Re: [PATCH] Fix buffer underrun in i386-dis.c.

H.J. Lu
2014-01-09T18:31:18Z

On Thu, Jan 9, 2014 at 10:26 AM, Roland McGrath <mcgrathr@google.com> wrote:
> When disassembling any instruction without a REX prefix, the print_insn
> function touches all_prefixes[-1]. This is usually harmless in most
> builds, because the word preceding all_prefixes will probably be the
> last_seg_prefix variable and it was usually zero already. But in some
> kinds of builds, all buffer underruns are caught and cause a crash.
>
> AFAICT the obvious local workaround is in fact the proper fix. In the
> similar cases nearby, there is a PREFIX_FOO bit in the "prefixes" bitmask
> that guards use of last_foo_prefix. But there is no such bit for the REX
> prefixes. We could test "rex != 0" instead, I suppose.
>
> OK for trunk and binutils-2.24 branch and gdb-7.7 branch?

OK for trunk and binutils-2.24 branch.

Thanks.

>
> Thanks,
> Roland
>
>
> opcodes/
> 2014-01-09 Bradley Nelson <bradnelson@google.com>
> Roland McGrath <mcgrathr@google.com>
>
> * i386-dis.c (print_insn): Do not touch all_prefixes[-1] when
> last_rex_prefix is -1.
>
> --- a/opcodes/i386-dis.c
> +++ b/opcodes/i386-dis.c
> @@ -12645,7 +12645,7 @@ print_insn (bfd_vma pc, disassemble_info *info)
> }
>
> /* Check if the REX prefix is used. */
> - if (rex_ignored == 0 && (rex ^ rex_used) == 0)
> + if (rex_ignored == 0 && (rex ^ rex_used) == 0 && last_rex_prefix >= 0)
> all_prefixes[last_rex_prefix] = 0;
>
> /* Check if the SEG prefix is used. */

--
H.J.