[Bug binutils/17898] New: rdrand/rdseed should not disassmble with f2/f3 prefixes present.

[markcharney at gmail dot com]

https://sourceware.org/bugzilla/show_bug.cgi?id=17898

            Bug ID: 17898
           Summary: rdrand/rdseed should not disassmble with f2/f3
                    prefixes present.
           Product: binutils
           Version: 2.24
            Status: NEW
          Severity: normal
          Priority: P2
         Component: binutils
          Assignee: unassigned at sourceware dot org
          Reporter: markcharney at gmail dot com

binutils/objdump should do the software equivalent of a #UD (emit "(bad)" or
whatever is the binutils convention for unrecognized instructions) if f2/f3
prefixes are present before rdrand/rdseed. Intel SDM says so.  Binutils
disassembles them as if F2/F3 were legal legacy prefixes for these
instructions. 

I demonstrated this with binutils 2.24. I did not check later versions.


% objdump -v
GNU objdump (GNU Binutils) 2.24
Copyright 2013 Free Software Foundation, Inc.
This program is free software; you may redistribute it under the terms of
the GNU General Public License version 3 or (at your option) any later version.
This program has absolutely no warranty.



% cat foo.c
int main()
{
    asm volatile(".byte 0xF3, 0x0F, 0xC7, 0xF8");
    asm volatile(".byte 0xF3, 0x0F, 0xC7, 0xF0");
    return 0;
}

% gcc foo.c

% objdump -D a.out | grep -A10 'main>:'
0000000000400496 <main>:
  400496:    55                       push   rbp
  400497:    48 89 e5                 mov    rbp,rsp
  40049a:    f3 0f c7 f8              repz rdseed eax
  40049e:    f3 0f c7 f0              repz rdrand eax
  4004a2:    b8 00 00 00 00           mov    eax,0x0
  4004a7:    5d                       pop    rbp
  4004a8:    c3                       ret    
  4004a9:    0f 1f 80 00 00 00 00     nop    DWORD PTR [rax+0x0]

00000000004004b0 <__libc_csu_init>:

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