ftrace/x86: Tell objtool to ignore nondeterministic ftrace stack layout

Objtool complains about the new ftrace direct trampoline code:

  arch/x86/kernel/ftrace_64.o: warning: objtool: ftrace_regs_caller()+0x190: stack state mismatch: cfa1=7+16 cfa2=7+24

Typically, code has a deterministic stack layout, such that at a given
instruction address, the stack frame size is always the same.

That's not the case for the new ftrace_regs_caller() code after it
adjusts the stack for the direct case.  Just plead ignorance and assume
it's always the non-direct path.  Note this creates a tiny window for
ORC to get confused.

Link: http://lkml.kernel.org/r/20191108225100.ea3bhsbdf6oerj6g@treble

Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Josh Poimboeuf <jpoimboe@redhat.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>

arch/x86/include/asm/unwind_hints.h

@@ -86,6 +86,14 @@
 	UNWIND_HINT sp_offset=\sp_offset
 .endm
 
+.macro UNWIND_HINT_SAVE
+	UNWIND_HINT type=UNWIND_HINT_TYPE_SAVE
+.endm
+
+.macro UNWIND_HINT_RESTORE
+	UNWIND_HINT type=UNWIND_HINT_TYPE_RESTORE
+.endm
+
 #else /* !__ASSEMBLY__ */
 
 #define UNWIND_HINT(sp_reg, sp_offset, type, end)		\

arch/x86/kernel/ftrace_64.S

@@ -178,6 +178,8 @@ ENTRY(ftrace_regs_caller)
 	/* Save the current flags before any operations that can change them */
 	pushfq
 
+	UNWIND_HINT_SAVE
+
 	/* added 8 bytes to save flags */
 	save_mcount_regs 8
 	/* save_mcount_regs fills in first two parameters */
@@ -250,8 +252,16 @@ GLOBAL(ftrace_regs_call)
 1:	restore_mcount_regs
 
 
+2:
+	/*
+	 * The stack layout is nondetermistic here, depending on which path was
+	 * taken.  This confuses objtool and ORC, rightfully so.  For now,
+	 * pretend the stack always looks like the non-direct case.
+	 */
+	UNWIND_HINT_RESTORE
+
 	/* Restore flags */
-2:	popfq
+	popfq
 
 	/*
 	 * As this jmp to ftrace_epilogue can be a short jump