objtool: Support conditional retpolines

A Clang-built kernel is showing the following warning:

  arch/x86/kernel/platform-quirks.o: warning: objtool: x86_early_init_platform_quirks()+0x84: unreachable instruction

That corresponds to this code:

  7e:   0f 85 00 00 00 00       jne    84 <x86_early_init_platform_quirks+0x84>
                        80: R_X86_64_PC32       __x86_indirect_thunk_r11-0x4
  84:   c3                      retq

This is a conditional retpoline sibling call, which is now possible
thanks to retpolines.  Objtool hasn't seen that before.  It's
incorrectly interpreting the conditional jump as an unconditional
dynamic jump.

Reported-by: Nick Desaulniers <ndesaulniers@google.com>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/30d4c758b267ef487fb97e6ecb2f148ad007b554.1563413318.git.jpoimboe@redhat.com

2 files changed, 11 insertions, 2 deletions

diff --git a/tools/objtool/arch.h b/tools/objtool/arch.h
index 50448c0c4bca..ced3765c4f44 100644
--- a/tools/objtool/arch.h
+++ b/tools/objtool/arch.h
@@ -15,6 +15,7 @@ enum insn_type {
         INSN_JUMP_CONDITIONAL,
         INSN_JUMP_UNCONDITIONAL,
         INSN_JUMP_DYNAMIC,
+        INSN_JUMP_DYNAMIC_CONDITIONAL,
         INSN_CALL,
         INSN_CALL_DYNAMIC,
         INSN_RETURN,
diff --git a/tools/objtool/check.c b/tools/objtool/check.c
index 04572a049cfc..5f26620f13f5 100644
--- a/tools/objtool/check.c
+++ b/tools/objtool/check.c
@@ -575,7 +575,11 @@ static int add_jump_destinations(struct objtool_file *file)
                          * Retpoline jumps are really dynamic jumps in
                          * disguise, so convert them accordingly.
                          */
-                        insn->type = INSN_JUMP_DYNAMIC;
+                        if (insn->type == INSN_JUMP_UNCONDITIONAL)
+                                insn->type = INSN_JUMP_DYNAMIC;
+                        else
+                                insn->type = INSN_JUMP_DYNAMIC_CONDITIONAL;
+
                         insn->retpoline_safe = true;
                         continue;
                 } else {
@@ -2114,13 +2118,17 @@ static int validate_branch(struct objtool_file *file, struct symbol *func,
                         break;
 
                 case INSN_JUMP_DYNAMIC:
+                case INSN_JUMP_DYNAMIC_CONDITIONAL:
                         if (func && is_sibling_call(insn)) {
                                 ret = validate_sibling_call(insn, &state);
                                 if (ret)
                                         return ret;
                         }
 
-                        return 0;
+                        if (insn->type == INSN_JUMP_DYNAMIC)
+                                return 0;
+
+                        break;
 
                 case INSN_CONTEXT_SWITCH:
                         if (func && (!next_insn || !next_insn->hint)) {