objtool/x86: Fix NOP decode

For x86_64 the kernel consistently uses 2 instructions for all NOPs:

  90       - NOP
  0f 1f /0 - NOPL

Notably:

 - REP NOP is PAUSE, not a NOP instruction.

 - 0f {0c...0f} is reserved space,
   except for 0f 0d /1, which is PREFETCHW, not a NOP.

 - 0f {19,1c...1f} is reserved space,
   except for 0f 1f /0, which is NOPL.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>

diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index ef6e96db8ce4424e146953a3bf96f10b192aa480..204e2ad1fadaeaa9faf557b7d7eb5cab74d0ce83 100644 (file)
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -494,6 +494,12 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
                break;
 
        case 0x90:
+               if (rex_b) /* XCHG %r8, %rax */
+                       break;
+
+               if (prefix == 0xf3) /* REP NOP := PAUSE */
+                       break;
+
                insn->type = INSN_NOP;
                break;
 
@@ -547,13 +553,14 @@ int arch_decode_instruction(struct objtool_file *file, const struct section *sec
 
                } else if (op2 == 0x0b || op2 == 0xb9) {
 
-                       /* ud2 */
+                       /* ud2, ud1 */
                        insn->type = INSN_BUG;
 
-               } else if (op2 == 0x0d || op2 == 0x1f) {
+               } else if (op2 == 0x1f) {
 
-                       /* nopl/nopw */
-                       insn->type = INSN_NOP;
+                       /* 0f 1f /0 := NOPL */
+                       if (modrm_reg == 0)
+                               insn->type = INSN_NOP;
 
                } else if (op2 == 0x1e) {