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userfaultfd.c

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  • userfaultfd.c 51.16 KiB
    // SPDX-License-Identifier: GPL-2.0-only
    /*
     *  fs/userfaultfd.c
     *
     *  Copyright (C) 2007  Davide Libenzi <davidel@xmailserver.org>
     *  Copyright (C) 2008-2009 Red Hat, Inc.
     *  Copyright (C) 2015  Red Hat, Inc.
     *
     *  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>
    
    int sysctl_unprivileged_userfaultfd __read_mostly = 1;
    
    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.
     *
     * Locking order:
     *	fd_wqh.lock
     *		fault_pending_wqh.lock
     *			fault_wqh.lock
     *		event_wqh.lock
     *
     * To avoid deadlocks, IRQs must be disabled when taking any of the above locks,
     * since fd_wqh.lock is taken by aio_poll() while it's holding a lock that's
     * also taken in IRQ context.
     */
    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 */
    	refcount_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)
    {
    	refcount_inc(&ctx->refcount);
    }
    
    /**
     * 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 (refcount_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_irq(&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_irq(&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_irq(&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_irq(&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_irq(&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_irq(&ctx->event_wqh.lock);
    
    		wake_up_poll(&ctx->fd_wqh, EPOLLIN);
    		schedule();
    
    		spin_lock_irq(&ctx->event_wqh.lock);
    	}
    	__set_current_state(TASK_RUNNING);
    	spin_unlock_irq(&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);
    		/* no task can run (and in turn coredump) yet */
    		VM_WARN_ON(!mmget_still_valid(mm));
    		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;
    		}
    
    		refcount_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)
    		return;
    
    	if (ctx->features & UFFD_FEATURE_EVENT_REMAP) {
    		vm_ctx->ctx = ctx;
    		userfaultfd_ctx_get(ctx);
    		WRITE_ONCE(ctx->mmap_changing, true);
    	} else {
    		/* Drop uffd context if remap feature not enabled */
    		vma->vm_userfaultfd_ctx = NULL_VM_UFFD_CTX;
    		vma->vm_flags &= ~(VM_UFFD_WP | VM_UFFD_MISSING);
    	}
    }
    
    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;
    	bool still_valid;
    
    	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);
    	still_valid = mmget_still_valid(mm);
    	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);
    		if (still_valid) {
    			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_irq(&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_irq(&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;
    
    	lockdep_assert_held(&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_irq(&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_irq(&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_irq(&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_irq(&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);
    	if (!mmget_still_valid(mm))
    		goto out_unlock;
    	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;
    
    		/*
    		 * UFFDIO_COPY will fill file holes even without
    		 * PROT_WRITE. This check enforces that if this is a
    		 * MAP_SHARED, the process has write permission to the backing
    		 * file. If VM_MAYWRITE is set it also enforces that on a
    		 * MAP_SHARED vma: there is no F_WRITE_SEAL and no further
    		 * F_WRITE_SEAL can be taken until the vma is destroyed.
    		 */
    		ret = -EPERM;
    		if (unlikely(!(cur->vm_flags & VM_MAYWRITE)))
    			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);
    		WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
    
    		/*
    		 * 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);
    	if (!mmget_still_valid(mm))
    		goto out_unlock;
    	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;
    
    		WARN_ON(!(vma->vm_flags & VM_MAYWRITE));
    
    		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_irq(&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_irq(&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;
    
    	if (!sysctl_unprivileged_userfaultfd && !capable(CAP_SYS_PTRACE))
    		return -EPERM;
    
    	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;
    
    	refcount_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);