[PATCH v2] x86: Ignore CS, DS, ES, and SS segment-override prefixes in 64-bit mode

[H.J. Lu]

On Sun, Nov 1, 2020 at 2:46 PM Borislav Petkov <bp@suse.de> wrote:
>
> On Wed, Oct 28, 2020 at 09:59:24PM +0100, Borislav Petkov wrote:
> > On Sun, Oct 25, 2020 at 11:23:12AM -0700, H.J. Lu wrote:
> > > Yes, this looks right.  Please add a testcase to verify it.
> >
> > Yeah, I need to analyze and address a couple of testcases failures
> > first. Will send v2 when ready.
>
> Ok, here's v2. I've fixed all test cases that started failing due to the
> change so that they accept the new format, see below. With that, there's
> no need for adding a new testcase as the old ones are simply made to
> check for the new dumping of prefixes before the insn.
>
> Also, I've added
>
> gas/testsuite/ChangeLog
> ld/testsuite/ChangeLog
>
> because there weren't changelog files for this year. I hope that is the
> correct way to do that.
>
> What also needed adjustment is the NOTRACK_Fixup() handling, see also
> below.
>
> Now all testcases pass again.
>
> Thx.
>
> ---
> From: Borislav Petkov <bp@suse.de>
> Date: Sun, 25 Oct 2020 10:23:26 +0100
>
> In 64-bit mode, the four segment override prefixes are ignored:
>
> "In 64-bit mode, the CS, DS, ES, and SS segment-override prefixes have
> no effect. These four prefixes are not treated as segment-override
> prefixes for the purposes of multiple-prefix rules. Instead, they are
> treated as null prefixes." (AMD APM v2).
>
> However, objdump disassembles instructions containing those ignored
> prefixes by still generating that segment override:
>
>   66 66 2e 0f 1f 84 00  data16 nopw %cs:0x0(%rax,%rax,1)
>   00 00 00 00
>
> Print those segment override prefixes as excessive ones:
>
>   66 66 2e 0f 1f 84 00    data16 cs nopw 0x0(%rax,%rax,1)
>   00 00 00 00
>
> which is what they actually are - they have no effect and the decoding
> hardware ignores them.
> ---
>  gas/testsuite/ChangeLog                       |  37 ++++
>  .../gas/i386/ilp32/x86-64-nops-1-core2.d      |  12 +-
>  .../gas/i386/ilp32/x86-64-nops-1-k8.d         |  12 +-
>  gas/testsuite/gas/i386/ilp32/x86-64-nops-1.d  |  12 +-
>  gas/testsuite/gas/i386/ilp32/x86-64-nops-2.d  |  14 +-
>  gas/testsuite/gas/i386/ilp32/x86-64-nops-3.d  |   6 +-
>  .../gas/i386/ilp32/x86-64-nops-4-core2.d      |  54 +++---
>  .../gas/i386/ilp32/x86-64-nops-4-k8.d         |  54 +++---
>  gas/testsuite/gas/i386/ilp32/x86-64-nops-4.d  |  54 +++---
>  .../gas/i386/ilp32/x86-64-nops-5-k8.d         |  16 +-
>  gas/testsuite/gas/i386/ilp32/x86-64-nops-5.d  |  16 +-
>  gas/testsuite/gas/i386/ilp32/x86-64-nops.d    |   2 +-
>  .../gas/i386/x86-64-align-branch-1a.d         |   4 +-
>  .../gas/i386/x86-64-align-branch-1b.d         |   4 +-
>  .../gas/i386/x86-64-align-branch-1c.d         |   4 +-
>  .../gas/i386/x86-64-align-branch-1d.d         |   2 +-
>  .../gas/i386/x86-64-align-branch-1g.d         |   4 +-
>  .../gas/i386/x86-64-align-branch-2c.d         |   4 +-
>  .../gas/i386/x86-64-align-branch-6.d          |  10 +-
>  .../gas/i386/x86-64-align-branch-7.d          |   2 +-
>  .../gas/i386/x86-64-align-branch-8.d          |   2 +-
>  gas/testsuite/gas/i386/x86-64-nop-1.d         |  28 +--
>  gas/testsuite/gas/i386/x86-64-nops-1-core2.d  |  12 +-
>  gas/testsuite/gas/i386/x86-64-nops-1-g64.d    |  12 +-
>  gas/testsuite/gas/i386/x86-64-nops-1-k8.d     |  12 +-
>  gas/testsuite/gas/i386/x86-64-nops-1.d        |  12 +-
>  gas/testsuite/gas/i386/x86-64-nops-2.d        |  14 +-
>  gas/testsuite/gas/i386/x86-64-nops-3.d        |   6 +-
>  gas/testsuite/gas/i386/x86-64-nops-4-core2.d  |  54 +++---
>  gas/testsuite/gas/i386/x86-64-nops-4-k8.d     |  54 +++---
>  gas/testsuite/gas/i386/x86-64-nops-4.d        |  54 +++---
>  gas/testsuite/gas/i386/x86-64-nops-5-k8.d     |  16 +-
>  gas/testsuite/gas/i386/x86-64-nops-5.d        |  16 +-
>  gas/testsuite/gas/i386/x86-64-nops-7.d        | 172 +++++++++---------
>  gas/testsuite/gas/i386/x86-64-nops.d          |   2 +-
>  gas/testsuite/gas/i386/x86-64-segovr.d        |  32 ++--
>  ld/testsuite/ChangeLog                        |   6 +
>  ld/testsuite/ld-x86-64/align-branch-1.d       |   2 +-
>  ld/testsuite/ld-x86-64/pe-x86-64-4.od         |   2 +-
>  ld/testsuite/ld-x86-64/tlsld3.dd              |   2 +-
>  ld/testsuite/ld-x86-64/tlsld4.dd              |   2 +-
>  opcodes/ChangeLog                             |   5 +
>  opcodes/i386-dis.c                            |  25 ++-
>  43 files changed, 464 insertions(+), 401 deletions(-)
>  create mode 100644 gas/testsuite/ChangeLog
>  create mode 100644 ld/testsuite/ChangeLog
>

+2020-10-26 Borislav Petkov <bp@suse.de>
+
+ * opcodes/i386-dis.c: Ignore CS,DS,FS,ES segment override prefix in
+ 64-bit mode.

Please mention what you changed in which functions in ChangeLog
entry.  OK with these changes.

Thanks.

-- 
H.J.