[PATCH] x86: Do not dump DS/CS segment overrides for branch hints

[Borislav Petkov]

Hi,

the change in this thread caused one regression, here's a fix. Sorry
about that.

---
From: Borislav Petkov <bp@suse.de>

The previous change

  "x86: Ignore CS/DS/ES/SS segment-override prefixes in 64-bit mode"

to ignore segment override prefixes in 64-bit mode lead to dumping
branch hints as excessive prefixes:

  ffffffff8109d5a0 <vmx_get_rflags>:
  ...
  ffffffff8109d601:       3e 77 0a                ds ja,pt ffffffff8109d60e <vmx_get_rflags+0x6e>
  						^^^^^

In this particular case, those prefixes are not excessive but are used
to provide branch hints - taken/not-taken - to the CPU.

Assign active_seg_prefix in that particular case to consume them.

opcodes /

2020-11-28 Borislav Petkov  <bp@suse.de>

        * i386-dis.c (print_insn): Set active_seg_prefix for branch hint insns
        to not dump branch hint prefixes 0x2E and 0x3E as unused prefixes.
---
 opcodes/ChangeLog  |  5 +++++
 opcodes/i386-dis.c | 10 ++++++++--
 2 files changed, 13 insertions(+), 2 deletions(-)

diff --git a/opcodes/ChangeLog b/opcodes/ChangeLog
index 1e7e40fd9664..02fd2f5b5837 100644
--- a/opcodes/ChangeLog
+++ b/opcodes/ChangeLog
@@ -1,3 +1,8 @@
+2020-11-28 Borislav Petkov  <bp@suse.de>
+
+	* i386-dis.c (print_insn): Set active_seg_prefix for branch hint insns
+	to not dump branch hint prefixes 0x2E and 0x3E as unused prefixes.
+
 2020-11-16  Przemyslaw Wirkus  <przemyslaw.wirkus@arm.com>
 
 	* aarch64-tbl.h (FLAGM): Handle for FLAGM feature.
diff --git a/opcodes/i386-dis.c b/opcodes/i386-dis.c
index 58cf001080bc..d1b3f50f7b70 100644
--- a/opcodes/i386-dis.c
+++ b/opcodes/i386-dis.c
@@ -10910,10 +10910,16 @@ putop (const char *in_template, int sizeflag)
 	      used_prefixes |= prefixes & (PREFIX_CS | PREFIX_DS);
 	      *obufp++ = ',';
 	      *obufp++ = 'p';
-	      if (prefixes & PREFIX_DS)
+
+	      /* Set active_seg_prefix even if not set in 64-bit mode because
+	       * here it is a valid branch hint. */
+	      if (prefixes & PREFIX_DS) {
+		active_seg_prefix = PREFIX_DS;
 		*obufp++ = 't';
-	      else
+	      } else {
+		active_seg_prefix = PREFIX_CS;
 		*obufp++ = 'n';
+	      }
 	    }
 	  break;
 	case 'K':
-- 
2.21.0

-- 
Regards/Gruss,
    Boris.

SUSE Software Solutions Germany GmbH, GF: Felix Imendörffer, HRB 36809, AG Nürnberg