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

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

On Wednesday, April 17 2013, I wrote:

Ping.

> 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;"

-- 
Sergio