding T.  INSN1 is
    the first 16-bit of instruction, and INSN2 is the second 16-bit of
diff --git a/gdb/testsuite/gdb.arch/arm-bl-branch-dest.c b/gdb/testsuite/gdb.arch/arm-bl-branch-dest.c
new file mode 100644
index 0000000..98269d0
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/arm-bl-branch-dest.c
@@ -0,0 +1,29 @@
+/* This testcase is part of GDB, the GNU debugger.
+
+   Copyright 2013 Free Software Foundation, Inc.
+
+   This program is free software; you can redistribute it and/or modify
+   it under the terms of the GNU General Public License as published by
+   the Free Software Foundation; either version 3 of the License, or
+   (at your option) any later version.
+
+   This program is distributed in the hope that it will be useful,
+   but WITHOUT ANY WARRANTY; without even the implied warranty of
+   MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+   GNU General Public License for more details.
+
+   You should have received a copy of the GNU General Public License
+   along with this program.  If not, see <http://www.gnu.org/licenses/>.  */
+
+static void
+foo (int a)
+{
+  ++a;
+}
+
+int
+main (int argc, char *argv[])
+{
+  foo (10);
+  return 0;
+}
diff --git a/gdb/testsuite/gdb.arch/arm-bl-branch-dest.exp b/gdb/testsuite/gdb.arch/arm-bl-branch-dest.exp
new file mode 100644
index 0000000..1e433f3
--- /dev/null
+++ b/gdb/testsuite/gdb.arch/arm-bl-branch-dest.exp
@@ -0,0 +1,37 @@
+# Copyright (C) 2013 Free Software Foundation, Inc.
+#
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+if { ![istarget "arm*-*-*"] } {
+    verbose "Skipping ${testfile}."
+    return
+}
+
+standard_testfile
+
+# We need to load the text segment in a high address.  This is because
+# the bug we are dealing with happened when GDB sign-extended the PC
+# on ARM, causing the PC to acquire a wrong value.  That's why we use
+# the "-Wl,-Ttext-segment" option compile the binary.
+
+if { [prepare_for_testing ${testfile}.exp ${testfile} ${srcfile} \
+    [list debug ldflags=-Wl,-Ttext-segment=0xb0000000]] } {
+    return -1
+}
+
+if { ![runto_main] } {
+    return -1
+}
+
+gdb_test "next" "\[0-9\]+\\s+return 0;"