text: /*
   *  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/list.h>
  #include <linux/hashtable.h>
  #include <linux/sched/signal.h>
  #include <linux/sched/mm.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>
  #include <linux/hugetlb.h>
  
  static struct kmem_cache *userfaultfd_ctx_cachep __read_mostly;
  
  enum userfaultfd_state {
  	UFFD_STATE_WAIT_API,
  	UFFD_STATE_RUNNING,
  };
  
  /*
   * Start with fault_pending_wqh and fault_wqh so they're more likely
   * to be in the same cacheline.
   */
  struct userfaultfd_ctx {
  	/* waitqueue head for the pending (i.e. not read) userfaults */
  	wait_queue_head_t fault_pending_wqh;
  	/* waitqueue head for the userfaults */
  	wait_queue_head_t fault_wqh;
  	/* waitqueue head for the pseudo fd to wakeup poll/read */
  	wait_queue_head_t fd_wqh;
  	/* waitqueue head for events */
  	wait_queue_head_t event_wqh;
  	/* a refile sequence protected by fault_pending_wqh lock */
  	struct seqcount refile_seq;
  	/* pseudo fd refcounting */
  	atomic_t refcount;
  	/* userfaultfd syscall flags */
  	unsigned int flags;
  	/* features requested from the userspace */
  	unsigned int features;
  	/* state machine */
  	enum userfaultfd_state state;
  	/* released */
  	bool released;
  	/* memory mappings are changing because of non-cooperative event */
  	bool mmap_changing;
  	/* mm with one ore more vmas attached to this userfaultfd_ctx */
  	struct mm_struct *mm;
  };
  
  struct userfaultfd_fork_ctx {
  	struct userfaultfd_ctx *orig;
  	struct userfaultfd_ctx *new;
  	struct list_head list;
  };
  
  struct userfaultfd_unmap_ctx {
  	struct userfaultfd_ctx *ctx;
  	unsigned long start;
  	unsigned long end;
  	struct list_head list;
  };
  
  struct userfaultfd_wait_queue {
  	struct uffd_msg msg;
  	wait_queue_entry_t wq;
  	struct userfaultfd_ctx *ctx;
  	bool waken;
  };
  
  struct userfaultfd_wake_range {
  	unsigned long start;
  	unsigned long len;
  };
  
  static int userfaultfd_wake_function(wait_queue_entry_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;
  	/* len == 0 means wake all */
  	start = range->start;
  	len = range->len;
  	if (len && (start > uwq->msg.arg.pagefault.address ||
  		    start + len <= uwq->msg.arg.pagefault.address))
  		goto out;
  	WRITE_ONCE(uwq->waken, true);
  	/*
  	 * The Program-Order guarantees provided by the scheduler
  	 * ensure uwq->waken is visible before the task is woken.
  	 */
  	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(), and the
  		 * wq->private is read before calling the extern function
  		 * "wake_up_state" (which in turns calls try_to_wake_up).
  		 */
  		list_del_init(&wq->entry);
  	}
  out:
  	return ret;
  }
  
  /**
   * userfaultfd_ctx_get - Acquires a reference to the internal userfaultfd
   * context.
   * @ctx: [in] Pointer to the userfaultfd context.
   */
  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->event_wqh.lock));
  		VM_BUG_ON(waitqueue_active(&ctx->event_wqh));
  		VM_BUG_ON(spin_is_locked(&ctx->fd_wqh.lock));
  		VM_BUG_ON(waitqueue_active(&ctx->fd_wqh));
  		mmdrop(ctx->mm);
  		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
  	}
  }
  
  static inline void msg_init(struct uffd_msg *msg)
  {
  	BUILD_BUG_ON(sizeof(struct uffd_msg) != 32);
  	/*
  	 * Must use memset to zero out the paddings or kernel data is
  	 * leaked to userland.
  	 */
  	memset(msg, 0, sizeof(struct uffd_msg));
  }
  
  static inline struct uffd_msg userfault_msg(unsigned long address,
  					    unsigned int flags,
  					    unsigned long reason,
  					    unsigned int features)
  {
  	struct uffd_msg msg;
  	msg_init(&msg);
  	msg.event = UFFD_EVENT_PAGEFAULT;
  	msg.arg.pagefault.address = address;
  	if (flags & FAULT_FLAG_WRITE)
  		/*
  		 * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
  		 * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WRITE
  		 * was not set in a UFFD_EVENT_PAGEFAULT, it means it
  		 * was a read fault, otherwise if set it means it's
  		 * a write fault.
  		 */
  		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WRITE;
  	if (reason & VM_UFFD_WP)
  		/*
  		 * If UFFD_FEATURE_PAGEFAULT_FLAG_WP was set in the
  		 * uffdio_api.features and UFFD_PAGEFAULT_FLAG_WP was
  		 * not set in a UFFD_EVENT_PAGEFAULT, it means it was
  		 * a missing fault, otherwise if set it means it's a
  		 * write protect fault.
  		 */
  		msg.arg.pagefault.flags |= UFFD_PAGEFAULT_FLAG_WP;
  	if (features & UFFD_FEATURE_THREAD_ID)
  		msg.arg.pagefault.feat.ptid = task_pid_vnr(current);
  	return msg;
  }
  
  #ifdef CONFIG_HUGETLB_PAGE
  /*
   * Same functionality as userfaultfd_must_wait below with modifications for
   * hugepmd ranges.
   */
  static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
  					 struct vm_area_struct *vma,
  					 unsigned long address,
  					 unsigned long flags,
  					 unsigned long reason)
  {
  	struct mm_struct *mm = ctx->mm;
  	pte_t *ptep, pte;
  	bool ret = true;
  
  	VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
  
  	ptep = huge_pte_offset(mm, address, vma_mmu_pagesize(vma));
  
  	if (!ptep)
  		goto out;
  
  	ret = false;
  	pte = huge_ptep_get(ptep);
  
  	/*
  	 * Lockless access: we're in a wait_event so it's ok if it
  	 * changes under us.
  	 */
  	if (huge_pte_none(pte))
  		ret = true;
  	if (!huge_pte_write(pte) && (reason & VM_UFFD_WP))
  		ret = true;
  out:
  	return ret;
  }
  #else
  static inline bool userfaultfd_huge_must_wait(struct userfaultfd_ctx *ctx,
  					 struct vm_area_struct *vma,
  					 unsigned long address,
  					 unsigned long flags,
  					 unsigned long reason)
  {
  	return false;	/* should never get here */
  }
  #endif /* CONFIG_HUGETLB_PAGE */
  
  /*
   * Verify the pagetables are still not ok after having reigstered into
   * the fault_pending_wqh to avoid userland having to UFFDIO_WAKE any
   * userfault that has already been resolved, if userfaultfd_read and
   * UFFDIO_COPY|ZEROPAGE are being run simultaneously on two different
   * threads.
   */
  static inline bool userfaultfd_must_wait(struct userfaultfd_ctx *ctx,
  					 unsigned long address,
  					 unsigned long flags,
  					 unsigned long reason)
  {
  	struct mm_struct *mm = ctx->mm;
  	pgd_t *pgd;
  	p4d_t *p4d;
  	pud_t *pud;
  	pmd_t *pmd, _pmd;
  	pte_t *pte;
  	bool ret = true;
  
  	VM_BUG_ON(!rwsem_is_locked(&mm->mmap_sem));
  
  	pgd = pgd_offset(mm, address);
  	if (!pgd_present(*pgd))
  		goto out;
  	p4d = p4d_offset(pgd, address);
  	if (!p4d_present(*p4d))
  		goto out;
  	pud = pud_offset(p4d, address);
  	if (!pud_present(*pud))
  		goto out;
  	pmd = pmd_offset(pud, address);
  	/*
  	 * READ_ONCE must function as a barrier with narrower scope
  	 * and it must be equivalent to:
  	 *	_pmd = *pmd; barrier();
  	 *
  	 * This is to deal with the instability (as in
  	 * pmd_trans_unstable) of the pmd.
  	 */
  	_pmd = READ_ONCE(*pmd);
  	if (pmd_none(_pmd))
  		goto out;
  
  	ret = false;
  	if (!pmd_present(_pmd))
  		goto out;
  
  	if (pmd_trans_huge(_pmd))
  		goto out;
  
  	/*
  	 * the pmd is stable (as in !pmd_trans_unstable) so we can re-read it
  	 * and use the standard pte_offset_map() instead of parsing _pmd.
  	 */
  	pte = pte_offset_map(pmd, address);
  	/*
  	 * Lockless access: we're in a wait_event so it's ok if it
  	 * changes under us.
  	 */
  	if (pte_none(*pte))
  		ret = true;
  	pte_unmap(pte);
  
  out:
  	return ret;
  }
  
  /*
   * 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.
   */
  vm_fault_t handle_userfault(struct vm_fault *vmf, unsigned long reason)
  {
  	struct mm_struct *mm = vmf->vma->vm_mm;
  	struct userfaultfd_ctx *ctx;
  	struct userfaultfd_wait_queue uwq;
  	vm_fault_t ret = VM_FAULT_SIGBUS;
  	bool must_wait, return_to_userland;
  	long blocking_state;
  
  	/*
  	 * We don't do userfault handling for the final child pid update.
  	 *
  	 * We also don't do userfault handling during
  	 * coredumping. hugetlbfs has the special
  	 * follow_hugetlb_page() to skip missing pages in the
  	 * FOLL_DUMP case, anon memory also checks for FOLL_DUMP with
  	 * the no_page_table() helper in follow_page_mask(), but the
  	 * shmem_vm_ops->fault method is invoked even during
  	 * coredumping without mmap_sem and it ends up here.
  	 */
  	if (current->flags & (PF_EXITING|PF_DUMPCORE))
  		goto out;
  
  	/*
  	 * Coredumping runs without mmap_sem so we can only check that
  	 * the mmap_sem is held, if PF_DUMPCORE was not set.
  	 */
  	WARN_ON_ONCE(!rwsem_is_locked(&mm->mmap_sem));
  
  	ctx = vmf->vma->vm_userfaultfd_ctx.ctx;
  	if (!ctx)
  		goto out;
  
  	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 (ctx->features & UFFD_FEATURE_SIGBUS)
  		goto out;
  
  	/*
  	 * 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(READ_ONCE(ctx->released))) {
  		/*
  		 * Don't return VM_FAULT_SIGBUS in this case, so a non
  		 * cooperative manager can close the uffd after the
  		 * last UFFDIO_COPY, without risking to trigger an
  		 * involuntary SIGBUS if the process was starting the
  		 * userfaultfd while the userfaultfd was still armed
  		 * (but after the last UFFDIO_COPY). If the uffd
  		 * wasn't already closed when the userfault reached
  		 * this point, that would normally be solved by
  		 * userfaultfd_must_wait returning 'false'.
  		 *
  		 * If we were to return VM_FAULT_SIGBUS here, the non
  		 * cooperative manager would be instead forced to
  		 * always call UFFDIO_UNREGISTER before it can safely
  		 * close the uffd.
  		 */
  		ret = VM_FAULT_NOPAGE;
  		goto out;
  	}
  
  	/*
  	 * 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(!(vmf->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(vmf->flags & FAULT_FLAG_RETRY_NOWAIT);
  #ifdef CONFIG_DEBUG_VM
  		if (printk_ratelimit()) {
  			printk(KERN_WARNING
  			       "FAULT_FLAG_ALLOW_RETRY missing %x\n",
  			       vmf->flags);
  			dump_stack();
  		}
  #endif
  		goto out;
  	}
  
  	/*
  	 * Handle nowait, not much to do other than tell it to retry
  	 * and wait.
  	 */
  	ret = VM_FAULT_RETRY;
  	if (vmf->flags & FAULT_FLAG_RETRY_NOWAIT)
  		goto out;
  
  	/* take the reference before dropping the mmap_sem */
  	userfaultfd_ctx_get(ctx);
  
  	init_waitqueue_func_entry(&uwq.wq, userfaultfd_wake_function);
  	uwq.wq.private = current;
  	uwq.msg = userfault_msg(vmf->address, vmf->flags, reason,
  			ctx->features);
  	uwq.ctx = ctx;
  	uwq.waken = false;
  
  	return_to_userland =
  		(vmf->flags & (FAULT_FLAG_USER|FAULT_FLAG_KILLABLE)) ==
  		(FAULT_FLAG_USER|FAULT_FLAG_KILLABLE);
  	blocking_state = return_to_userland ? TASK_INTERRUPTIBLE :
  			 TASK_KILLABLE;
  
  	spin_lock(&ctx->fault_pending_wqh.lock);
  	/*
  	 * After the __add_wait_queue the uwq is visible to userland
  	 * through poll/read().
  	 */
  	__add_wait_queue(&ctx->fault_pending_wqh, &uwq.wq);
  	/*
  	 * The smp_mb() after __set_current_state prevents the reads
  	 * following the spin_unlock to happen before the list_add in
  	 * __add_wait_queue.
  	 */
  	set_current_state(blocking_state);
  	spin_unlock(&ctx->fault_pending_wqh.lock);
  
  	if (!is_vm_hugetlb_page(vmf->vma))
  		must_wait = userfaultfd_must_wait(ctx, vmf->address, vmf->flags,
  						  reason);
  	else
  		must_wait = userfaultfd_huge_must_wait(ctx, vmf->vma,
  						       vmf->address,
  						       vmf->flags, reason);
  	up_read(&mm->mmap_sem);
  
  	if (likely(must_wait && !READ_ONCE(ctx->released) &&
  		   (return_to_userland ? !signal_pending(current) :
  		    !fatal_signal_pending(current)))) {
  		wake_up_poll(&ctx->fd_wqh, EPOLLIN);
  		schedule();
  		ret |= VM_FAULT_MAJOR;
  
  		/*
  		 * False wakeups can orginate even from rwsem before
  		 * up_read() however userfaults will wait either for a
  		 * targeted wakeup on the specific uwq waitqueue from
  		 * wake_userfault() or for signals or for uffd
  		 * release.
  		 */
  		while (!READ_ONCE(uwq.waken)) {
  			/*
  			 * This needs the full smp_store_mb()
  			 * guarantee as the state write must be
  			 * visible to other CPUs before reading
  			 * uwq.waken from other CPUs.
  			 */
  			set_current_state(blocking_state);
  			if (READ_ONCE(uwq.waken) ||
  			    READ_ONCE(ctx->released) ||
  			    (return_to_userland ? signal_pending(current) :
  			     fatal_signal_pending(current)))
  				break;
  			schedule();
  		}
  	}
  
  	__set_current_state(TASK_RUNNING);
  
  	if (return_to_userland) {
  		if (signal_pending(current) &&
  		    !fatal_signal_pending(current)) {
  			/*
  			 * If we got a SIGSTOP or SIGCONT and this is
  			 * a normal userland page fault, just let
  			 * userland return so the signal will be
  			 * handled and gdb debugging works.  The page
  			 * fault code immediately after we return from
  			 * this function is going to release the
  			 * mmap_sem and it's not depending on it
  			 * (unlike gup would if we were not to return
  			 * VM_FAULT_RETRY).
  			 *
  			 * If a fatal signal is pending we still take
  			 * the streamlined VM_FAULT_RETRY failure path
  			 * and there's no need to retake the mmap_sem
  			 * in such case.
  			 */
  			down_read(&mm->mmap_sem);
  			ret = VM_FAULT_NOPAGE;
  		}
  	}
  
  	/*
  	 * Here we race with the list_del; list_add in
  	 * userfaultfd_ctx_read(), however because we don't ever run
  	 * list_del_init() to refile across the two lists, the prev
  	 * and next pointers will never point to self. list_add also
  	 * would never let any of the two pointers to point to
  	 * self. So list_empty_careful won't risk to see both pointers
  	 * pointing to self at any time during the list refile. The
  	 * only case where list_del_init() is called is the full
  	 * removal in the wake function and there we don't re-list_add
  	 * and it's fine not to block on the spinlock. The uwq on this
  	 * kernel stack can be released after the list_del_init.
  	 */
  	if (!list_empty_careful(&uwq.wq.entry)) {
  		spin_lock(&ctx->fault_pending_wqh.lock);
  		/*
  		 * No need of list_del_init(), the uwq on the stack
  		 * will be freed shortly anyway.
  		 */
  		list_del(&uwq.wq.entry);
  		spin_unlock(&ctx->fault_pending_wqh.lock);
  	}
  
  	/*
  	 * ctx may go away after this if the userfault pseudo fd is
  	 * already released.
  	 */
  	userfaultfd_ctx_put(ctx);
  
  out:
  	return ret;
  }
  
  static void userfaultfd_event_wait_completion(struct userfaultfd_ctx *ctx,
  					      struct userfaultfd_wait_queue *ewq)
  {
  	struct userfaultfd_ctx *release_new_ctx;
  
  	if (WARN_ON_ONCE(current->flags & PF_EXITING))
  		goto out;
  
  	ewq->ctx = ctx;
  	init_waitqueue_entry(&ewq->wq, current);
  	release_new_ctx = NULL;
  
  	spin_lock(&ctx->event_wqh.lock);
  	/*
  	 * After the __add_wait_queue the uwq is visible to userland
  	 * through poll/read().
  	 */
  	__add_wait_queue(&ctx->event_wqh, &ewq->wq);
  	for (;;) {
  		set_current_state(TASK_KILLABLE);
  		if (ewq->msg.event == 0)
  			break;
  		if (READ_ONCE(ctx->released) ||
  		    fatal_signal_pending(current)) {
  			/*
  			 * &ewq->wq may be queued in fork_event, but
  			 * __remove_wait_queue ignores the head
  			 * parameter. It would be a problem if it
  			 * didn't.
  			 */
  			__remove_wait_queue(&ctx->event_wqh, &ewq->wq);
  			if (ewq->msg.event == UFFD_EVENT_FORK) {
  				struct userfaultfd_ctx *new;
  
  				new = (struct userfaultfd_ctx *)
  					(unsigned long)
  					ewq->msg.arg.reserved.reserved1;
  				release_new_ctx = new;
  			}
  			break;
  		}
  
  		spin_unlock(&ctx->event_wqh.lock);
  
  		wake_up_poll(&ctx->fd_wqh, EPOLLIN);
  		schedule();
  
  		spin_lock(&ctx->event_wqh.lock);
  	}
  	__set_current_state(TASK_RUNNING);
  	spin_unlock(&ctx->event_wqh.lock);
  
  	if (release_new_ctx) {
  		struct vm_area_struct *vma;
  		struct mm_struct *mm = release_new_ctx->mm;
  
  		/* the various vma->vm_userfaultfd_ctx still points to it */
  		down_write(&mm->mmap_sem);
  		for (vma = mm->mmap; vma; vma = vma->vm_next)
  			if (vma->vm_userfaultfd_ctx.ctx == release_new_ctx) {
  				vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
  				vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
  			}
  		up_write(&mm->mmap_sem);
  
  		userfaultfd_ctx_put(release_new_ctx);
  	}
  
  	/*
  	 * ctx may go away after this if the userfault pseudo fd is
  	 * already released.
  	 */
  out:
  	WRITE_ONCE(ctx->mmap_changing, false);
  	userfaultfd_ctx_put(ctx);
  }
  
  static void userfaultfd_event_complete(struct userfaultfd_ctx *ctx,
  				       struct userfaultfd_wait_queue *ewq)
  {
  	ewq->msg.event = 0;
  	wake_up_locked(&ctx->event_wqh);
  	__remove_wait_queue(&ctx->event_wqh, &ewq->wq);
  }
  
  int dup_userfaultfd(struct vm_area_struct *vma, struct list_head *fcs)
  {
  	struct userfaultfd_ctx *ctx = NULL, *octx;
  	struct userfaultfd_fork_ctx *fctx;
  
  	octx = vma->vm_userfaultfd_ctx.ctx;
  	if (!octx || !(octx->features & UFFD_FEATURE_EVENT_FORK)) {
  		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
  		vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
  		return 0;
  	}
  
  	list_for_each_entry(fctx, fcs, list)
  		if (fctx->orig == octx) {
  			ctx = fctx->new;
  			break;
  		}
  
  	if (!ctx) {
  		fctx = kmalloc(sizeof(*fctx), GFP_KERNEL);
  		if (!fctx)
  			return -ENOMEM;
  
  		ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
  		if (!ctx) {
  			kfree(fctx);
  			return -ENOMEM;
  		}
  
  		atomic_set(&ctx->refcount, 1);
  		ctx->flags = octx->flags;
  		ctx->state = UFFD_STATE_RUNNING;
  		ctx->features = octx->features;
  		ctx->released = false;
  		ctx->mmap_changing = false;
  		ctx->mm = vma->vm_mm;
  		mmgrab(ctx->mm);
  
  		userfaultfd_ctx_get(octx);
  		WRITE_ONCE(octx->mmap_changing, true);
  		fctx->orig = octx;
  		fctx->new = ctx;
  		list_add_tail(&fctx->list, fcs);
  	}
  
  	vma->vm_userfaultfd_ctx.ctx = ctx;
  	return 0;
  }
  
  static void dup_fctx(struct userfaultfd_fork_ctx *fctx)
  {
  	struct userfaultfd_ctx *ctx = fctx->orig;
  	struct userfaultfd_wait_queue ewq;
  
  	msg_init(&ewq.msg);
  
  	ewq.msg.event = UFFD_EVENT_FORK;
  	ewq.msg.arg.reserved.reserved1 = (unsigned long)fctx->new;
  
  	userfaultfd_event_wait_completion(ctx, &ewq);
  }
  
  void dup_userfaultfd_complete(struct list_head *fcs)
  {
  	struct userfaultfd_fork_ctx *fctx, *n;
  
  	list_for_each_entry_safe(fctx, n, fcs, list) {
  		dup_fctx(fctx);
  		list_del(&fctx->list);
  		kfree(fctx);
  	}
  }
  
  void mremap_userfaultfd_prep(struct vm_area_struct *vma,
  			     struct vm_userfaultfd_ctx *vm_ctx)
  {
  	struct userfaultfd_ctx *ctx;
  
  	ctx = vma->vm_userfaultfd_ctx.ctx;
  	if (ctx && (ctx->features & UFFD_FEATURE_EVENT_REMAP)) {
  		vm_ctx->ctx = ctx;
  		userfaultfd_ctx_get(ctx);
  		WRITE_ONCE(ctx->mmap_changing, true);
  	}
  }
  
  void mremap_userfaultfd_complete(struct vm_userfaultfd_ctx *vm_ctx,
  				 unsigned long from, unsigned long to,
  				 unsigned long len)
  {
  	struct userfaultfd_ctx *ctx = vm_ctx->ctx;
  	struct userfaultfd_wait_queue ewq;
  
  	if (!ctx)
  		return;
  
  	if (to & ~PAGE_MASK) {
  		userfaultfd_ctx_put(ctx);
  		return;
  	}
  
  	msg_init(&ewq.msg);
  
  	ewq.msg.event = UFFD_EVENT_REMAP;
  	ewq.msg.arg.remap.from = from;
  	ewq.msg.arg.remap.to = to;
  	ewq.msg.arg.remap.len = len;
  
  	userfaultfd_event_wait_completion(ctx, &ewq);
  }
  
  bool userfaultfd_remove(struct vm_area_struct *vma,
  			unsigned long start, unsigned long end)
  {
  	struct mm_struct *mm = vma->vm_mm;
  	struct userfaultfd_ctx *ctx;
  	struct userfaultfd_wait_queue ewq;
  
  	ctx = vma->vm_userfaultfd_ctx.ctx;
  	if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_REMOVE))
  		return true;
  
  	userfaultfd_ctx_get(ctx);
  	WRITE_ONCE(ctx->mmap_changing, true);
  	up_read(&mm->mmap_sem);
  
  	msg_init(&ewq.msg);
  
  	ewq.msg.event = UFFD_EVENT_REMOVE;
  	ewq.msg.arg.remove.start = start;
  	ewq.msg.arg.remove.end = end;
  
  	userfaultfd_event_wait_completion(ctx, &ewq);
  
  	return false;
  }
  
  static bool has_unmap_ctx(struct userfaultfd_ctx *ctx, struct list_head *unmaps,
  			  unsigned long start, unsigned long end)
  {
  	struct userfaultfd_unmap_ctx *unmap_ctx;
  
  	list_for_each_entry(unmap_ctx, unmaps, list)
  		if (unmap_ctx->ctx == ctx && unmap_ctx->start == start &&
  		    unmap_ctx->end == end)
  			return true;
  
  	return false;
  }
  
  int userfaultfd_unmap_prep(struct vm_area_struct *vma,
  			   unsigned long start, unsigned long end,
  			   struct list_head *unmaps)
  {
  	for ( ; vma && vma->vm_start < end; vma = vma->vm_next) {
  		struct userfaultfd_unmap_ctx *unmap_ctx;
  		struct userfaultfd_ctx *ctx = vma->vm_userfaultfd_ctx.ctx;
  
  		if (!ctx || !(ctx->features & UFFD_FEATURE_EVENT_UNMAP) ||
  		    has_unmap_ctx(ctx, unmaps, start, end))
  			continue;
  
  		unmap_ctx = kzalloc(sizeof(*unmap_ctx), GFP_KERNEL);
  		if (!unmap_ctx)
  			return -ENOMEM;
  
  		userfaultfd_ctx_get(ctx);
  		WRITE_ONCE(ctx->mmap_changing, true);
  		unmap_ctx->ctx = ctx;
  		unmap_ctx->start = start;
  		unmap_ctx->end = end;
  		list_add_tail(&unmap_ctx->list, unmaps);
  	}
  
  	return 0;
  }
  
  void userfaultfd_unmap_complete(struct mm_struct *mm, struct list_head *uf)
  {
  	struct userfaultfd_unmap_ctx *ctx, *n;
  	struct userfaultfd_wait_queue ewq;
  
  	list_for_each_entry_safe(ctx, n, uf, list) {
  		msg_init(&ewq.msg);
  
  		ewq.msg.event = UFFD_EVENT_UNMAP;
  		ewq.msg.arg.remove.start = ctx->start;
  		ewq.msg.arg.remove.end = ctx->end;
  
  		userfaultfd_event_wait_completion(ctx->ctx, &ewq);
  
  		list_del(&ctx->list);
  		kfree(ctx);
  	}
  }
  
  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;
  
  	WRITE_ONCE(ctx->released, true);
  
  	if (!mmget_not_zero(mm))
  		goto wakeup;
  
  	/*
  	 * 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);
  	mmput(mm);
  wakeup:
  	/*
  	 * 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_pending_wqh.lock);
  	__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL, &range);
  	__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, &range);
  	spin_unlock(&ctx->fault_pending_wqh.lock);
  
  	/* Flush pending events that may still wait on event_wqh */
  	wake_up_all(&ctx->event_wqh);
  
  	wake_up_poll(&ctx->fd_wqh, EPOLLHUP);
  	userfaultfd_ctx_put(ctx);
  	return 0;
  }
  
  /* fault_pending_wqh.lock must be hold by the caller */
  static inline struct userfaultfd_wait_queue *find_userfault_in(
  		wait_queue_head_t *wqh)
  {
  	wait_queue_entry_t *wq;
  	struct userfaultfd_wait_queue *uwq;
  
  	VM_BUG_ON(!spin_is_locked(&wqh->lock));
  
  	uwq = NULL;
  	if (!waitqueue_active(wqh))
  		goto out;
  	/* walk in reverse to provide FIFO behavior to read userfaults */
  	wq = list_last_entry(&wqh->head, typeof(*wq), entry);
  	uwq = container_of(wq, struct userfaultfd_wait_queue, wq);
  out:
  	return uwq;
  }
  
  static inline struct userfaultfd_wait_queue *find_userfault(
  		struct userfaultfd_ctx *ctx)
  {
  	return find_userfault_in(&ctx->fault_pending_wqh);
  }
  
  static inline struct userfaultfd_wait_queue *find_userfault_evt(
  		struct userfaultfd_ctx *ctx)
  {
  	return find_userfault_in(&ctx->event_wqh);
  }
  
  static __poll_t userfaultfd_poll(struct file *file, poll_table *wait)
  {
  	struct userfaultfd_ctx *ctx = file->private_data;
  	__poll_t ret;
  
  	poll_wait(file, &ctx->fd_wqh, wait);
  
  	switch (ctx->state) {
  	case UFFD_STATE_WAIT_API:
  		return EPOLLERR;
  	case UFFD_STATE_RUNNING:
  		/*
  		 * poll() never guarantees that read won't block.
  		 * userfaults can be waken before they're read().
  		 */
  		if (unlikely(!(file->f_flags & O_NONBLOCK)))
  			return EPOLLERR;
  		/*
  		 * lockless access to see if there are pending faults
  		 * __pollwait last action is the add_wait_queue but
  		 * the spin_unlock would allow the waitqueue_active to
  		 * pass above the actual list_add inside
  		 * add_wait_queue critical section. So use a full
  		 * memory barrier to serialize the list_add write of
  		 * add_wait_queue() with the waitqueue_active read
  		 * below.
  		 */
  		ret = 0;
  		smp_mb();
  		if (waitqueue_active(&ctx->fault_pending_wqh))
  			ret = EPOLLIN;
  		else if (waitqueue_active(&ctx->event_wqh))
  			ret = EPOLLIN;
  
  		return ret;
  	default:
  		WARN_ON_ONCE(1);
  		return EPOLLERR;
  	}
  }
  
  static const struct file_operations userfaultfd_fops;
  
  static int resolve_userfault_fork(struct userfaultfd_ctx *ctx,
  				  struct userfaultfd_ctx *new,
  				  struct uffd_msg *msg)
  {
  	int fd;
  
  	fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, new,
  			      O_RDWR | (new->flags & UFFD_SHARED_FCNTL_FLAGS));
  	if (fd < 0)
  		return fd;
  
  	msg->arg.reserved.reserved1 = 0;
  	msg->arg.fork.ufd = fd;
  	return 0;
  }
  
  static ssize_t userfaultfd_ctx_read(struct userfaultfd_ctx *ctx, int no_wait,
  				    struct uffd_msg *msg)
  {
  	ssize_t ret;
  	DECLARE_WAITQUEUE(wait, current);
  	struct userfaultfd_wait_queue *uwq;
  	/*
  	 * Handling fork event requires sleeping operations, so
  	 * we drop the event_wqh lock, then do these ops, then
  	 * lock it back and wake up the waiter. While the lock is
  	 * dropped the ewq may go away so we keep track of it
  	 * carefully.
  	 */
  	LIST_HEAD(fork_event);
  	struct userfaultfd_ctx *fork_nctx = NULL;
  
  	/* always take the fd_wqh lock before the fault_pending_wqh lock */
  	spin_lock_irq(&ctx->fd_wqh.lock);
  	__add_wait_queue(&ctx->fd_wqh, &wait);
  	for (;;) {
  		set_current_state(TASK_INTERRUPTIBLE);
  		spin_lock(&ctx->fault_pending_wqh.lock);
  		uwq = find_userfault(ctx);
  		if (uwq) {
  			/*
  			 * Use a seqcount to repeat the lockless check
  			 * in wake_userfault() to avoid missing
  			 * wakeups because during the refile both
  			 * waitqueue could become empty if this is the
  			 * only userfault.
  			 */
  			write_seqcount_begin(&ctx->refile_seq);
  
  			/*
  			 * The fault_pending_wqh.lock prevents the uwq
  			 * to disappear from under us.
  			 *
  			 * Refile this userfault from
  			 * fault_pending_wqh to fault_wqh, it's not
  			 * pending anymore after we read it.
  			 *
  			 * Use list_del() by hand (as
  			 * userfaultfd_wake_function also uses
  			 * list_del_init() by hand) to be sure nobody
  			 * changes __remove_wait_queue() to use
  			 * list_del_init() in turn breaking the
  			 * !list_empty_careful() check in
  			 * handle_userfault(). The uwq->wq.head list
  			 * must never be empty at any time during the
  			 * refile, or the waitqueue could disappear
  			 * from under us. The "wait_queue_head_t"
  			 * parameter of __remove_wait_queue() is unused
  			 * anyway.
  			 */
  			list_del(&uwq->wq.entry);
  			add_wait_queue(&ctx->fault_wqh, &uwq->wq);
  
  			write_seqcount_end(&ctx->refile_seq);
  
  			/* careful to always initialize msg if ret == 0 */
  			*msg = uwq->msg;
  			spin_unlock(&ctx->fault_pending_wqh.lock);
  			ret = 0;
  			break;
  		}
  		spin_unlock(&ctx->fault_pending_wqh.lock);
  
  		spin_lock(&ctx->event_wqh.lock);
  		uwq = find_userfault_evt(ctx);
  		if (uwq) {
  			*msg = uwq->msg;
  
  			if (uwq->msg.event == UFFD_EVENT_FORK) {
  				fork_nctx = (struct userfaultfd_ctx *)
  					(unsigned long)
  					uwq->msg.arg.reserved.reserved1;
  				list_move(&uwq->wq.entry, &fork_event);
  				/*
  				 * fork_nctx can be freed as soon as
  				 * we drop the lock, unless we take a
  				 * reference on it.
  				 */
  				userfaultfd_ctx_get(fork_nctx);
  				spin_unlock(&ctx->event_wqh.lock);
  				ret = 0;
  				break;
  			}
  
  			userfaultfd_event_complete(ctx, uwq);
  			spin_unlock(&ctx->event_wqh.lock);
  			ret = 0;
  			break;
  		}
  		spin_unlock(&ctx->event_wqh.lock);
  
  		if (signal_pending(current)) {
  			ret = -ERESTARTSYS;
  			break;
  		}
  		if (no_wait) {
  			ret = -EAGAIN;
  			break;
  		}
  		spin_unlock_irq(&ctx->fd_wqh.lock);
  		schedule();
  		spin_lock_irq(&ctx->fd_wqh.lock);
  	}
  	__remove_wait_queue(&ctx->fd_wqh, &wait);
  	__set_current_state(TASK_RUNNING);
  	spin_unlock_irq(&ctx->fd_wqh.lock);
  
  	if (!ret && msg->event == UFFD_EVENT_FORK) {
  		ret = resolve_userfault_fork(ctx, fork_nctx, msg);
  		spin_lock(&ctx->event_wqh.lock);
  		if (!list_empty(&fork_event)) {
  			/*
  			 * The fork thread didn't abort, so we can
  			 * drop the temporary refcount.
  			 */
  			userfaultfd_ctx_put(fork_nctx);
  
  			uwq = list_first_entry(&fork_event,
  					       typeof(*uwq),
  					       wq.entry);
  			/*
  			 * If fork_event list wasn't empty and in turn
  			 * the event wasn't already released by fork
  			 * (the event is allocated on fork kernel
  			 * stack), put the event back to its place in
  			 * the event_wq. fork_event head will be freed
  			 * as soon as we return so the event cannot
  			 * stay queued there no matter the current
  			 * "ret" value.
  			 */
  			list_del(&uwq->wq.entry);
  			__add_wait_queue(&ctx->event_wqh, &uwq->wq);
  
  			/*
  			 * Leave the event in the waitqueue and report
  			 * error to userland if we failed to resolve
  			 * the userfault fork.
  			 */
  			if (likely(!ret))
  				userfaultfd_event_complete(ctx, uwq);
  		} else {
  			/*
  			 * Here the fork thread aborted and the
  			 * refcount from the fork thread on fork_nctx
  			 * has already been released. We still hold
  			 * the reference we took before releasing the
  			 * lock above. If resolve_userfault_fork
  			 * failed we've to drop it because the
  			 * fork_nctx has to be freed in such case. If
  			 * it succeeded we'll hold it because the new
  			 * uffd references it.
  			 */
  			if (ret)
  				userfaultfd_ctx_put(fork_nctx);
  		}
  		spin_unlock(&ctx->event_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;
  	struct uffd_msg msg;
  	int no_wait = file->f_flags & O_NONBLOCK;
  
  	if (ctx->state == UFFD_STATE_WAIT_API)
  		return -EINVAL;
  
  	for (;;) {
  		if (count < sizeof(msg))
  			return ret ? ret : -EINVAL;
  		_ret = userfaultfd_ctx_read(ctx, no_wait, &msg);
  		if (_ret < 0)
  			return ret ? ret : _ret;
  		if (copy_to_user((__u64 __user *) buf, &msg, sizeof(msg)))
  			return ret ? ret : -EFAULT;
  		ret += sizeof(msg);
  		buf += sizeof(msg);
  		count -= sizeof(msg);
  		/*
  		 * 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)
  {
  	spin_lock(&ctx->fault_pending_wqh.lock);
  	/* wake all in the range and autoremove */
  	if (waitqueue_active(&ctx->fault_pending_wqh))
  		__wake_up_locked_key(&ctx->fault_pending_wqh, TASK_NORMAL,
  				     range);
  	if (waitqueue_active(&ctx->fault_wqh))
  		__wake_up(&ctx->fault_wqh, TASK_NORMAL, 1, range);
  	spin_unlock(&ctx->fault_pending_wqh.lock);
  }
  
  static __always_inline void wake_userfault(struct userfaultfd_ctx *ctx,
  					   struct userfaultfd_wake_range *range)
  {
  	unsigned seq;
  	bool need_wakeup;
  
  	/*
  	 * 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.
  	 */
  	do {
  		seq = read_seqcount_begin(&ctx->refile_seq);
  		need_wakeup = waitqueue_active(&ctx->fault_pending_wqh) ||
  			waitqueue_active(&ctx->fault_wqh);
  		cond_resched();
  	} while (read_seqcount_retry(&ctx->refile_seq, seq));
  	if (need_wakeup)
  		__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 inline bool vma_can_userfault(struct vm_area_struct *vma)
  {
  	return vma_is_anonymous(vma) || is_vm_hugetlb_page(vma) ||
  		vma_is_shmem(vma);
  }
  
  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;
  	bool basic_ioctls;
  	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;
  
  	ret = -ENOMEM;
  	if (!mmget_not_zero(mm))
  		goto out;
  
  	down_write(&mm->mmap_sem);
  	vma = find_vma_prev(mm, start, &prev);
  	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;
  
  	/*
  	 * If the first vma contains huge pages, make sure start address
  	 * is aligned to huge page size.
  	 */
  	if (is_vm_hugetlb_page(vma)) {
  		unsigned long vma_hpagesize = vma_kernel_pagesize(vma);
  
  		if (start & (vma_hpagesize - 1))
  			goto out_unlock;
  	}
  
  	/*
  	 * Search for not compatible vmas.
  	 */
  	found = false;
  	basic_ioctls = 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 (!vma_can_userfault(cur))
  			goto out_unlock;
  		/*
  		 * If this vma contains ending address, and huge pages
  		 * check alignment.
  		 */
  		if (is_vm_hugetlb_page(cur) && end <= cur->vm_end &&
  		    end > cur->vm_start) {
  			unsigned long vma_hpagesize = vma_kernel_pagesize(cur);
  
  			ret = -EINVAL;
  
  			if (end & (vma_hpagesize - 1))
  				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;
  
  		/*
  		 * Note vmas containing huge pages
  		 */
  		if (is_vm_hugetlb_page(cur))
  			basic_ioctls = true;
  
  		found = true;
  	}
  	BUG_ON(!found);
  
  	if (vma->vm_start < start)
  		prev = vma;
  
  	ret = 0;
  	do {
  		cond_resched();
  
  		BUG_ON(!vma_can_userfault(vma));
  		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);
  	mmput(mm);
  	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(basic_ioctls ? UFFD_API_RANGE_IOCTLS_BASIC :
  			     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;
  
  	ret = -ENOMEM;
  	if (!mmget_not_zero(mm))
  		goto out;
  
  	down_write(&mm->mmap_sem);
  	vma = find_vma_prev(mm, start, &prev);
  	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;
  
  	/*
  	 * If the first vma contains huge pages, make sure start address
  	 * is aligned to huge page size.
  	 */
  	if (is_vm_hugetlb_page(vma)) {
  		unsigned long vma_hpagesize = vma_kernel_pagesize(vma);
  
  		if (start & (vma_hpagesize - 1))
  			goto out_unlock;
  	}
  
  	/*
  	 * Search for not compatible 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 (!vma_can_userfault(cur))
  			goto out_unlock;
  
  		found = true;
  	}
  	BUG_ON(!found);
  
  	if (vma->vm_start < start)
  		prev = vma;
  
  	ret = 0;
  	do {
  		cond_resched();
  
  		BUG_ON(!vma_can_userfault(vma));
  
  		/*
  		 * 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);
  
  		if (userfaultfd_missing(vma)) {
  			/*
  			 * Wake any concurrent pending userfault while
  			 * we unregister, so they will not hang
  			 * permanently and it avoids userland to call
  			 * UFFDIO_WAKE explicitly.
  			 */
  			struct userfaultfd_wake_range range;
  			range.start = start;
  			range.len = vma_end - start;
  			wake_userfault(vma->vm_userfaultfd_ctx.ctx, &range);
  		}
  
  		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);
  	mmput(mm);
  out:
  	return ret;
  }
  
  /*
   * userfaultfd_wake may be used in combination with the
   * UFFDIO_*_MODE_DONTWAKE to wakeup userfaults in batches.
   */
  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;
  }
  
  static int userfaultfd_copy(struct userfaultfd_ctx *ctx,
  			    unsigned long arg)
  {
  	__s64 ret;
  	struct uffdio_copy uffdio_copy;
  	struct uffdio_copy __user *user_uffdio_copy;
  	struct userfaultfd_wake_range range;
  
  	user_uffdio_copy = (struct uffdio_copy __user *) arg;
  
  	ret = -EAGAIN;
  	if (READ_ONCE(ctx->mmap_changing))
  		goto out;
  
  	ret = -EFAULT;
  	if (copy_from_user(&uffdio_copy, user_uffdio_copy,
  			   /* don't copy "copy" last field */
  			   sizeof(uffdio_copy)-sizeof(__s64)))
  		goto out;
  
  	ret = validate_range(ctx->mm, uffdio_copy.dst, uffdio_copy.len);
  	if (ret)
  		goto out;
  	/*
  	 * double check for wraparound just in case. copy_from_user()
  	 * will later check uffdio_copy.src + uffdio_copy.len to fit
  	 * in the userland range.
  	 */
  	ret = -EINVAL;
  	if (uffdio_copy.src + uffdio_copy.len <= uffdio_copy.src)
  		goto out;
  	if (uffdio_copy.mode & ~UFFDIO_COPY_MODE_DONTWAKE)
  		goto out;
  	if (mmget_not_zero(ctx->mm)) {
  		ret = mcopy_atomic(ctx->mm, uffdio_copy.dst, uffdio_copy.src,
  				   uffdio_copy.len, &ctx->mmap_changing);
  		mmput(ctx->mm);
  	} else {
  		return -ESRCH;
  	}
  	if (unlikely(put_user(ret, &user_uffdio_copy->copy)))
  		return -EFAULT;
  	if (ret < 0)
  		goto out;
  	BUG_ON(!ret);
  	/* len == 0 would wake all */
  	range.len = ret;
  	if (!(uffdio_copy.mode & UFFDIO_COPY_MODE_DONTWAKE)) {
  		range.start = uffdio_copy.dst;
  		wake_userfault(ctx, &range);
  	}
  	ret = range.len == uffdio_copy.len ? 0 : -EAGAIN;
  out:
  	return ret;
  }
  
  static int userfaultfd_zeropage(struct userfaultfd_ctx *ctx,
  				unsigned long arg)
  {
  	__s64 ret;
  	struct uffdio_zeropage uffdio_zeropage;
  	struct uffdio_zeropage __user *user_uffdio_zeropage;
  	struct userfaultfd_wake_range range;
  
  	user_uffdio_zeropage = (struct uffdio_zeropage __user *) arg;
  
  	ret = -EAGAIN;
  	if (READ_ONCE(ctx->mmap_changing))
  		goto out;
  
  	ret = -EFAULT;
  	if (copy_from_user(&uffdio_zeropage, user_uffdio_zeropage,
  			   /* don't copy "zeropage" last field */
  			   sizeof(uffdio_zeropage)-sizeof(__s64)))
  		goto out;
  
  	ret = validate_range(ctx->mm, uffdio_zeropage.range.start,
  			     uffdio_zeropage.range.len);
  	if (ret)
  		goto out;
  	ret = -EINVAL;
  	if (uffdio_zeropage.mode & ~UFFDIO_ZEROPAGE_MODE_DONTWAKE)
  		goto out;
  
  	if (mmget_not_zero(ctx->mm)) {
  		ret = mfill_zeropage(ctx->mm, uffdio_zeropage.range.start,
  				     uffdio_zeropage.range.len,
  				     &ctx->mmap_changing);
  		mmput(ctx->mm);
  	} else {
  		return -ESRCH;
  	}
  	if (unlikely(put_user(ret, &user_uffdio_zeropage->zeropage)))
  		return -EFAULT;
  	if (ret < 0)
  		goto out;
  	/* len == 0 would wake all */
  	BUG_ON(!ret);
  	range.len = ret;
  	if (!(uffdio_zeropage.mode & UFFDIO_ZEROPAGE_MODE_DONTWAKE)) {
  		range.start = uffdio_zeropage.range.start;
  		wake_userfault(ctx, &range);
  	}
  	ret = range.len == uffdio_zeropage.range.len ? 0 : -EAGAIN;
  out:
  	return ret;
  }
  
  static inline unsigned int uffd_ctx_features(__u64 user_features)
  {
  	/*
  	 * For the current set of features the bits just coincide
  	 */
  	return (unsigned int)user_features;
  }
  
  /*
   * 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;
  	__u64 features;
  
  	ret = -EINVAL;
  	if (ctx->state != UFFD_STATE_WAIT_API)
  		goto out;
  	ret = -EFAULT;
  	if (copy_from_user(&uffdio_api, buf, sizeof(uffdio_api)))
  		goto out;
  	features = uffdio_api.features;
  	if (uffdio_api.api != UFFD_API || (features & ~UFFD_API_FEATURES)) {
  		memset(&uffdio_api, 0, sizeof(uffdio_api));
  		if (copy_to_user(buf, &uffdio_api, sizeof(uffdio_api)))
  			goto out;
  		ret = -EINVAL;
  		goto out;
  	}
  	/* report all available features and ioctls to userland */
  	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;
  	/* only enable the requested features for this uffd context */
  	ctx->features = uffd_ctx_features(features);
  	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;
  
  	if (cmd != UFFDIO_API && ctx->state == UFFD_STATE_WAIT_API)
  		return -EINVAL;
  
  	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;
  	case UFFDIO_COPY:
  		ret = userfaultfd_copy(ctx, arg);
  		break;
  	case UFFDIO_ZEROPAGE:
  		ret = userfaultfd_zeropage(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_entry_t *wq;
  	unsigned long pending = 0, total = 0;
  
  	spin_lock(&ctx->fault_pending_wqh.lock);
  	list_for_each_entry(wq, &ctx->fault_pending_wqh.head, entry) {
  		pending++;
  		total++;
  	}
  	list_for_each_entry(wq, &ctx->fault_wqh.head, entry) {
  		total++;
  	}
  	spin_unlock(&ctx->fault_pending_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, ctx->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,
  };
  
  static void init_once_userfaultfd_ctx(void *mem)
  {
  	struct userfaultfd_ctx *ctx = (struct userfaultfd_ctx *) mem;
  
  	init_waitqueue_head(&ctx->fault_pending_wqh);
  	init_waitqueue_head(&ctx->fault_wqh);
  	init_waitqueue_head(&ctx->event_wqh);
  	init_waitqueue_head(&ctx->fd_wqh);
  	seqcount_init(&ctx->refile_seq);
  }
  
  SYSCALL_DEFINE1(userfaultfd, int, flags)
  {
  	struct userfaultfd_ctx *ctx;
  	int fd;
  
  	BUG_ON(!current->mm);
  
  	/* Check the UFFD_* constants for consistency.  */
  	BUILD_BUG_ON(UFFD_CLOEXEC != O_CLOEXEC);
  	BUILD_BUG_ON(UFFD_NONBLOCK != O_NONBLOCK);
  
  	if (flags & ~UFFD_SHARED_FCNTL_FLAGS)
  		return -EINVAL;
  
  	ctx = kmem_cache_alloc(userfaultfd_ctx_cachep, GFP_KERNEL);
  	if (!ctx)
  		return -ENOMEM;
  
  	atomic_set(&ctx->refcount, 1);
  	ctx->flags = flags;
  	ctx->features = 0;
  	ctx->state = UFFD_STATE_WAIT_API;
  	ctx->released = false;
  	ctx->mmap_changing = false;
  	ctx->mm = current->mm;
  	/* prevent the mm struct to be freed */
  	mmgrab(ctx->mm);
  
  	fd = anon_inode_getfd("[userfaultfd]", &userfaultfd_fops, ctx,
  			      O_RDWR | (flags & UFFD_SHARED_FCNTL_FLAGS));
  	if (fd < 0) {
  		mmdrop(ctx->mm);
  		kmem_cache_free(userfaultfd_ctx_cachep, ctx);
  	}
  	return fd;
  }
  
  static int __init userfaultfd_init(void)
  {
  	userfaultfd_ctx_cachep = kmem_cache_create("userfaultfd_ctx_cache",
  						sizeof(struct userfaultfd_ctx),
  						0,
  						SLAB_HWCACHE_ALIGN|SLAB_PANIC,
  						init_once_userfaultfd_ctx);
  	return 0;
  }
  __initcall(userfaultfd_init);
  