[RFC][PATCH] objtool: WARN about ENDBR instructions

[Peter Zijlstra <peterz@infradead.org>]

Given all the ENDBR fun we recently had, do we want the below? Until
someone comes and fixes up kprobes/ftrace/livepatch etc.. having them is
a giant pain and we'd better warn about it.

---
diff --git a/tools/objtool/arch/x86/decode.c b/tools/objtool/arch/x86/decode.c
index 931cef78b857..e708731b10cd 100644
--- a/tools/objtool/arch/x86/decode.c
+++ b/tools/objtool/arch/x86/decode.c
@@ -91,7 +91,7 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
 {
 	struct insn insn;
 	int x86_64, sign;
-	unsigned char op1, op2, rex = 0, rex_b = 0, rex_r = 0, rex_w = 0,
+	unsigned char op1, op2, pfx = 0, rex = 0, rex_b = 0, rex_r = 0, rex_w = 0,
 		      rex_x = 0, modrm = 0, modrm_mod = 0, modrm_rm = 0,
 		      modrm_reg = 0, sib = 0;
 	struct stack_op *op = NULL;
@@ -118,6 +118,9 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
 	op1 = insn.opcode.bytes[0];
 	op2 = insn.opcode.bytes[1];
 
+	if (insn.prefixes.nbytes)
+		pfx = insn.prefixes.bytes[0];
+
 	if (insn.rex_prefix.nbytes) {
 		rex = insn.rex_prefix.bytes[0];
 		rex_w = X86_REX_W(rex) >> 3;
@@ -444,6 +447,11 @@ int arch_decode_instruction(const struct elf *elf, const struct section *sec,
 			/* nopl/nopw */
 			*type = INSN_NOP;
 
+		} else if (op2 == 0x1e && pfx == 0xf3 && (modrm == 0xfa || modrm == 0xfb)) {
+
+			/* endbr32/endbr64 */
+			WARN("endbr32/64 instruction at %s:0x%lx", sec->name, offset);
+
 		} else if (op2 == 0xa0 || op2 == 0xa8) {
 
 			/* push fs/gs */