objtool: Fix SLS validation for kcov tail-call replacement

Since not all compilers have a function attribute to disable KCOV
instrumentation, objtool can rewrite KCOV instrumentation in noinstr
functions as per commit:

  f56dae88 ("objtool: Handle __sanitize_cov*() tail calls")

However, this has subtle interaction with the SLS validation from
commit:

  1cc1e4c8 ("objtool: Add straight-line-speculation validation")

In that when a tail-call instrucion is replaced with a RET an
additional INT3 instruction is also written, but is not represented in
the decoded instruction stream.

This then leads to false positive missing INT3 objtool warnings in
noinstr code.

Instead of adding additional struct instruction objects, mark the RET
instruction with retpoline_safe to suppress the warning (since we know
there really is an INT3).

Fixes: 1cc1e4c8 ("objtool: Add straight-line-speculation validation")
Signed-off-by: NPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220323230712.GA8939@worktop.programming.kicks-ass.net

tools/objtool/check.c

@@ -1155,6 +1155,17 @@ static void annotate_call_site(struct objtool_file *file,
 			               : arch_nop_insn(insn->len));
 
 		insn->type = sibling ? INSN_RETURN : INSN_NOP;
+
+		if (sibling) {
+			/*
+			 * We've replaced the tail-call JMP insn by two new
+			 * insn: RET; INT3, except we only have a single struct
+			 * insn here. Mark it retpoline_safe to avoid the SLS
+			 * warning, instead of adding another insn.
+			 */
+			insn->retpoline_safe = true;
+		}
+
 		return;
 	}