entry: Fix noinstr warning in __enter_from_user_mode()

__enter_from_user_mode() is triggering noinstr warnings with
CONFIG_DEBUG_PREEMPT due to its call of preempt_count_add() via
ct_state().

The preemption disable isn't needed as interrupts are already disabled.
And the context_tracking_enabled() check in ct_state() also isn't needed
as that's already being done by the CT_WARN_ON().

Just use __ct_state() instead.

Fixes the following warnings:

  vmlinux.o: warning: objtool: enter_from_user_mode+0xba: call to preempt_count_add() leaves .noinstr.text section
  vmlinux.o: warning: objtool: syscall_enter_from_user_mode+0xf9: call to preempt_count_add() leaves .noinstr.text section
  vmlinux.o: warning: objtool: syscall_enter_from_user_mode_prepare+0xc7: call to preempt_count_add() leaves .noinstr.text section
  vmlinux.o: warning: objtool: irqentry_enter_from_user_mode+0xba: call to preempt_count_add() leaves .noinstr.text section

Fixes: 1714767 ("context_tracking: Convert state to atomic_t")
Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/d8955fa6d68dc955dda19baf13ae014ae27926f5.1677369694.git.jpoimboe@kernel.org

Author: jpoimboe

include/linux/context_tracking.h

@@ -96,6 +96,7 @@ static inline void user_exit_irqoff(void) { }
 static inline int exception_enter(void) { return 0; }
 static inline void exception_exit(enum ctx_state prev_ctx) { }
 static inline int ct_state(void) { return -1; }
+static inline int __ct_state(void) { return -1; }
 static __always_inline bool context_tracking_guest_enter(void) { return false; }
 static inline void context_tracking_guest_exit(void) { }
 #define CT_WARN_ON(cond) do { } while (0)

include/linux/context_tracking_state.h

@@ -46,7 +46,9 @@ struct context_tracking {
 
 #ifdef CONFIG_CONTEXT_TRACKING
 DECLARE_PER_CPU(struct context_tracking, context_tracking);
+#endif
 
+#ifdef CONFIG_CONTEXT_TRACKING_USER
 static __always_inline int __ct_state(void)
 {
 	return arch_atomic_read(this_cpu_ptr(&context_tracking.state)) & CT_STATE_MASK;

kernel/entry/common.c

@@ -21,7 +21,7 @@ static __always_inline void __enter_from_user_mode(struct pt_regs *regs)
 	arch_enter_from_user_mode(regs);
 	lockdep_hardirqs_off(CALLER_ADDR0);
 
-	CT_WARN_ON(ct_state() != CONTEXT_USER);
+	CT_WARN_ON(__ct_state() != CONTEXT_USER);
 	user_exit_irqoff();
 
 	instrumentation_begin();