ftrace,kcfi: Separate ftrace_stub() and ftrace_stub_graph()

Different function signatures means they needs to be different
functions; otherwise CFI gets upset.

As triggered by the ftrace boot tests:

  [] CFI failure at ftrace_return_to_handler+0xac/0x16c (target: ftrace_stub+0x0/0x14; expected type: 0x0a5d5347)

Fixes: 3c516f89 ("x86: Add support for CONFIG_CFI_CLANG")
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Tested-by: Mark Rutland <mark.rutland@arm.com>
Link: https://lkml.kernel.org/r/Y06dg4e1xF6JTdQq@hirez.programming.kicks-ass.net

arch/arm64/kernel/entry-ftrace.S

@@ -7,6 +7,7 @@
  */
 
 #include <linux/linkage.h>
+#include <linux/cfi_types.h>
 #include <asm/asm-offsets.h>
 #include <asm/assembler.h>
 #include <asm/ftrace.h>
@@ -294,10 +295,14 @@ SYM_FUNC_END(ftrace_graph_caller)
 #endif /* CONFIG_FUNCTION_GRAPH_TRACER */
 #endif /* CONFIG_DYNAMIC_FTRACE_WITH_REGS */
 
-SYM_FUNC_START(ftrace_stub)
+SYM_TYPED_FUNC_START(ftrace_stub)
 	ret
 SYM_FUNC_END(ftrace_stub)
 
+SYM_TYPED_FUNC_START(ftrace_stub_graph)
+	ret
+SYM_FUNC_END(ftrace_stub_graph)
+
 #ifdef CONFIG_FUNCTION_GRAPH_TRACER
 /*
  * void return_to_handler(void)

arch/x86/kernel/ftrace_64.S

@@ -4,6 +4,7 @@
  */
 
 #include <linux/linkage.h>
+#include <linux/cfi_types.h>
 #include <asm/ptrace.h>
 #include <asm/ftrace.h>
 #include <asm/export.h>
@@ -129,6 +130,14 @@
 
 	.endm
 
+SYM_TYPED_FUNC_START(ftrace_stub)
+	RET
+SYM_FUNC_END(ftrace_stub)
+
+SYM_TYPED_FUNC_START(ftrace_stub_graph)
+	RET
+SYM_FUNC_END(ftrace_stub_graph)
+
 #ifdef CONFIG_DYNAMIC_FTRACE
 
 SYM_FUNC_START(__fentry__)
@@ -176,11 +185,6 @@ SYM_INNER_LABEL(ftrace_caller_end, SYM_L_GLOBAL)
 SYM_FUNC_END(ftrace_caller);
 STACK_FRAME_NON_STANDARD_FP(ftrace_caller)
 
-SYM_FUNC_START(ftrace_stub)
-	UNWIND_HINT_FUNC
-	RET
-SYM_FUNC_END(ftrace_stub)
-
 SYM_FUNC_START(ftrace_regs_caller)
 	/* Save the current flags before any operations that can change them */
 	pushfq
@@ -282,9 +286,6 @@ STACK_FRAME_NON_STANDARD_FP(ftrace_regs_caller)
 SYM_FUNC_START(__fentry__)
 	cmpq $ftrace_stub, ftrace_trace_function
 	jnz trace
-
-SYM_INNER_LABEL(ftrace_stub, SYM_L_GLOBAL)
-	ENDBR
 	RET
 
 trace:

include/asm-generic/vmlinux.lds.h

@@ -162,6 +162,16 @@
 #define PATCHABLE_DISCARDS	*(__patchable_function_entries)
 #endif
 
+#ifndef CONFIG_ARCH_SUPPORTS_CFI_CLANG
+/*
+ * Simply points to ftrace_stub, but with the proper protocol.
+ * Defined by the linker script in linux/vmlinux.lds.h
+ */
+#define	FTRACE_STUB_HACK	ftrace_stub_graph = ftrace_stub;
+#else
+#define FTRACE_STUB_HACK
+#endif
+
 #ifdef CONFIG_FTRACE_MCOUNT_RECORD
 /*
  * The ftrace call sites are logged to a section whose name depends on the
@@ -169,10 +179,6 @@
  * FTRACE_CALLSITE_SECTION. We capture all of them here to avoid header
  * dependencies for FTRACE_CALLSITE_SECTION's definition.
  *
- * Need to also make ftrace_stub_graph point to ftrace_stub
- * so that the same stub location may have different protocols
- * and not mess up with C verifiers.
- *
  * ftrace_ops_list_func will be defined as arch_ftrace_ops_list_func
  * as some archs will have a different prototype for that function
  * but ftrace_ops_list_func() will have a single prototype.
@@ -182,11 +188,11 @@
 			KEEP(*(__mcount_loc))			\
 			KEEP_PATCHABLE				\
 			__stop_mcount_loc = .;			\
-			ftrace_stub_graph = ftrace_stub;	\
+			FTRACE_STUB_HACK			\
 			ftrace_ops_list_func = arch_ftrace_ops_list_func;
 #else
 # ifdef CONFIG_FUNCTION_TRACER
-#  define MCOUNT_REC()	ftrace_stub_graph = ftrace_stub;	\
+#  define MCOUNT_REC()	FTRACE_STUB_HACK			\
 			ftrace_ops_list_func = arch_ftrace_ops_list_func;
 # else
 #  define MCOUNT_REC()