entry: Add exit_to_user_mode() wrapper

Called from architecture specific code when syscall_exit_to_user_mode() is
not suitable. It simply calls __exit_to_user_mode().

This way __exit_to_user_mode() can still be inlined because it is declared
static __always_inline.

[ tglx: Amended comments and moved it to a different place in the header ]
Signed-off-by: Sven Schnelle <svens@linux.ibm.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20201201142755.31931-5-svens@linux.ibm.com

include/linux/entry-common.h

@@ -300,6 +300,25 @@ static inline void arch_syscall_exit_tracehook(struct pt_regs *regs, bool step)
 }
 #endif
 
+/**
+ * exit_to_user_mode - Fixup state when exiting to user mode
+ *
+ * Syscall/interrupt exit enables interrupts, but the kernel state is
+ * interrupts disabled when this is invoked. Also tell RCU about it.
+ *
+ * 1) Trace interrupts on state
+ * 2) Invoke context tracking if enabled to adjust RCU state
+ * 3) Invoke architecture specific last minute exit code, e.g. speculation
+ *    mitigations, etc.: arch_exit_to_user_mode()
+ * 4) Tell lockdep that interrupts are enabled
+ *
+ * Invoked from architecture specific code when syscall_exit_to_user_mode()
+ * is not suitable as the last step before returning to userspace. Must be
+ * invoked with interrupts disabled and the caller must be
+ * non-instrumentable.
+ */
+void exit_to_user_mode(void);
+
 /**
  * syscall_exit_to_user_mode - Handle work before returning to user mode
  * @regs:	Pointer to currents pt_regs
@@ -322,8 +341,8 @@ static inline void arch_syscall_exit_tracehook(struct pt_regs *regs, bool step)
  *	- Architecture specific one time work arch_exit_to_user_mode_prepare()
  *	- Address limit and lockdep checks
  *
- *  3) Final transition (lockdep, tracing, context tracking, RCU). Invokes
- *     arch_exit_to_user_mode() to handle e.g. speculation mitigations
+ *  3) Final transition (lockdep, tracing, context tracking, RCU), i.e. the
+ *     functionality in exit_to_user_mode().
  */
 void syscall_exit_to_user_mode(struct pt_regs *regs);
 

kernel/entry/common.c

@@ -117,18 +117,7 @@ noinstr void syscall_enter_from_user_mode_prepare(struct pt_regs *regs)
 	instrumentation_end();
 }
 
-/**
- * __exit_to_user_mode - Fixup state when exiting to user mode
- *
- * Syscall/interupt exit enables interrupts, but the kernel state is
- * interrupts disabled when this is invoked. Also tell RCU about it.
- *
- * 1) Trace interrupts on state
- * 2) Invoke context tracking if enabled to adjust RCU state
- * 3) Invoke architecture specific last minute exit code, e.g. speculation
- *    mitigations, etc.
- * 4) Tell lockdep that interrupts are enabled
- */
+/* See comment for exit_to_user_mode() in entry-common.h */
 static __always_inline void __exit_to_user_mode(void)
 {
 	instrumentation_begin();
@@ -141,6 +130,11 @@ static __always_inline void __exit_to_user_mode(void)
 	lockdep_hardirqs_on(CALLER_ADDR0);
 }
 
+void noinstr exit_to_user_mode(void)
+{
+	__exit_to_user_mode();
+}
+
 /* Workaround to allow gradual conversion of architecture code */
 void __weak arch_do_signal_or_restart(struct pt_regs *regs, bool has_signal) { }