[PATCH 22/22] objtool: Support conditional retpolines

[Josh Poimboeuf]

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@xxxxxxxxxx>
Signed-off-by: Josh Poimboeuf <jpoimboe@xxxxxxxxxx>
---
 tools/objtool/arch.h  |  1 +
 tools/objtool/check.c | 12 ++++++++++--
 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 5a237fc05cdc..a8411355a80d 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)) {
-- 
2.20.1