[PATCH v4 4/9] kentry: Simplify the common syscall API

[Andy Lutomirski <luto@kernel.org>]

The new common syscall API had a large and confusing API surface.  Simplify
it.  Now there is exactly one way to use it.  It's a bit more verbose than
the old way for the simple x86_64 native case, but it's much easier to use
right, and the diffstat should speak for itself.

Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Andy Lutomirski <luto@kernel.org>
---
 arch/x86/entry/common.c      | 57 +++++++++++++++---------
 include/linux/entry-common.h | 86 ++++++------------------------------
 kernel/entry/common.c        | 57 +++---------------------
 3 files changed, 54 insertions(+), 146 deletions(-)

diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
index ef1c65938a6b..8710b2300b8d 100644
--- a/arch/x86/entry/common.c
+++ b/arch/x86/entry/common.c
@@ -38,9 +38,12 @@
 #ifdef CONFIG_X86_64
 __visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
 {
-	nr = syscall_enter_from_user_mode(regs, nr);
-
+	kentry_enter_from_user_mode(regs);
+	local_irq_enable();
 	instrumentation_begin();
+
+	nr = kentry_syscall_begin(regs, nr);
+
 	if (likely(nr < NR_syscalls)) {
 		nr = array_index_nospec(nr, NR_syscalls);
 		regs->ax = sys_call_table[nr](regs);
@@ -52,8 +55,12 @@ __visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
 		regs->ax = x32_sys_call_table[nr](regs);
 #endif
 	}
+
+	kentry_syscall_end(regs);
+
+	local_irq_disable();
 	instrumentation_end();
-	syscall_exit_to_user_mode(regs);
+	kentry_exit_to_user_mode(regs);
 }
 #endif
 
@@ -83,33 +90,34 @@ __visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
 {
 	unsigned int nr = syscall_32_enter(regs);
 
+	kentry_enter_from_user_mode(regs);
+	local_irq_enable();
+	instrumentation_begin();
+
 	/*
 	 * Subtlety here: if ptrace pokes something larger than 2^32-1 into
 	 * orig_ax, the unsigned int return value truncates it.  This may
 	 * or may not be necessary, but it matches the old asm behavior.
 	 */
-	nr = (unsigned int)syscall_enter_from_user_mode(regs, nr);
-	instrumentation_begin();
-
+	nr = (unsigned int)kentry_syscall_begin(regs, nr);
 	do_syscall_32_irqs_on(regs, nr);
+	kentry_syscall_end(regs);
 
+	local_irq_disable();
 	instrumentation_end();
-	syscall_exit_to_user_mode(regs);
+	kentry_exit_to_user_mode(regs);
 }
 
 static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
 {
 	unsigned int nr = syscall_32_enter(regs);
+	bool ret;
 	int res;
 
-	/*
-	 * This cannot use syscall_enter_from_user_mode() as it has to
-	 * fetch EBP before invoking any of the syscall entry work
-	 * functions.
-	 */
-	syscall_enter_from_user_mode_prepare(regs);
-
+	kentry_enter_from_user_mode(regs);
+	local_irq_enable();
 	instrumentation_begin();
+
 	/* Fetch EBP from where the vDSO stashed it. */
 	if (IS_ENABLED(CONFIG_X86_64)) {
 		/*
@@ -126,21 +134,23 @@ static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)
 	if (res) {
 		/* User code screwed up. */
 		regs->ax = -EFAULT;
-
-		instrumentation_end();
-		local_irq_disable();
-		irqentry_exit_to_user_mode(regs);
-		return false;
+		ret = false;
+		goto out;
 	}
 
 	/* The case truncates any ptrace induced syscall nr > 2^32 -1 */
-	nr = (unsigned int)syscall_enter_from_user_mode_work(regs, nr);
+	nr = (unsigned int)kentry_syscall_begin(regs, nr);
 
 	/* Now this is just like a normal syscall. */
 	do_syscall_32_irqs_on(regs, nr);
 
+	kentry_syscall_end(regs);
+	ret = true;
+
+out:
+	local_irq_disable();
 	instrumentation_end();
-	syscall_exit_to_user_mode(regs);
+	kentry_exit_to_user_mode(regs);
 	return true;
 }
 
@@ -233,8 +243,11 @@ __visible void noinstr ret_from_fork(struct task_struct *prev,
 		user_regs->ax = 0;
 	}
 
+	kentry_syscall_end(user_regs);
+
+	local_irq_disable();
 	instrumentation_end();
-	syscall_exit_to_user_mode(user_regs);
+	kentry_exit_to_user_mode(user_regs);
 }
 
 #ifdef CONFIG_XEN_PV
diff --git a/include/linux/entry-common.h b/include/linux/entry-common.h
index fd2d7c35670a..5287c6c15a66 100644
--- a/include/linux/entry-common.h
+++ b/include/linux/entry-common.h
@@ -119,31 +119,12 @@ static inline __must_check int arch_syscall_enter_tracehook(struct pt_regs *regs
 void enter_from_user_mode(struct pt_regs *regs);
 
 /**
- * syscall_enter_from_user_mode_prepare - Establish state and enable interrupts
- * @regs:	Pointer to currents pt_regs
- *
- * 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, interrupts are enabled and the
- * subsequent functions can be instrumented.
- *
- * This handles lockdep, RCU (context tracking) and tracing state, i.e.
- * the functionality provided by enter_from_user_mode().
- *
- * This is invoked when there is extra architecture specific functionality
- * to be done between establishing state and handling user mode entry work.
- */
-void syscall_enter_from_user_mode_prepare(struct pt_regs *regs);
-
-/**
- * syscall_enter_from_user_mode_work - Check and handle work before invoking
- *				       a syscall
+ * kentry_syscall_begin - Prepare to invoke a syscall handler
  * @regs:	Pointer to currents pt_regs
  * @syscall:	The syscall number
  *
  * Invoked from architecture specific syscall entry code with interrupts
- * enabled after invoking syscall_enter_from_user_mode_prepare() and extra
- * architecture specific work.
+ * enabled after kentry_enter_from_usermode or a similar function.
  *
  * Returns: The original or a modified syscall number
  *
@@ -152,32 +133,16 @@ void syscall_enter_from_user_mode_prepare(struct pt_regs *regs);
  * syscall_set_return_value() first.  If neither of those are called and -1
  * is returned, then the syscall will fail with ENOSYS.
  *
+ * After calling kentry_syscall_begin(), regardless of the return value,
+ * the caller must call kentry_syscall_end().
+ *
  * It handles the following work items:
  *
  *  1) syscall_work flag dependent invocations of
  *     arch_syscall_enter_tracehook(), __secure_computing(), trace_sys_enter()
  *  2) Invocation of audit_syscall_entry()
  */
-long syscall_enter_from_user_mode_work(struct pt_regs *regs, long syscall);
-
-/**
- * syscall_enter_from_user_mode - Establish state and check and handle work
- *				  before invoking a syscall
- * @regs:	Pointer to currents pt_regs
- * @syscall:	The syscall number
- *
- * 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, interrupts are enabled and the
- * subsequent functions can be instrumented.
- *
- * This is combination of syscall_enter_from_user_mode_prepare() and
- * syscall_enter_from_user_mode_work().
- *
- * Returns: The original or a modified syscall number. See
- * syscall_enter_from_user_mode_work() for further explanation.
- */
-long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall);
+long kentry_syscall_begin(struct pt_regs *regs, long syscall);
 
 /**
  * local_irq_enable_exit_to_user - Exit to user variant of local_irq_enable()
@@ -317,28 +282,16 @@ static inline void arch_syscall_exit_tracehook(struct pt_regs *regs, bool step)
 void exit_to_user_mode(void);
 
 /**
- * syscall_exit_to_user_mode_work - Handle work before returning to user mode
+ * kentry_syscall_end - Finish syscall processing
  * @regs:	Pointer to currents pt_regs
  *
- * Same as step 1 and 2 of syscall_exit_to_user_mode() but without calling
- * exit_to_user_mode() to perform the final transition to user mode.
  *
- * Calling convention is the same as for syscall_exit_to_user_mode() and it
- * returns with all work handled and interrupts disabled. The caller must
- * invoke exit_to_user_mode() before actually switching to user mode to
- * make the final state transitions. Interrupts must stay disabled between
- * return from this function and the invocation of exit_to_user_mode().
- */
-void syscall_exit_to_user_mode_work(struct pt_regs *regs);
-
-/**
- * syscall_exit_to_user_mode - Handle work before returning to user mode
- * @regs:	Pointer to currents pt_regs
+ * This must be called after arch code calls kentry_syscall_begin()
+ * and invoking a syscall handler, if any.  This must also be called when
+ * returning from fork() to user mode, since return-from-fork is considered
+ * to be a syscall return.
  *
- * Invoked with interrupts enabled and fully valid regs. Returns with all
- * work handled, interrupts disabled such that the caller can immediately
- * switch to user mode. Called from architecture specific syscall and ret
- * from fork code.
+ * Called with IRQs on.  Returns with IRQs still on.
  *
  * The call order is:
  *  1) One-time syscall exit work:
@@ -346,21 +299,8 @@ void syscall_exit_to_user_mode_work(struct pt_regs *regs);
  *      - audit
  *	- syscall tracing
  *	- tracehook (single stepping)
- *
- *  2) Preparatory work
- *	- Exit to user mode loop (common TIF handling). Invokes
- *	  arch_exit_to_user_mode_work() for architecture specific TIF work
- *	- Architecture specific one time work arch_exit_to_user_mode_prepare()
- *	- Address limit and lockdep checks
- *
- *  3) Final transition (lockdep, tracing, context tracking, RCU), i.e. the
- *     functionality in exit_to_user_mode().
- *
- * This is a combination of syscall_exit_to_user_mode_work() (1,2) and
- * exit_to_user_mode(). This function is preferred unless there is a
- * compelling architectural reason to use the seperate functions.
  */
-void syscall_exit_to_user_mode(struct pt_regs *regs);
+void kentry_syscall_end(struct pt_regs *regs);
 
 /**
  * kentry_enter_from_user_mode - Establish state before invoking the irq handler
diff --git a/kernel/entry/common.c b/kernel/entry/common.c
index 269766a8f981..800ad406431b 100644
--- a/kernel/entry/common.c
+++ b/kernel/entry/common.c
@@ -80,44 +80,19 @@ static long syscall_trace_enter(struct pt_regs *regs, long syscall,
 	return ret ? : syscall;
 }
 
-static __always_inline long
-__syscall_enter_from_user_work(struct pt_regs *regs, long syscall)
+long kentry_syscall_begin(struct pt_regs *regs, long syscall)
 {
 	unsigned long work = READ_ONCE(current_thread_info()->syscall_work);
 
+	CT_WARN_ON(ct_state() != CONTEXT_KERNEL);
+	lockdep_assert_irqs_enabled();
+
 	if (work & SYSCALL_WORK_ENTER)
 		syscall = syscall_trace_enter(regs, syscall, work);
 
 	return syscall;
 }
 
-long syscall_enter_from_user_mode_work(struct pt_regs *regs, long syscall)
-{
-	return __syscall_enter_from_user_work(regs, syscall);
-}
-
-noinstr long syscall_enter_from_user_mode(struct pt_regs *regs, long syscall)
-{
-	long ret;
-
-	__enter_from_user_mode(regs);
-
-	instrumentation_begin();
-	local_irq_enable();
-	ret = __syscall_enter_from_user_work(regs, syscall);
-	instrumentation_end();
-
-	return ret;
-}
-
-noinstr void syscall_enter_from_user_mode_prepare(struct pt_regs *regs)
-{
-	__enter_from_user_mode(regs);
-	instrumentation_begin();
-	local_irq_enable();
-	instrumentation_end();
-}
-
 /* See comment for exit_to_user_mode() in entry-common.h */
 static __always_inline void __exit_to_user_mode(void)
 {
@@ -218,7 +193,7 @@ static inline bool report_single_step(unsigned long work)
 /*
  * If SYSCALL_EMU is set, then the only reason to report is when
  * TIF_SINGLESTEP is set (i.e. PTRACE_SYSEMU_SINGLESTEP).  This syscall
- * instruction has been already reported in syscall_enter_from_user_mode().
+ * instruction has been already reported in kentry_syscall_begin().
  */
 static inline bool report_single_step(unsigned long work)
 {
@@ -261,7 +236,7 @@ static void syscall_exit_work(struct pt_regs *regs, unsigned long work)
  * Syscall specific exit to user mode preparation. Runs with interrupts
  * enabled.
  */
-static void syscall_exit_to_user_mode_prepare(struct pt_regs *regs)
+void kentry_syscall_end(struct pt_regs *regs)
 {
 	unsigned long work = READ_ONCE(current_thread_info()->syscall_work);
 	unsigned long nr = syscall_get_nr(current, regs);
@@ -284,26 +259,6 @@ static void syscall_exit_to_user_mode_prepare(struct pt_regs *regs)
 		syscall_exit_work(regs, work);
 }
 
-static __always_inline void __syscall_exit_to_user_mode_work(struct pt_regs *regs)
-{
-	syscall_exit_to_user_mode_prepare(regs);
-	local_irq_disable_exit_to_user();
-	exit_to_user_mode_prepare(regs);
-}
-
-void syscall_exit_to_user_mode_work(struct pt_regs *regs)
-{
-	__syscall_exit_to_user_mode_work(regs);
-}
-
-__visible noinstr void syscall_exit_to_user_mode(struct pt_regs *regs)
-{
-	instrumentation_begin();
-	__syscall_exit_to_user_mode_work(regs);
-	instrumentation_end();
-	__exit_to_user_mode();
-}
-
 noinstr void kentry_enter_from_user_mode(struct pt_regs *regs)
 {
 	__enter_from_user_mode(regs);
-- 
2.30.2