alistair23-linux/fs/userfaultfd.c

/*
 *  fs/userfaultfd.c
 *
 *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
 *  Copyright (C) 2008-2009 Red Hat, Inc.
 *  Copyright (C) 2015  Red Hat, Inc.
 *
 *  This work is licensed under the terms of the GNU GPL, version 2. See
 *  the COPYING file in the top-level directory.
 *
 *  Some part derived from fs/eventfd.c (anon inode setup) and
 *  mm/ksm.c (mm hashing).
 */

#include <linux/hashtable.h>
#include <linux/sched.h>
#include <linux/mm.h>
#include <linux/poll.h>
#include <linux/slab.h>
#include <linux/seq_file.h>
#include <linux/file.h>
#include <linux/bug.h>
#include <linux/anon_inodes.h>
#include <linux/syscalls.h>
#include <linux/userfaultfd_k.h>
#include <linux/mempolicy.h>
#include <linux/ioctl.h>
#include <linux/security.h>

enum userfaultfd_state {
	UFFD_STATE_WAIT_API,
	UFFD_STATE_RUNNING,
};

struct userfaultfd_ctx {
	/* pseudo fd refcounting */
	atomic_t refcount;
	/* waitqueue head for the userfaultfd page faults */
	wait_queue_head_t fault_wqh;
	/* waitqueue head for the pseudo fd to wakeup poll/read */
	wait_queue_head_t fd_wqh;
	/* userfaultfd syscall flags */
	unsigned int flags;
	/* state machine */
	enum userfaultfd_state state;
	/* released */
	bool released;
	/* mm with one ore more vmas attached to this userfaultfd_ctx */
	struct mm_struct *mm;
};

struct userfaultfd_wait_queue {
	unsigned long address;
	wait_queue_t wq;
	bool pending;
	struct userfaultfd_ctx *ctx;
};

struct userfaultfd_wake_range {
	unsigned long start;
	unsigned long len;
};

static int userfaultfd_wake_function(wait_queue_t *wq, unsigned mode,
				     int wake_flags, void *key)
{
	struct userfaultfd_wake_range *range = key;
	int ret;
	struct userfaultfd_wait_queue *uwq;
	unsigned long start, len;

	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
	ret = 0;
	/* don't wake the pending ones to avoid reads to block */
	if (uwq->pending && !ACCESS_ONCE(uwq->ctx->released))
		goto out;
	/* len == 0 means wake all */
	start = range->start;
	len = range->len;
	if (len && (start > uwq->address || start + len <= uwq->address))
		goto out;
	ret = wake_up_state(wq->private, mode);
	if (ret)
		/*
		 * Wake only once, autoremove behavior.
		 *
		 * After the effect of list_del_init is visible to the
		 * other CPUs, the waitqueue may disappear from under
		 * us, see the !list_empty_careful() in
		 * handle_userfault(). try_to_wake_up() has an
		 * implicit smp_mb__before_spinlock, and the
		 * wq->private is read before calling the extern
		 * function "wake_up_state" (which in turns calls
		 * try_to_wake_up). While the spin_lock;spin_unlock;
		 * wouldn't be enough, the smp_mb__before_spinlock is
		 * enough to avoid an explicit smp_mb() here.
		 */
		list_del_init(&wq->task_list);
out:
	return ret;
}

/**
 * userfaultfd_ctx_get - Acquires a reference to the internal userfaultfd
 * context.
 * @ctx: [in] Pointer to the userfaultfd context.
 *
 * Returns: In case of success, returns not zero.
 */
static void userfaultfd_ctx_get(struct userfaultfd_ctx *ctx)
{
	if (!atomic_inc_not_zero(&ctx->refcount))
		BUG();
}

/**
 * userfaultfd_ctx_put - Releases a reference to the internal userfaultfd
 * context.
 * @ctx: [in] Pointer to userfaultfd context.
 *
 * The userfaultfd context reference must have been previously acquired either
 * with userfaultfd_ctx_get() or userfaultfd_ctx_fdget().
 */
static void userfaultfd_ctx_put(struct userfaultfd_ctx *ctx)
{
	if (atomic_dec_and_test(&ctx->refcount)) {
		VM_BUG_ON(spin_is_locked(&ctx->fault_pending_wqh.lock));
		VM_BUG_ON(waitqueue_active(&ctx->fault_pending_wqh));
		VM_BUG_ON(spin_is_locked(&ctx->fault_wqh.lock));
		VM_BUG_ON(waitqueue_active(&ctx->fault_wqh));
		VM_BUG_ON(spin_is_locked(&ctx->fd_wqh.lock));
		VM_BUG_ON(waitqueue_active(&ctx->fd_wqh));
		mmput(ctx->mm);
		kfree(ctx);
	}
}

static inline unsigned long userfault_address(unsigned long address,
					      unsigned int flags,
					      unsigned long reason)
{
	BUILD_BUG_ON(PAGE_SHIFT < UFFD_BITS);
	address &= PAGE_MASK;
	if (flags & FAULT_FLAG_WRITE)
		/*
		 * Encode "write" fault information in the LSB of the
		 * address read by userland, without depending on
		 * FAULT_FLAG_WRITE kernel internal value.
		 */
		address |= UFFD_BIT_WRITE;
	if (reason & VM_UFFD_WP)
		/*
		 * Encode "reason" fault information as bit number 1
		 * in the address read by userland. If bit number 1 is
		 * clear it means the reason is a VM_FAULT_MISSING
		 * fault.
		 */
		address |= UFFD_BIT_WP;
	return address;
}

/*
 * The locking rules involved in returning VM_FAULT_RETRY depending on
 * FAULT_FLAG_ALLOW_RETRY, FAULT_FLAG_RETRY_NOWAIT and
 * FAULT_FLAG_KILLABLE are not straightforward. The "Caution"
 * recommendation in __lock_page_or_retry is not an understatement.
 *
 * If FAULT_FLAG_ALLOW_RETRY is set, the mmap_sem must be released
 * before returning VM_FAULT_RETRY only if FAULT_FLAG_RETRY_NOWAIT is
 * not set.
 *
 * If FAULT_FLAG_ALLOW_RETRY is set but FAULT_FLAG_KILLABLE is not
 * set, VM_FAULT_RETRY can still be returned if and only if there are
 * fatal_signal_pending()s, and the mmap_sem must be released before
 * returning it.
 */
int handle_userfault(struct vm_area_struct *vma, unsigned long address,
		     unsigned int flags, unsigned long reason)
{
	struct mm_struct *mm = vma->vm_mm;
	struct userfaultfd_ctx *ctx;
	struct userfaultfd_wait_queue uwq;

	BUG_ON(!rwsem_is_locked(&mm->mmap_sem));

	ctx = vma->vm_userfaultfd_ctx.ctx;
	if (!ctx)
		return VM_FAULT_SIGBUS;

	BUG_ON(ctx->mm != mm);

	VM_BUG_ON(reason & ~(VM_UFFD_MISSING|VM_UFFD_WP));
	VM_BUG_ON(!(reason & VM_UFFD_MISSING) ^ !!(reason & VM_UFFD_WP));

	/*
	 * If it's already released don't get it. This avoids to loop
	 * in __get_user_pages if userfaultfd_release waits on the
	 * caller of handle_userfault to release the mmap_sem.
	 */
	if (unlikely(ACCESS_ONCE(ctx->released)))
		return VM_FAULT_SIGBUS;

	/*
	 * Check that we can return VM_FAULT_RETRY.
	 *
	 * NOTE: it should become possible to return VM_FAULT_RETRY
	 * even if FAULT_FLAG_TRIED is set without leading to gup()
	 * -EBUSY failures, if the userfaultfd is to be extended for
	 * VM_UFFD_WP tracking and we intend to arm the userfault
	 * without first stopping userland access to the memory. For
	 * VM_UFFD_MISSING userfaults this is enough for now.
	 */
	if (unlikely(!(flags & FAULT_FLAG_ALLOW_RETRY))) {
		/*
		 * Validate the invariant that nowait must allow retry
		 * to be sure not to return SIGBUS erroneously on
		 * nowait invocations.
		 */
		BUG_ON(flags & FAULT_FLAG_RETRY_NOWAIT);
#ifdef CONFIG_DEBUG_VM
		if (printk_ratelimit()) {
			printk(KERN_WARNING
			       "FAULT_FLAG_ALLOW_RETRY missing %x\n", flags);
			dump_stack();
		}
#endif
		return VM_FAULT_SIGBUS;
	}

	/*
	 * Handle nowait, not much to do other than tell it to retry
	 * and wait.
	 */
	if (flags & FAULT_FLAG_RETRY_NOWAIT)
		return VM_FAULT_RETRY;

	/* take the reference before dropping the mmap_sem */
	userfaultfd_ctx_get(ctx);

	/* be gentle and immediately relinquish the mmap_sem */
	up_read(&mm->mmap_sem);

	init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
	uwq.wq.private = current;
	uwq.address = userfault_address(address, flags, reason);
	uwq.pending = true;
	uwq.ctx = ctx;

	spin_lock(&ctx->fault_wqh.lock);
	/*
	 * After the __add_wait_queue the uwq is visible to userland
	 * through poll/read().
	 */
	__add_wait_queue(&ctx->fault_wqh, &uwq.wq);
	for (;;) {
		set_current_state(TASK_KILLABLE);
		if (!uwq.pending || ACCESS_ONCE(ctx->released) ||
		    fatal_signal_pending(current))
			break;
		spin_unlock(&ctx->fault_wqh.lock);

		wake_up_poll(&ctx->fd_wqh, POLLIN);
		schedule();

		spin_lock(&ctx->fault_wqh.lock);
	}
	__remove_wait_queue(&ctx->fault_wqh, &uwq.wq);
	__set_current_state(TASK_RUNNING);
	spin_unlock(&ctx->fault_wqh.lock);

	/*
	 * ctx may go away after this if the userfault pseudo fd is
	 * already released.
	 */
	userfaultfd_ctx_put(ctx);

	return VM_FAULT_RETRY;
}

static int userfaultfd_release(struct inode *inode, struct file *file)
{
	struct userfaultfd_ctx *ctx = file->private_data;
	struct mm_struct *mm = ctx->mm;
	struct vm_area_struct *vma, *prev;
	/* len == 0 means wake all */
	struct userfaultfd_wake_range range = { .len = 0, };
	unsigned long new_flags;

	ACCESS_ONCE(ctx->released) = true;

	/*
	 * Flush page faults out of all CPUs. NOTE: all page faults
	 * must be retried without returning VM_FAULT_SIGBUS if
	 * userfaultfd_ctx_get() succeeds but vma->vma_userfault_ctx
	 * changes while handle_userfault released the mmap_sem. So
	 * it's critical that released is set to true (above), before
	 * taking the mmap_sem for writing.
	 */
	down_write(&mm->mmap_sem);
	prev = NULL;
	for (vma = mm->mmap; vma; vma = vma->vm_next) {
		cond_resched();
		BUG_ON(!!vma->vm_userfaultfd_ctx.ctx ^
		       !!(vma->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));
		if (vma->vm_userfaultfd_ctx.ctx != ctx) {
			prev = vma;
			continue;
		}
		new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
		prev = vma_merge(mm, prev, vma->vm_start, vma->vm_end,
				 new_flags, vma->anon_vma,
				 vma->vm_file, vma->vm_pgoff,
				 vma_policy(vma),
				 NULL_VM_UFFD_CTX);
		if (prev)
			vma = prev;
		else
			prev = vma;
		vma->vm_flags = new_flags;
		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
	}
	up_write(&mm->mmap_sem);

	/*
	 * After no new page faults can wait on this fault_wqh, flush
	 * the last page faults that may have been already waiting on
	 * the fault_wqh.
	 */
	spin_lock(&ctx->fault_wqh.lock);
	__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, 0, &range);
	spin_unlock(&ctx->fault_wqh.lock);

	wake_up_poll(&ctx->fd_wqh, POLLHUP);
	userfaultfd_ctx_put(ctx);
	return 0;
}

/* fault_wqh.lock must be hold by the caller */
static inline unsigned int find_userfault(struct userfaultfd_ctx *ctx,
					  struct userfaultfd_wait_queue **uwq)
{
	wait_queue_t *wq;
	struct userfaultfd_wait_queue *_uwq;
	unsigned int ret = 0;

	VM_BUG_ON(!spin_is_locked(&ctx->fault_wqh.lock));

	list_for_each_entry(wq, &ctx->fault_wqh.task_list, task_list) {
		_uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
		if (_uwq->pending) {
			ret = POLLIN;
			if (!uwq)
				/*
				 * If there's at least a pending and
				 * we don't care which one it is,
				 * break immediately and leverage the
				 * efficiency of the LIFO walk.
				 */
				break;
			/*
			 * If we need to find which one was pending we
			 * keep walking until we find the first not
			 * pending one, so we read() them in FIFO order.
			 */
			*uwq = _uwq;
		} else
			/*
			 * break the loop at the first not pending
			 * one, there cannot be pending userfaults
			 * after the first not pending one, because
			 * all new pending ones are inserted at the
			 * head and we walk it in LIFO.
			 */
			break;
	}

	return ret;
}

static unsigned int userfaultfd_poll(struct file *file, poll_table *wait)
{
	struct userfaultfd_ctx *ctx = file->private_data;
	unsigned int ret;

	poll_wait(file, &ctx->fd_wqh, wait);

	switch (ctx->state) {
	case UFFD_STATE_WAIT_API:
		return POLLERR;
	case UFFD_STATE_RUNNING:
		spin_lock(&ctx->fault_wqh.lock);
		ret = find_userfault(ctx, NULL);
		spin_unlock(&ctx->fault_wqh.lock);
		return ret;
	default:
		BUG();
	}
}

static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
				    __u64 *addr)
{
	ssize_t ret;
	DECLARE_WAITQUEUE(wait, current);
	struct userfaultfd_wait_queue *uwq = NULL;

	/* always take the fd_wqh lock before the fault_wqh lock */
	spin_lock(&ctx->fd_wqh.lock);
	__add_wait_queue(&ctx->fd_wqh, &wait);
	for (;;) {
		set_current_state(TASK_INTERRUPTIBLE);
		spin_lock(&ctx->fault_wqh.lock);
		if (find_userfault(ctx, &uwq)) {
			/*
			 * The fault_wqh.lock prevents the uwq to
			 * disappear from under us.
			 */
			uwq->pending = false;
			/* careful to always initialize addr if ret == 0 */
			*addr = uwq->address;
			spin_unlock(&ctx->fault_wqh.lock);
			ret = 0;
			break;
		}
		spin_unlock(&ctx->fault_wqh.lock);
		if (signal_pending(current)) {
			ret = -ERESTARTSYS;
			break;
		}
		if (no_wait) {
			ret = -EAGAIN;
			break;
		}
		spin_unlock(&ctx->fd_wqh.lock);
		schedule();
		spin_lock(&ctx->fd_wqh.lock);
	}
	__remove_wait_queue(&ctx->fd_wqh, &wait);
	__set_current_state(TASK_RUNNING);
	spin_unlock(&ctx->fd_wqh.lock);

	return ret;
}

static ssize_t userfaultfd_read(struct file *file, char __user *buf,
				size_t count, loff_t *ppos)
{
	struct userfaultfd_ctx *ctx = file->private_data;
	ssize_t _ret, ret = 0;
	/* careful to always initialize addr if ret == 0 */
	__u64 uninitialized_var(addr);
	int no_wait = file->f_flags & O_NONBLOCK;

	if (ctx->state == UFFD_STATE_WAIT_API)
		return -EINVAL;
	BUG_ON(ctx->state != UFFD_STATE_RUNNING);

	for (;;) {
		if (count < sizeof(addr))
			return ret ? ret : -EINVAL;
		_ret = userfaultfd_ctx_read(ctx, no_wait, &addr);
		if (_ret < 0)
			return ret ? ret : _ret;
		if (put_user(addr, (__u64 __user *) buf))
			return ret ? ret : -EFAULT;
		ret += sizeof(addr);
		buf += sizeof(addr);
		count -= sizeof(addr);
		/*
		 * Allow to read more than one fault at time but only
		 * block if waiting for the very first one.
		 */
		no_wait = O_NONBLOCK;
	}
}

static void __wake_userfault(struct userfaultfd_ctx *ctx,
			     struct userfaultfd_wake_range *range)
{
	unsigned long start, end;

	start = range->start;
	end = range->start + range->len;

	spin_lock(&ctx->fault_wqh.lock);
	/* wake all in the range and autoremove */
	__wake_up_locked_key(&ctx->fault_wqh, TASK_NORMAL, 0, range);
	spin_unlock(&ctx->fault_wqh.lock);
}

static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
					   struct userfaultfd_wake_range *range)
{
	/*
	 * To be sure waitqueue_active() is not reordered by the CPU
	 * before the pagetable update, use an explicit SMP memory
	 * barrier here. PT lock release or up_read(mmap_sem) still
	 * have release semantics that can allow the
	 * waitqueue_active() to be reordered before the pte update.
	 */
	smp_mb();

	/*
	 * Use waitqueue_active because it's very frequent to
	 * change the address space atomically even if there are no
	 * userfaults yet. So we take the spinlock only when we're
	 * sure we've userfaults to wake.
	 */
	if (waitqueue_active(&ctx->fault_wqh))
		__wake_userfault(ctx, range);
}

static __always_inline int validate_range(struct mm_struct *mm,
					  __u64 start, __u64 len)
{
	__u64 task_size = mm->task_size;

	if (start & ~PAGE_MASK)
		return -EINVAL;
	if (len & ~PAGE_MASK)
		return -EINVAL;
	if (!len)
		return -EINVAL;
	if (start < mmap_min_addr)
		return -EINVAL;
	if (start >= task_size)
		return -EINVAL;
	if (len > task_size - start)
		return -EINVAL;
	return 0;
}

static int userfaultfd_register(struct userfaultfd_ctx *ctx,
				unsigned long arg)
{
	struct mm_struct *mm = ctx->mm;
	struct vm_area_struct *vma, *prev, *cur;
	int ret;
	struct uffdio_register uffdio_register;
	struct uffdio_register __user *user_uffdio_register;
	unsigned long vm_flags, new_flags;
	bool found;
	unsigned long start, end, vma_end;

	user_uffdio_register = (struct uffdio_register __user *) arg;

	ret = -EFAULT;
	if (copy_from_user(&uffdio_register, user_uffdio_register,
			   sizeof(uffdio_register)-sizeof(__u64)))
		goto out;

	ret = -EINVAL;
	if (!uffdio_register.mode)
		goto out;
	if (uffdio_register.mode & ~(UFFDIO_REGISTER_MODE_MISSING|
				     UFFDIO_REGISTER_MODE_WP))
		goto out;
	vm_flags = 0;
	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_MISSING)
		vm_flags |= VM_UFFD_MISSING;
	if (uffdio_register.mode & UFFDIO_REGISTER_MODE_WP) {
		vm_flags |= VM_UFFD_WP;
		/*
		 * FIXME: remove the below error constraint by
		 * implementing the wprotect tracking mode.
		 */
		ret = -EINVAL;
		goto out;
	}

	ret = validate_range(mm, uffdio_register.range.start,
			     uffdio_register.range.len);
	if (ret)
		goto out;

	start = uffdio_register.range.start;
	end = start + uffdio_register.range.len;

	down_write(&mm->mmap_sem);
	vma = find_vma_prev(mm, start, &prev);

	ret = -ENOMEM;
	if (!vma)
		goto out_unlock;

	/* check that there's at least one vma in the range */
	ret = -EINVAL;
	if (vma->vm_start >= end)
		goto out_unlock;

	/*
	 * Search for not compatible vmas.
	 *
	 * FIXME: this shall be relaxed later so that it doesn't fail
	 * on tmpfs backed vmas (in addition to the current allowance
	 * on anonymous vmas).
	 */
	found = false;
	for (cur = vma; cur && cur->vm_start < end; cur = cur->vm_next) {
		cond_resched();

		BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
		       !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));

		/* check not compatible vmas */
		ret = -EINVAL;
		if (cur->vm_ops)
			goto out_unlock;

		/*
		 * Check that this vma isn't already owned by a
		 * different userfaultfd. We can't allow more than one
		 * userfaultfd to own a single vma simultaneously or we
		 * wouldn't know which one to deliver the userfaults to.
		 */
		ret = -EBUSY;
		if (cur->vm_userfaultfd_ctx.ctx &&
		    cur->vm_userfaultfd_ctx.ctx != ctx)
			goto out_unlock;

		found = true;
	}
	BUG_ON(!found);

	if (vma->vm_start < start)
		prev = vma;

	ret = 0;
	do {
		cond_resched();

		BUG_ON(vma->vm_ops);
		BUG_ON(vma->vm_userfaultfd_ctx.ctx &&
		       vma->vm_userfaultfd_ctx.ctx != ctx);

		/*
		 * Nothing to do: this vma is already registered into this
		 * userfaultfd and with the right tracking mode too.
		 */
		if (vma->vm_userfaultfd_ctx.ctx == ctx &&
		    (vma->vm_flags & vm_flags) == vm_flags)
			goto skip;

		if (vma->vm_start > start)
			start = vma->vm_start;
		vma_end = min(end, vma->vm_end);

		new_flags = (vma->vm_flags & ~vm_flags) | vm_flags;
		prev = vma_merge(mm, prev, start, vma_end, new_flags,
				 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
				 vma_policy(vma),
				 ((struct vm_userfaultfd_ctx){ ctx }));
		if (prev) {
			vma = prev;
			goto next;
		}
		if (vma->vm_start < start) {
			ret = split_vma(mm, vma, start, 1);
			if (ret)
				break;
		}
		if (vma->vm_end > end) {
			ret = split_vma(mm, vma, end, 0);
			if (ret)
				break;
		}
	next:
		/*
		 * In the vma_merge() successful mprotect-like case 8:
		 * the next vma was merged into the current one and
		 * the current one has not been updated yet.
		 */
		vma->vm_flags = new_flags;
		vma->vm_userfaultfd_ctx.ctx = ctx;

	skip:
		prev = vma;
		start = vma->vm_end;
		vma = vma->vm_next;
	} while (vma && vma->vm_start < end);
out_unlock:
	up_write(&mm->mmap_sem);
	if (!ret) {
		/*
		 * Now that we scanned all vmas we can already tell
		 * userland which ioctls methods are guaranteed to
		 * succeed on this range.
		 */
		if (put_user(UFFD_API_RANGE_IOCTLS,
			     &user_uffdio_register->ioctls))
			ret = -EFAULT;
	}
out:
	return ret;
}

static int userfaultfd_unregister(struct userfaultfd_ctx *ctx,
				  unsigned long arg)
{
	struct mm_struct *mm = ctx->mm;
	struct vm_area_struct *vma, *prev, *cur;
	int ret;
	struct uffdio_range uffdio_unregister;
	unsigned long new_flags;
	bool found;
	unsigned long start, end, vma_end;
	const void __user *buf = (void __user *)arg;

	ret = -EFAULT;
	if (copy_from_user(&uffdio_unregister, buf, sizeof(uffdio_unregister)))
		goto out;

	ret = validate_range(mm, uffdio_unregister.start,
			     uffdio_unregister.len);
	if (ret)
		goto out;

	start = uffdio_unregister.start;
	end = start + uffdio_unregister.len;

	down_write(&mm->mmap_sem);
	vma = find_vma_prev(mm, start, &prev);

	ret = -ENOMEM;
	if (!vma)
		goto out_unlock;

	/* check that there's at least one vma in the range */
	ret = -EINVAL;
	if (vma->vm_start >= end)
		goto out_unlock;

	/*
	 * Search for not compatible vmas.
	 *
	 * FIXME: this shall be relaxed later so that it doesn't fail
	 * on tmpfs backed vmas (in addition to the current allowance
	 * on anonymous vmas).
	 */
	found = false;
	ret = -EINVAL;
	for (cur = vma; cur && cur->vm_start < end; cur = cur->vm_next) {
		cond_resched();

		BUG_ON(!!cur->vm_userfaultfd_ctx.ctx ^
		       !!(cur->vm_flags & (VM_UFFD_MISSING | VM_UFFD_WP)));

		/*
		 * Check not compatible vmas, not strictly required
		 * here as not compatible vmas cannot have an
		 * userfaultfd_ctx registered on them, but this
		 * provides for more strict behavior to notice
		 * unregistration errors.
		 */
		if (cur->vm_ops)
			goto out_unlock;

		found = true;
	}
	BUG_ON(!found);

	if (vma->vm_start < start)
		prev = vma;

	ret = 0;
	do {
		cond_resched();

		BUG_ON(vma->vm_ops);

		/*
		 * Nothing to do: this vma is already registered into this
		 * userfaultfd and with the right tracking mode too.
		 */
		if (!vma->vm_userfaultfd_ctx.ctx)
			goto skip;

		if (vma->vm_start > start)
			start = vma->vm_start;
		vma_end = min(end, vma->vm_end);

		new_flags = vma->vm_flags & ~(VM_UFFD_MISSING | VM_UFFD_WP);
		prev = vma_merge(mm, prev, start, vma_end, new_flags,
				 vma->anon_vma, vma->vm_file, vma->vm_pgoff,
				 vma_policy(vma),
				 NULL_VM_UFFD_CTX);
		if (prev) {
			vma = prev;
			goto next;
		}
		if (vma->vm_start < start) {
			ret = split_vma(mm, vma, start, 1);
			if (ret)
				break;
		}
		if (vma->vm_end > end) {
			ret = split_vma(mm, vma, end, 0);
			if (ret)
				break;
		}
	next:
		/*
		 * In the vma_merge() successful mprotect-like case 8:
		 * the next vma was merged into the current one and
		 * the current one has not been updated yet.
		 */
		vma->vm_flags = new_flags;
		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;

	skip:
		prev = vma;
		start = vma->vm_end;
		vma = vma->vm_next;
	} while (vma && vma->vm_start < end);
out_unlock:
	up_write(&mm->mmap_sem);
out:
	return ret;
}

/*
 * This is mostly needed to re-wakeup those userfaults that were still
 * pending when userland wake them up the first time. We don't wake
 * the pending one to avoid blocking reads to block, or non blocking
 * read to return -EAGAIN, if used with POLLIN, to avoid userland
 * doubts on why POLLIN wasn't reliable.
 */
static int userfaultfd_wake(struct userfaultfd_ctx *ctx,
			    unsigned long arg)
{
	int ret;
	struct uffdio_range uffdio_wake;
	struct userfaultfd_wake_range range;
	const void __user *buf = (void __user *)arg;

	ret = -EFAULT;
	if (copy_from_user(&uffdio_wake, buf, sizeof(uffdio_wake)))
		goto out;

	ret = validate_range(ctx->mm, uffdio_wake.start, uffdio_wake.len);
	if (ret)
		goto out;

	range.start = uffdio_wake.start;
	range.len = uffdio_wake.len;

	/*
	 * len == 0 means wake all and we don't want to wake all here,
	 * so check it again to be sure.
	 */
	VM_BUG_ON(!range.len);

	wake_userfault(ctx, &range);
	ret = 0;

out:
	return ret;
}

/*
 * userland asks for a certain API version and we return which bits
 * and ioctl commands are implemented in this kernel for such API
 * version or -EINVAL if unknown.
 */
static int userfaultfd_api(struct userfaultfd_ctx *ctx,
			   unsigned long arg)
{
	struct uffdio_api uffdio_api;
	void __user *buf = (void __user *)arg;
	int ret;

	ret = -EINVAL;
	if (ctx->state != UFFD_STATE_WAIT_API)
		goto out;
	ret = -EFAULT;
	if (copy_from_user(&uffdio_api, buf, sizeof(__u64)))
		goto out;
	if (uffdio_api.api != UFFD_API) {
		/* careful not to leak info, we only read the first 8 bytes */
		memset(&uffdio_api, 0, sizeof(uffdio_api));
		if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
			goto out;
		ret = -EINVAL;
		goto out;
	}
	/* careful not to leak info, we only read the first 8 bytes */
	uffdio_api.features = UFFD_API_FEATURES;
	uffdio_api.ioctls = UFFD_API_IOCTLS;
	ret = -EFAULT;
	if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
		goto out;
	ctx->state = UFFD_STATE_RUNNING;
	ret = 0;
out:
	return ret;
}

static long userfaultfd_ioctl(struct file *file, unsigned cmd,
			      unsigned long arg)
{
	int ret = -EINVAL;
	struct userfaultfd_ctx *ctx = file->private_data;

	switch(cmd) {
	case UFFDIO_API:
		ret = userfaultfd_api(ctx, arg);
		break;
	case UFFDIO_REGISTER:
		ret = userfaultfd_register(ctx, arg);
		break;
	case UFFDIO_UNREGISTER:
		ret = userfaultfd_unregister(ctx, arg);
		break;
	case UFFDIO_WAKE:
		ret = userfaultfd_wake(ctx, arg);
		break;
	}
	return ret;
}

#ifdef CONFIG_PROC_FS
static void userfaultfd_show_fdinfo(struct seq_file *m, struct file *f)
{
	struct userfaultfd_ctx *ctx = f->private_data;
	wait_queue_t *wq;
	struct userfaultfd_wait_queue *uwq;
	unsigned long pending = 0, total = 0;

	spin_lock(&ctx->fault_wqh.lock);
	list_for_each_entry(wq, &ctx->fault_wqh.task_list, task_list) {
		uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
		if (uwq->pending)
			pending++;
		total++;
	}
	spin_unlock(&ctx->fault_wqh.lock);

	/*
	 * If more protocols will be added, there will be all shown
	 * separated by a space. Like this:
	 *	protocols: aa:... bb:...
	 */
	seq_printf(m, "pending:\t%lu\ntotal:\t%lu\nAPI:\t%Lx:%x:%Lx\n",
		   pending, total, UFFD_API, UFFD_API_FEATURES,
		   UFFD_API_IOCTLS|UFFD_API_RANGE_IOCTLS);
}
#endif

static const struct file_operations userfaultfd_fops = {
#ifdef CONFIG_PROC_FS
	.show_fdinfo	= userfaultfd_show_fdinfo,
#endif
	.release	= userfaultfd_release,
	.poll		= userfaultfd_poll,
	.read		= userfaultfd_read,
	.unlocked_ioctl = userfaultfd_ioctl,
	.compat_ioctl	= userfaultfd_ioctl,
	.llseek		= noop_llseek,
};

/**
 * userfaultfd_file_create - Creates an userfaultfd file pointer.
 * @flags: Flags for the userfaultfd file.
 *
 * This function creates an userfaultfd file pointer, w/out installing
 * it into the fd table. This is useful when the userfaultfd file is
 * used during the initialization of data structures that require
 * extra setup after the userfaultfd creation. So the userfaultfd
 * creation is split into the file pointer creation phase, and the
 * file descriptor installation phase.  In this way races with
 * userspace closing the newly installed file descriptor can be
 * avoided.  Returns an userfaultfd file pointer, or a proper error
 * pointer.
 */
static struct file *userfaultfd_file_create(int flags)
{
	struct file *file;
	struct userfaultfd_ctx *ctx;

	BUG_ON(!current->mm);

	/* Check the UFFD_* constants for consistency.  */
	BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
	BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);

	file = ERR_PTR(-EINVAL);
	if (flags & ~UFFD_SHARED_FCNTL_FLAGS)
		goto out;

	file = ERR_PTR(-ENOMEM);
	ctx = kmalloc(sizeof(*ctx), GFP_KERNEL);
	if (!ctx)
		goto out;

	atomic_set(&ctx->refcount, 1);
	init_waitqueue_head(&ctx->fault_wqh);
	init_waitqueue_head(&ctx->fd_wqh);
	ctx->flags = flags;
	ctx->state = UFFD_STATE_WAIT_API;
	ctx->released = false;
	ctx->mm = current->mm;
	/* prevent the mm struct to be freed */
	atomic_inc(&ctx->mm->mm_users);

	file = anon_inode_getfile("[userfaultfd]", &userfaultfd_fops, ctx,
				  O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
	if (IS_ERR(file))
		kfree(ctx);
out:
	return file;
}

SYSCALL_DEFINE1(userfaultfd, int, flags)
{
	int fd, error;
	struct file *file;

	error = get_unused_fd_flags(flags & UFFD_SHARED_FCNTL_FLAGS);
	if (error < 0)
		return error;
	fd = error;

	file = userfaultfd_file_create(flags);
	if (IS_ERR(file)) {
		error = PTR_ERR(file);
		goto err_put_unused_fd;
	}
	fd_install(fd, file);

	return fd;

err_put_unused_fd:
	put_unused_fd(fd);

	return error;
}