x86/entry: Make entry_64_compat.S objtool clean

Currently entry_64_compat is exempt from objtool, but with vmlinux
mode there is no hiding it.

Make the following changes to make it pass:

 - change entry_SYSENTER_compat to STT_NOTYPE; it's not a function
   and doesn't have function type stack setup.

 - mark all STT_NOTYPE symbols with UNWIND_HINT_EMPTY; so we do
   validate them and don't treat them as unreachable.

 - don't abuse RSP as a temp register, this confuses objtool
   mightily as it (rightfully) thinks we're doing unspeakable
   things to the stack.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Acked-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200505134341.272248024@linutronix.de

arch/x86/entry/Makefile

@@ -11,8 +11,6 @@ CFLAGS_REMOVE_common.o = $(CC_FLAGS_FTRACE) -fstack-protector -fstack-protector-
 CFLAGS_REMOVE_syscall_32.o = $(CC_FLAGS_FTRACE) -fstack-protector -fstack-protector-strong
 CFLAGS_REMOVE_syscall_64.o = $(CC_FLAGS_FTRACE) -fstack-protector -fstack-protector-strong
 
-OBJECT_FILES_NON_STANDARD_entry_64_compat.o := y
-
 CFLAGS_syscall_64.o		+= $(call cc-option,-Wno-override-init,)
 CFLAGS_syscall_32.o		+= $(call cc-option,-Wno-override-init,)
 obj-y				:= entry_$(BITS).o thunk_$(BITS).o syscall_$(BITS).o

arch/x86/entry/entry_64_compat.S

@@ -46,12 +46,14 @@
  * ebp  user stack
  * 0(%ebp) arg6
  */
-SYM_FUNC_START(entry_SYSENTER_compat)
+SYM_CODE_START(entry_SYSENTER_compat)
+	UNWIND_HINT_EMPTY
 	/* Interrupts are off on entry. */
 	SWAPGS
 
-	/* We are about to clobber %rsp anyway, clobbering here is OK */
-	SWITCH_TO_KERNEL_CR3 scratch_reg=%rsp
+	pushq	%rax
+	SWITCH_TO_KERNEL_CR3 scratch_reg=%rax
+	popq	%rax
 
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
 
@@ -104,6 +106,9 @@ SYM_FUNC_START(entry_SYSENTER_compat)
 	xorl	%r14d, %r14d		/* nospec   r14 */
 	pushq   $0			/* pt_regs->r15 = 0 */
 	xorl	%r15d, %r15d		/* nospec   r15 */
+
+	UNWIND_HINT_REGS
+
 	cld
 
 	/*
@@ -141,7 +146,7 @@ SYM_FUNC_START(entry_SYSENTER_compat)
 	popfq
 	jmp	.Lsysenter_flags_fixed
 SYM_INNER_LABEL(__end_entry_SYSENTER_compat, SYM_L_GLOBAL)
-SYM_FUNC_END(entry_SYSENTER_compat)
+SYM_CODE_END(entry_SYSENTER_compat)
 
 /*
  * 32-bit SYSCALL entry.
@@ -191,6 +196,7 @@ SYM_FUNC_END(entry_SYSENTER_compat)
  * 0(%esp) arg6
  */
 SYM_CODE_START(entry_SYSCALL_compat)
+	UNWIND_HINT_EMPTY
 	/* Interrupts are off on entry. */
 	swapgs
 
@@ -241,6 +247,8 @@ SYM_INNER_LABEL(entry_SYSCALL_compat_after_hwframe, SYM_L_GLOBAL)
 	pushq   $0			/* pt_regs->r15 = 0 */
 	xorl	%r15d, %r15d		/* nospec   r15 */
 
+	UNWIND_HINT_REGS
+
 	movq	%rsp, %rdi
 	call	do_fast_syscall_32
 	/* XEN PV guests always use IRET path */
@@ -328,6 +336,7 @@ SYM_CODE_END(entry_SYSCALL_compat)
  * ebp  arg6
  */
 SYM_CODE_START(entry_INT80_compat)
+	UNWIND_HINT_EMPTY
 	/*
 	 * Interrupts are off on entry.
 	 */
@@ -349,8 +358,11 @@ SYM_CODE_START(entry_INT80_compat)
 
 	/* Need to switch before accessing the thread stack. */
 	SWITCH_TO_KERNEL_CR3 scratch_reg=%rdi
+
 	/* In the Xen PV case we already run on the thread stack. */
-	ALTERNATIVE "movq %rsp, %rdi", "jmp .Lint80_keep_stack", X86_FEATURE_XENPV
+	ALTERNATIVE "", "jmp .Lint80_keep_stack", X86_FEATURE_XENPV
+
+	movq	%rsp, %rdi
 	movq	PER_CPU_VAR(cpu_current_top_of_stack), %rsp
 
 	pushq	6*8(%rdi)		/* regs->ss */
@@ -389,6 +401,9 @@ SYM_CODE_START(entry_INT80_compat)
 	xorl	%r14d, %r14d		/* nospec   r14 */
 	pushq   %r15                    /* pt_regs->r15 */
 	xorl	%r15d, %r15d		/* nospec   r15 */
+
+	UNWIND_HINT_REGS
+
 	cld
 
 	movq	%rsp, %rdi