[RFC/INCOMPLETE 05/13] x86/entry: Add enter_from_user_mode and use it in syscalls

[Andy Lutomirski <luto@kernel.org>]

Changing the x86 context tracking hooks is dangerous because there
are no good checks that we track our context correctly.  Add a
helper to check that we're actually in CONTEXT_USER when we enter
from user mode and wire it up for syscall entries.

Subsequent patches will wire this up for all non-NMI entries as
well.  NMIs are their own special beast and cannot currently switch
overall context tracking state.  Instead, they have their own
special RCU hooks.

This is a tiny speedup if !CONFIG_CONTEXT_TRACKING (removes a
branch) and a tiny slowdown if CONFIG_CONTEXT_TRACING (adds a layer
of indirection).  Eventually, we should fix up the core context
tracking code to supply a function that does what we want (and can
be much simpler than user_exit), which will enable us to get rid of
the extra call.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/entry/common.c | 13 ++++++++++++-
 1 file changed, 12 insertions(+), 1 deletion(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index 348465473e55..54af0df50080 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -28,6 +28,15 @@
 #define CREATE_TRACE_POINTS
 #include <trace/events/syscalls.h>
 
+#ifdef CONFIG_CONTEXT_TRACKING
+/* Called on entry from user mode with IRQs off. */
+asmlinkage __visible void enter_from_user_mode(void)
+{
+	context_tracking_assert_state(CONTEXT_USER);
+	user_exit();
+}
+#endif
+
 static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
 {
 #ifdef CONFIG_X86_64
@@ -65,14 +74,16 @@ unsigned long syscall_trace_enter_phase1(struct pt_regs *regs, u32 arch)
 	work = ACCESS_ONCE(current_thread_info()->flags) &
 		_TIF_WORK_SYSCALL_ENTRY;
 
+#ifdef CONFIG_CONTEXT_TRACKING
 	/*
 	 * If TIF_NOHZ is set, we are required to call user_exit() before
 	 * doing anything that could touch RCU.
 	 */
 	if (work & _TIF_NOHZ) {
-		user_exit();
+		enter_from_user_mode();
 		work &= ~_TIF_NOHZ;
 	}
+#endif
 
 #ifdef CONFIG_SECCOMP
 	/*
-- 
2.4.3