x86/ftrace: Rebalance RSB

ftrace_regs_caller() uses a PUSH;RET pattern to tail-call into a
direct-call function, this unbalances the RSB, fix that.
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111148.823216933@infradead.org

arch/x86/kernel/ftrace_64.S

@@ -271,6 +271,17 @@ SYM_INNER_LABEL(ftrace_regs_caller_end, SYM_L_GLOBAL)
 	/* Restore flags */
 	popfq
 	UNWIND_HINT_FUNC
+
+	/*
+	 * The above left an extra return value on the stack; effectively
+	 * doing a tail-call without using a register. This PUSH;RET
+	 * pattern unbalances the RSB, inject a pointless CALL to rebalance.
+	 */
+	ANNOTATE_INTRA_FUNCTION_CALL
+	CALL .Ldo_rebalance
+	int3
+.Ldo_rebalance:
+	add $8, %rsp
 	RET
 
 SYM_FUNC_END(ftrace_regs_caller)