common.c

// SPDX-License-Identifier: GPL-2.0-only
/*
 * common.c - C code for kernel entry and exit
 * Copyright (c) 2015 Andrew Lutomirski
 *
 * Based on asm and ptrace code by many authors.  The code here originated
 * in ptrace.c and signal.c.
 */

#include <linux/kernel.h>
#include <linux/sched.h>
#include <linux/sched/task_stack.h>
#include <linux/mm.h>
#include <linux/smp.h>
#include <linux/errno.h>
#include <linux/kmsan.h>
#include <linux/ptrace.h>
#include <linux/tracehook.h>
#include <linux/audit.h>
#include <linux/seccomp.h>
#include <linux/signal.h>
#include <linux/export.h>
#include <linux/context_tracking.h>
#include <linux/user-return-notifier.h>
#include <linux/nospec.h>
#include <linux/uprobes.h>
#include <linux/livepatch.h>
#include <linux/syscalls.h>
#include <linux/uaccess.h>

#ifdef CONFIG_XEN_PV
#include <xen/xen-ops.h>
#include <xen/events.h>
#endif

#include <asm/desc.h>
#include <asm/traps.h>
#include <asm/vdso.h>
#include <asm/cpufeature.h>
#include <asm/fpu/api.h>
#include <asm/nospec-branch.h>
#include <asm/io_bitmap.h>
#include <asm/syscall.h>
#include <asm/irq_stack.h>

#define CREATE_TRACE_POINTS
#include <trace/events/syscalls.h>

/* Check that the stack and regs on entry from user mode are sane. */
static void check_user_regs(struct pt_regs *regs)
{
	if (IS_ENABLED(CONFIG_DEBUG_ENTRY)) {
		/*
		 * Make sure that the entry code gave us a sensible EFLAGS
		 * register.  Native because we want to check the actual CPU
		 * state, not the interrupt state as imagined by Xen.
		 */
		unsigned long flags = native_save_fl();
		WARN_ON_ONCE(flags & (X86_EFLAGS_AC | X86_EFLAGS_DF |
				      X86_EFLAGS_NT));

		/* We think we came from user mode. Make sure pt_regs agrees. */
		WARN_ON_ONCE(!user_mode(regs));

		/*
		 * All entries from user mode (except #DF) should be on the
		 * normal thread stack and should have user pt_regs in the
		 * correct location.
		 */
		WARN_ON_ONCE(!on_thread_stack());
		WARN_ON_ONCE(regs != task_pt_regs(current));
	}
}

#ifdef CONFIG_CONTEXT_TRACKING
/**
 * enter_from_user_mode - Establish state when coming from user mode
 *
 * Syscall entry disables interrupts, but user mode is traced as interrupts
 * enabled. Also with NO_HZ_FULL RCU might be idle.
 *
 * 1) Tell lockdep that interrupts are disabled
 * 2) Invoke context tracking if enabled to reactivate RCU
 * 3) Trace interrupts off state
 */
static noinstr void enter_from_user_mode(void)
{
	enum ctx_state state = ct_state();

	lockdep_hardirqs_off(CALLER_ADDR0);
	user_exit_irqoff();

	instrumentation_begin();
	CT_WARN_ON(state != CONTEXT_USER);
	trace_hardirqs_off_finish();
	instrumentation_end();
}
#else
static __always_inline void enter_from_user_mode(void)
{
	lockdep_hardirqs_off(CALLER_ADDR0);
	instrumentation_begin();
	trace_hardirqs_off_finish();
	instrumentation_end();
}
#endif

/**
 * exit_to_user_mode - Fixup state when exiting to user mode
 *
 * Syscall 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) Clear CPU buffers if CPU is affected by MDS and the migitation is on.
 * 4) Tell lockdep that interrupts are enabled
 */
static __always_inline void exit_to_user_mode(void)
{
	instrumentation_begin();
	trace_hardirqs_on_prepare();
	lockdep_hardirqs_on_prepare(CALLER_ADDR0);
	instrumentation_end();

	user_enter_irqoff();
	mds_user_clear_cpu_buffers();
	lockdep_hardirqs_on(CALLER_ADDR0);
}

static void do_audit_syscall_entry(struct pt_regs *regs, u32 arch)
{
#ifdef CONFIG_X86_64
	if (arch == AUDIT_ARCH_X86_64) {
		audit_syscall_entry(regs->orig_ax, regs->di,
				    regs->si, regs->dx, regs->r10);
	} else
#endif
	{
		audit_syscall_entry(regs->orig_ax, regs->bx,
				    regs->cx, regs->dx, regs->si);
	}
}

/*
 * Returns the syscall nr to run (which should match regs->orig_ax) or -1
 * to skip the syscall.
 */
static long syscall_trace_enter(struct pt_regs *regs)
{
	u32 arch = in_ia32_syscall() ? AUDIT_ARCH_I386 : AUDIT_ARCH_X86_64;

	struct thread_info *ti = current_thread_info();
	unsigned long ret = 0;
	u32 work;

	work = READ_ONCE(ti->flags);

	if (work & (_TIF_SYSCALL_TRACE | _TIF_SYSCALL_EMU)) {
		ret = tracehook_report_syscall_entry(regs);
		if (ret || (work & _TIF_SYSCALL_EMU))
			return -1L;
	}

#ifdef CONFIG_SECCOMP
	/*
	 * Do seccomp after ptrace, to catch any tracer changes.
	 */
	if (work & _TIF_SECCOMP) {
		struct seccomp_data sd;

		sd.arch = arch;
		sd.nr = regs->orig_ax;
		sd.instruction_pointer = regs->ip;
#ifdef CONFIG_X86_64
		if (arch == AUDIT_ARCH_X86_64) {
			sd.args[0] = regs->di;
			sd.args[1] = regs->si;
			sd.args[2] = regs->dx;
			sd.args[3] = regs->r10;
			sd.args[4] = regs->r8;
			sd.args[5] = regs->r9;
		} else
#endif
		{
			sd.args[0] = regs->bx;
			sd.args[1] = regs->cx;
			sd.args[2] = regs->dx;
			sd.args[3] = regs->si;
			sd.args[4] = regs->di;
			sd.args[5] = regs->bp;
		}

		ret = __secure_computing(&sd);
		if (ret == -1)
			return ret;
	}
#endif

	if (unlikely(test_thread_flag(TIF_SYSCALL_TRACEPOINT)))
		trace_sys_enter(regs, regs->orig_ax);

	do_audit_syscall_entry(regs, arch);

	return ret ?: regs->orig_ax;
}

#define EXIT_TO_USERMODE_LOOP_FLAGS				\
	(_TIF_SIGPENDING | _TIF_NOTIFY_RESUME | _TIF_UPROBE |	\
	 _TIF_NEED_RESCHED | _TIF_USER_RETURN_NOTIFY | _TIF_PATCH_PENDING)

static void exit_to_usermode_loop(struct pt_regs *regs, u32 cached_flags)
{
	/*
	 * In order to return to user mode, we need to have IRQs off with
	 * none of EXIT_TO_USERMODE_LOOP_FLAGS set.  Several of these flags
	 * can be set at any time on preemptible kernels if we have IRQs on,
	 * so we need to loop.  Disabling preemption wouldn't help: doing the
	 * work to clear some of the flags can sleep.
	 */
	while (true) {
		/* We have work to do. */
		local_irq_enable();

		if (cached_flags & _TIF_NEED_RESCHED)
			schedule();

		if (cached_flags & _TIF_UPROBE)
			uprobe_notify_resume(regs);

		if (cached_flags & _TIF_PATCH_PENDING)
			klp_update_patch_state(current);

		/* deal with pending signal delivery */
		if (cached_flags & _TIF_SIGPENDING)
			do_signal(regs);

		if (cached_flags & _TIF_NOTIFY_RESUME) {
			clear_thread_flag(TIF_NOTIFY_RESUME);
			tracehook_notify_resume(regs);
			rseq_handle_notify_resume(NULL, regs);
		}

		if (cached_flags & _TIF_USER_RETURN_NOTIFY)
			fire_user_return_notifiers();

		/* Disable IRQs and retry */
		local_irq_disable();

		cached_flags = READ_ONCE(current_thread_info()->flags);

		if (!(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
			break;
	}
}

static void __prepare_exit_to_usermode(struct pt_regs *regs)
{
	struct thread_info *ti = current_thread_info();
	u32 cached_flags;

	addr_limit_user_check();

	lockdep_assert_irqs_disabled();
	lockdep_sys_exit();

	cached_flags = READ_ONCE(ti->flags);

	if (unlikely(cached_flags & EXIT_TO_USERMODE_LOOP_FLAGS))
		exit_to_usermode_loop(regs, cached_flags);

	/* Reload ti->flags; we may have rescheduled above. */
	cached_flags = READ_ONCE(ti->flags);

	if (unlikely(cached_flags & _TIF_IO_BITMAP))
		tss_update_io_bitmap();

	fpregs_assert_state_consistent();
	if (unlikely(cached_flags & _TIF_NEED_FPU_LOAD))
		switch_fpu_return();

#ifdef CONFIG_COMPAT
	/*
	 * Compat syscalls set TS_COMPAT.  Make sure we clear it before
	 * returning to user mode.  We need to clear it *after* signal
	 * handling, because syscall restart has a fixup for compat
	 * syscalls.  The fixup is exercised by the ptrace_syscall_32
	 * selftest.
	 *
	 * We also need to clear TS_REGS_POKED_I386: the 32-bit tracer
	 * special case only applies after poking regs and before the
	 * very next return to user mode.
	 */
	ti->status &= ~(TS_COMPAT|TS_I386_REGS_POKED);
#endif
}

__visible noinstr void prepare_exit_to_usermode(struct pt_regs *regs)
{
	instrumentation_begin();
	__prepare_exit_to_usermode(regs);
	instrumentation_end();
	exit_to_user_mode();
}

#define SYSCALL_EXIT_WORK_FLAGS				\
	(_TIF_SYSCALL_TRACE | _TIF_SYSCALL_AUDIT |	\
	 _TIF_SINGLESTEP | _TIF_SYSCALL_TRACEPOINT)

static void syscall_slow_exit_work(struct pt_regs *regs, u32 cached_flags)
{
	bool step;

	audit_syscall_exit(regs);

	if (cached_flags & _TIF_SYSCALL_TRACEPOINT)
		trace_sys_exit(regs, regs->ax);

	/*
	 * If TIF_SYSCALL_EMU is set, we only get here because of
	 * TIF_SINGLESTEP (i.e. this is PTRACE_SYSEMU_SINGLESTEP).
	 * We already reported this syscall instruction in
	 * syscall_trace_enter().
	 */
	step = unlikely(
		(cached_flags & (_TIF_SINGLESTEP | _TIF_SYSCALL_EMU))
		== _TIF_SINGLESTEP);
	if (step || cached_flags & _TIF_SYSCALL_TRACE)
		tracehook_report_syscall_exit(regs, step);
}

static void __syscall_return_slowpath(struct pt_regs *regs)
{
	struct thread_info *ti = current_thread_info();
	u32 cached_flags = READ_ONCE(ti->flags);

	CT_WARN_ON(ct_state() != CONTEXT_KERNEL);

	if (IS_ENABLED(CONFIG_PROVE_LOCKING) &&
	    WARN(irqs_disabled(), "syscall %ld left IRQs disabled", regs->orig_ax))
		local_irq_enable();

	rseq_syscall(regs);

	/*
	 * First do one-time work.  If these work items are enabled, we
	 * want to run them exactly once per syscall exit with IRQs on.
	 */
	if (unlikely(cached_flags & SYSCALL_EXIT_WORK_FLAGS))
		syscall_slow_exit_work(regs, cached_flags);

	local_irq_disable();
	__prepare_exit_to_usermode(regs);
}

/*
 * Called with IRQs on and fully valid regs.  Returns with IRQs off in a
 * state such that we can immediately switch to user mode.
 */
__visible noinstr void syscall_return_slowpath(struct pt_regs *regs)
{
	instrumentation_begin();
	__syscall_return_slowpath(regs);
	instrumentation_end();
	exit_to_user_mode();
}

#ifdef CONFIG_X86_64
__visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
{
	struct thread_info *ti;

	kmsan_unpoison_pt_regs(regs);
	check_user_regs(regs);

	enter_from_user_mode();
	instrumentation_begin();

	local_irq_enable();
	ti = current_thread_info();
	if (READ_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY)
		nr = syscall_trace_enter(regs);

	if (likely(nr < NR_syscalls)) {
		nr = array_index_nospec(nr, NR_syscalls);
		regs->ax = sys_call_table[nr](regs);
#ifdef CONFIG_X86_X32_ABI
	} else if (likely((nr & __X32_SYSCALL_BIT) &&
			  (nr & ~__X32_SYSCALL_BIT) < X32_NR_syscalls)) {
		nr = array_index_nospec(nr & ~__X32_SYSCALL_BIT,
					X32_NR_syscalls);
		regs->ax = x32_sys_call_table[nr](regs);
#endif
	}
	__syscall_return_slowpath(regs);

	instrumentation_end();
	exit_to_user_mode();
}
#endif

#if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
/*
 * Does a 32-bit syscall.  Called with IRQs on in CONTEXT_KERNEL.  Does
 * all entry and exit work and returns with IRQs off.  This function is
 * extremely hot in workloads that use it, and it's usually called from
 * do_fast_syscall_32, so forcibly inline it to improve performance.
 */
static void do_syscall_32_irqs_on(struct pt_regs *regs)
{
	struct thread_info *ti = current_thread_info();
	unsigned int nr = (unsigned int)regs->orig_ax;

#ifdef CONFIG_IA32_EMULATION
	ti->status |= TS_COMPAT;
#endif

	if (READ_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY) {
		/*
		 * Subtlety here: if ptrace pokes something larger than
		 * 2^32-1 into orig_ax, this truncates it.  This may or
		 * may not be necessary, but it matches the old asm
		 * behavior.
		 */
		nr = syscall_trace_enter(regs);
	}

	if (likely(nr < IA32_NR_syscalls)) {
		nr = array_index_nospec(nr, IA32_NR_syscalls);
		regs->ax = ia32_sys_call_table[nr](regs);
	}

	__syscall_return_slowpath(regs);
}

/* Handles int $0x80 */
__visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
{
	check_user_regs(regs);

	enter_from_user_mode();
	instrumentation_begin();

	local_irq_enable();
	do_syscall_32_irqs_on(regs);

	instrumentation_end();
	exit_to_user_mode();
}

static bool __do_fast_syscall_32(struct pt_regs *regs)
{
	int res;

	/* Fetch EBP from where the vDSO stashed it. */
	if (IS_ENABLED(CONFIG_X86_64)) {
		/*
		 * Micro-optimization: the pointer we're following is
		 * explicitly 32 bits, so it can't be out of range.
		 */
		res = __get_user(*(u32 *)&regs->bp,
			 (u32 __user __force *)(unsigned long)(u32)regs->sp);
	} else {
		res = get_user(*(u32 *)&regs->bp,
		       (u32 __user __force *)(unsigned long)(u32)regs->sp);
	}

	if (res) {
		/* User code screwed up. */
		regs->ax = -EFAULT;
		local_irq_disable();
		__prepare_exit_to_usermode(regs);
		return false;
	}

	/* Now this is just like a normal syscall. */
	do_syscall_32_irqs_on(regs);
	return true;
}

/* Returns 0 to return using IRET or 1 to return using SYSEXIT/SYSRETL. */
__visible noinstr long do_fast_syscall_32(struct pt_regs *regs)
{
	/*
	 * Called using the internal vDSO SYSENTER/SYSCALL32 calling
	 * convention.  Adjust regs so it looks like we entered using int80.
	 */
	unsigned long landing_pad = (unsigned long)current->mm->context.vdso +
					vdso_image_32.sym_int80_landing_pad;
	bool success;

	check_user_regs(regs);

	/*
	 * SYSENTER loses EIP, and even SYSCALL32 needs us to skip forward
	 * so that 'regs->ip -= 2' lands back on an int $0x80 instruction.
	 * Fix it up.
	 */
	regs->ip = landing_pad;

	enter_from_user_mode();
	instrumentation_begin();

	local_irq_enable();
	success = __do_fast_syscall_32(regs);

	instrumentation_end();
	exit_to_user_mode();

	/* If it failed, keep it simple: use IRET. */
	if (!success)
		return 0;

#ifdef CONFIG_X86_64
	/*
	 * Opportunistic SYSRETL: if possible, try to return using SYSRETL.
	 * SYSRETL is available on all 64-bit CPUs, so we don't need to
	 * bother with SYSEXIT.
	 *
	 * Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP,
	 * because the ECX fixup above will ensure that this is essentially
	 * never the case.
	 */
	return regs->cs == __USER32_CS && regs->ss == __USER_DS &&
		regs->ip == landing_pad &&
		(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF)) == 0;
#else
	/*
	 * Opportunistic SYSEXIT: if possible, try to return using SYSEXIT.
	 *
	 * Unlike 64-bit opportunistic SYSRET, we can't check that CX == IP,
	 * because the ECX fixup above will ensure that this is essentially
	 * never the case.
	 *
	 * We don't allow syscalls at all from VM86 mode, but we still
	 * need to check VM, because we might be returning from sys_vm86.
	 */
	return static_cpu_has(X86_FEATURE_SEP) &&
		regs->cs == __USER_CS && regs->ss == __USER_DS &&
		regs->ip == landing_pad &&
		(regs->flags & (X86_EFLAGS_RF | X86_EFLAGS_TF | X86_EFLAGS_VM)) == 0;
#endif
}

/* Returns 0 to return using IRET or 1 to return using SYSEXIT/SYSRETL. */
__visible noinstr long do_SYSENTER_32(struct pt_regs *regs)
{
	/* SYSENTER loses RSP, but the vDSO saved it in RBP. */
	regs->sp = regs->bp;

	/* SYSENTER clobbers EFLAGS.IF.  Assume it was set in usermode. */
	regs->flags |= X86_EFLAGS_IF;

	return do_fast_syscall_32(regs);
}
#endif

SYSCALL_DEFINE0(ni_syscall)
{
	return -ENOSYS;
}

/**
 * idtentry_enter_cond_rcu - Handle state tracking on idtentry with conditional
 *			     RCU handling
 * @regs:	Pointer to pt_regs of interrupted context
 *
 * Invokes:
 *  - lockdep irqflag state tracking as low level ASM entry disabled
 *    interrupts.
 *
 *  - Context tracking if the exception hit user mode.
 *
 *  - The hardirq tracer to keep the state consistent as low level ASM
 *    entry disabled interrupts.
 *
 * For kernel mode entries RCU handling is done conditional. If RCU is
 * watching then the only RCU requirement is to check whether the tick has
 * to be restarted. If RCU is not watching then rcu_irq_enter() has to be
 * invoked on entry and rcu_irq_exit() on exit.
 *
 * Avoiding the rcu_irq_enter/exit() calls is an optimization but also
 * solves the problem of kernel mode pagefaults which can schedule, which
 * is not possible after invoking rcu_irq_enter() without undoing it.
 *
 * For user mode entries enter_from_user_mode() must be invoked to
 * establish the proper context for NOHZ_FULL. Otherwise scheduling on exit
 * would not be possible.
 *
 * Returns: True if RCU has been adjusted on a kernel entry
 *	    False otherwise
 *
 * The return value must be fed into the rcu_exit argument of
 * idtentry_exit_cond_rcu().
 */
bool noinstr idtentry_enter_cond_rcu(struct pt_regs *regs)
{
	if (user_mode(regs)) {
		check_user_regs(regs);
		enter_from_user_mode();
		return false;
	}

	/*
	 * If this entry hit the idle task invoke rcu_irq_enter() whether
	 * RCU is watching or not.
	 *
	 * Interupts can nest when the first interrupt invokes softirq
	 * processing on return which enables interrupts.
	 *
	 * Scheduler ticks in the idle task can mark quiescent state and
	 * terminate a grace period, if and only if the timer interrupt is
	 * not nested into another interrupt.
	 *
	 * Checking for __rcu_is_watching() here would prevent the nesting
	 * interrupt to invoke rcu_irq_enter(). If that nested interrupt is
	 * the tick then rcu_flavor_sched_clock_irq() would wrongfully
	 * assume that it is the first interupt and eventually claim
	 * quiescient state and end grace periods prematurely.
	 *
	 * Unconditionally invoke rcu_irq_enter() so RCU state stays
	 * consistent.
	 *
	 * TINY_RCU does not support EQS, so let the compiler eliminate
	 * this part when enabled.
	 */
	if (!IS_ENABLED(CONFIG_TINY_RCU) && is_idle_task(current)) {
		/*
		 * If RCU is not watching then the same careful
		 * sequence vs. lockdep and tracing is required
		 * as in enter_from_user_mode().
		 */
		lockdep_hardirqs_off(CALLER_ADDR0);
		rcu_irq_enter();
		instrumentation_begin();
		trace_hardirqs_off_finish();
		instrumentation_end();

		return true;
	}

	/*
	 * If RCU is watching then RCU only wants to check whether it needs
	 * to restart the tick in NOHZ mode. rcu_irq_enter_check_tick()
	 * already contains a warning when RCU is not watching, so no point
	 * in having another one here.
	 */
	instrumentation_begin();
	rcu_irq_enter_check_tick();
	/* Use the combo lockdep/tracing function */
	trace_hardirqs_off();
	instrumentation_end();

	return false;
}

static void idtentry_exit_cond_resched(struct pt_regs *regs, bool may_sched)
{
	if (may_sched && !preempt_count()) {
		/* Sanity check RCU and thread stack */
		rcu_irq_exit_check_preempt();
		if (IS_ENABLED(CONFIG_DEBUG_ENTRY))
			WARN_ON_ONCE(!on_thread_stack());
		if (need_resched())
			preempt_schedule_irq();
	}
	/* Covers both tracing and lockdep */
	trace_hardirqs_on();
}

/**
 * idtentry_exit_cond_rcu - Handle return from exception with conditional RCU
 *			    handling
 * @regs:	Pointer to pt_regs (exception entry regs)
 * @rcu_exit:	Invoke rcu_irq_exit() if true
 *
 * Depending on the return target (kernel/user) this runs the necessary
 * preemption and work checks if possible and reguired and returns to
 * the caller with interrupts disabled and no further work pending.
 *
 * This is the last action before returning to the low level ASM code which
 * just needs to return to the appropriate context.
 *
 * Counterpart to idtentry_enter_cond_rcu(). The return value of the entry
 * function must be fed into the @rcu_exit argument.
 */
void noinstr idtentry_exit_cond_rcu(struct pt_regs *regs, bool rcu_exit)
{
	lockdep_assert_irqs_disabled();

	/* Check whether this returns to user mode */
	if (user_mode(regs)) {
		prepare_exit_to_usermode(regs);
	} else if (regs->flags & X86_EFLAGS_IF) {
		/*
		 * If RCU was not watching on entry this needs to be done
		 * carefully and needs the same ordering of lockdep/tracing
		 * and RCU as the return to user mode path.
		 */
		if (rcu_exit) {
			instrumentation_begin();
			/* Tell the tracer that IRET will enable interrupts */
			trace_hardirqs_on_prepare();
			lockdep_hardirqs_on_prepare(CALLER_ADDR0);
			instrumentation_end();
			rcu_irq_exit();
			lockdep_hardirqs_on(CALLER_ADDR0);
			return;
		}

		instrumentation_begin();
		idtentry_exit_cond_resched(regs, IS_ENABLED(CONFIG_PREEMPTION));
		instrumentation_end();
	} else {
		/*
		 * IRQ flags state is correct already. Just tell RCU if it
		 * was not watching on entry.
		 */
		if (rcu_exit)
			rcu_irq_exit();
	}
}

/**
 * idtentry_enter_user - Handle state tracking on idtentry from user mode
 * @regs:	Pointer to pt_regs of interrupted context
 *
 * Invokes enter_from_user_mode() to establish the proper context for
 * NOHZ_FULL. Otherwise scheduling on exit would not be possible.
 */
void noinstr idtentry_enter_user(struct pt_regs *regs)
{
	check_user_regs(regs);
	enter_from_user_mode();
}

/**
 * idtentry_exit_user - Handle return from exception to user mode
 * @regs:	Pointer to pt_regs (exception entry regs)
 *
 * Runs the necessary preemption and work checks and returns to the caller
 * with interrupts disabled and no further work pending.
 *
 * This is the last action before returning to the low level ASM code which
 * just needs to return to the appropriate context.
 *
 * Counterpart to idtentry_enter_user().
 */
void noinstr idtentry_exit_user(struct pt_regs *regs)
{
	lockdep_assert_irqs_disabled();

	prepare_exit_to_usermode(regs);
}

#ifdef CONFIG_XEN_PV
#ifndef CONFIG_PREEMPTION
/*
 * Some hypercalls issued by the toolstack can take many 10s of
 * seconds. Allow tasks running hypercalls via the privcmd driver to
 * be voluntarily preempted even if full kernel preemption is
 * disabled.
 *
 * Such preemptible hypercalls are bracketed by
 * xen_preemptible_hcall_begin() and xen_preemptible_hcall_end()
 * calls.
 */
DEFINE_PER_CPU(bool, xen_in_preemptible_hcall);
EXPORT_SYMBOL_GPL(xen_in_preemptible_hcall);

/*
 * In case of scheduling the flag must be cleared and restored after
 * returning from schedule as the task might move to a different CPU.
 */
static __always_inline bool get_and_clear_inhcall(void)
{
	bool inhcall = __this_cpu_read(xen_in_preemptible_hcall);

	__this_cpu_write(xen_in_preemptible_hcall, false);
	return inhcall;
}

static __always_inline void restore_inhcall(bool inhcall)
{
	__this_cpu_write(xen_in_preemptible_hcall, inhcall);
}
#else
static __always_inline bool get_and_clear_inhcall(void) { return false; }
static __always_inline void restore_inhcall(bool inhcall) { }
#endif

static void __xen_pv_evtchn_do_upcall(void)
{
	irq_enter_rcu();
	inc_irq_stat(irq_hv_callback_count);

	xen_hvm_evtchn_do_upcall();

	irq_exit_rcu();
}

__visible noinstr void xen_pv_evtchn_do_upcall(struct pt_regs *regs)
{
	struct pt_regs *old_regs;
	bool inhcall, rcu_exit;

	rcu_exit = idtentry_enter_cond_rcu(regs);
	old_regs = set_irq_regs(regs);

	instrumentation_begin();
	run_on_irqstack_cond(__xen_pv_evtchn_do_upcall, NULL, regs);
	instrumentation_begin();

	set_irq_regs(old_regs);

	inhcall = get_and_clear_inhcall();
	if (inhcall && !WARN_ON_ONCE(rcu_exit)) {
		instrumentation_begin();
		idtentry_exit_cond_resched(regs, true);
		instrumentation_end();
		restore_inhcall(inhcall);
	} else {
		idtentry_exit_cond_rcu(regs, rcu_exit);
	}
}
#endif /* CONFIG_XEN_PV */