linux-kernel - [PATCH v3 07/11] kentry: Make entry/exit_to_user

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Message-Id: <e8ad39b0e268caec5cc9ff52371438badedd0737.1614884673.git.luto@kernel.org>
Date:   Thu,  4 Mar 2021 11:06:00 -0800
From:   Andy Lutomirski <luto@...nel.org>
To:     x86@...nel.org
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Andy Lutomirski <luto@...nel.org>
Subject: [PATCH v3 07/11] kentry: Make entry/exit_to_user_mode() arm64-only

exit_to_user_mode() does part, but not all, of the exit-to-user-mode work.
It's used only by arm64, and arm64 should stop using it (hint, hint!).
Compile it out on other architectures to minimize the chance of error.

enter_from_user_mode() is a legacy way to spell
kentry_enter_from_user_mode().  It's also only used by arm64.  Give it
the same treatment.

Signed-off-by: Andy Lutomirski <luto@...nel.org>
---
 include/linux/entry-common.h | 34 ++++++----------------------------
 kernel/entry/common.c        |  4 ++++
 2 files changed, 10 insertions(+), 28 deletions(-)

diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index 5287c6c15a66..a75374f87258 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -97,26 +97,15 @@ static inline __must_check int arch_syscall_enter_tracehook(struct pt_regs *regs
 }
 #endif
 
+#ifdef CONFIG_ARM64
 /**
  * enter_from_user_mode - Establish state when coming from user mode
  *
- * Syscall/interrupt entry disables interrupts, but user mode is traced as
- * interrupts enabled. Also with NO_HZ_FULL RCU might be idle.
+ * Legacy variant of kentry_enter_from_user_mode().  Used only by arm64.
  *
- * 1) Tell lockdep that interrupts are disabled
- * 2) Invoke context tracking if enabled to reactivate RCU
- * 3) Trace interrupts off state
- *
- * Invoked from architecture specific syscall entry code with interrupts
- * disabled. The calling code has to be non-instrumentable. When the
- * function returns all state is correct and interrupts are still
- * disabled. The subsequent functions can be instrumented.
- *
- * This is invoked when there is architecture specific functionality to be
- * done between establishing state and enabling interrupts. The caller must
- * enable interrupts before invoking syscall_enter_from_user_mode_work().
  */
 void enter_from_user_mode(struct pt_regs *regs);
+#endif
 
 /**
  * kentry_syscall_begin - Prepare to invoke a syscall handler
@@ -261,25 +250,14 @@ static inline void arch_syscall_exit_tracehook(struct pt_regs *regs, bool step)
 }
 #endif
 
+#ifdef CONFIG_ARM64
 /**
  * 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.
- * The caller has to invoke syscall_exit_to_user_mode_work() before this.
+ * Does the latter part of irqentry_exit_to_user_mode().  Only used by arm64.
  */
 void exit_to_user_mode(void);
+#endif
 
 /**
  * kentry_syscall_end - Finish syscall processing
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 800ad406431b..4ba82c684189 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -25,10 +25,12 @@ static __always_inline void __enter_from_user_mode(struct pt_regs *regs)
 	instrumentation_end();
 }
 
+#ifdef CONFIG_ARM64
 void noinstr enter_from_user_mode(struct pt_regs *regs)
 {
 	__enter_from_user_mode(regs);
 }
+#endif
 
 static inline void syscall_enter_audit(struct pt_regs *regs, long syscall)
 {
@@ -106,10 +108,12 @@ static __always_inline void __exit_to_user_mode(void)
 	lockdep_hardirqs_on(CALLER_ADDR0);
 }
 
+#ifdef CONFIG_ARM64
 void noinstr exit_to_user_mode(void)
 {
 	__exit_to_user_mode();
 }
+#endif
 
 /* Workaround to allow gradual conversion of architecture code */
 void __weak arch_do_signal_or_restart(struct pt_regs *regs, bool has_signal) { }
-- 
2.29.2