// SPDX-License-Identifier: GPL-2.0
/*
 *      mm/mremap.c
 *
 *      (C) Copyright 1996 Linus Torvalds
 *
 *      Address space accounting code   <alan@lxorguk.ukuu.org.uk>
 *      (C) Copyright 2002 Red Hat Inc, All Rights Reserved
 */

#include <linux/mm.h>
#include <linux/hugetlb.h>
#include <linux/shm.h>
#include <linux/ksm.h>
#include <linux/mman.h>
#include <linux/swap.h>
#include <linux/capability.h>
#include <linux/fs.h>
#include <linux/swapops.h>
#include <linux/highmem.h>
#include <linux/security.h>
#include <linux/syscalls.h>
#include <linux/mmu_notifier.h>
#include <linux/uaccess.h>
#include <linux/mm-arch-hooks.h>
#include <linux/userfaultfd_k.h>

#include <asm/cacheflush.h>
#include <asm/tlbflush.h>

#include "internal.h"

static pmd_t *get_old_pmd(struct mm_struct *mm, unsigned long addr)
{
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;
        pmd_t *pmd;

        pgd = pgd_offset(mm, addr);
        if (pgd_none_or_clear_bad(pgd))
                return NULL;

        p4d = p4d_offset(pgd, addr);
        if (p4d_none_or_clear_bad(p4d))
                return NULL;

        pud = pud_offset(p4d, addr);
        if (pud_none_or_clear_bad(pud))
                return NULL;

        pmd = pmd_offset(pud, addr);
        if (pmd_none(*pmd))
                return NULL;

        return pmd;
}

static pmd_t *alloc_new_pmd(struct mm_struct *mm, struct vm_area_struct *vma,
                            unsigned long addr)
{
        pgd_t *pgd;
        p4d_t *p4d;
        pud_t *pud;
        pmd_t *pmd;

        pgd = pgd_offset(mm, addr);
        p4d = p4d_alloc(mm, pgd, addr);
        if (!p4d)
                return NULL;
        pud = pud_alloc(mm, p4d, addr);
        if (!pud)
                return NULL;

        pmd = pmd_alloc(mm, pud, addr);
        if (!pmd)
                return NULL;

        VM_BUG_ON(pmd_trans_huge(*pmd));

        return pmd;
}

static void take_rmap_locks(struct vm_area_struct *vma)
{
        if (vma->vm_file)
                i_mmap_lock_write(vma->vm_file->f_mapping);
        if (vma->anon_vma)
                anon_vma_lock_write(vma->anon_vma);
}

static void drop_rmap_locks(struct vm_area_struct *vma)
{
        if (vma->anon_vma)
                anon_vma_unlock_write(vma->anon_vma);
        if (vma->vm_file)
                i_mmap_unlock_write(vma->vm_file->f_mapping);
}

static pte_t move_soft_dirty_pte(pte_t pte)
{
        /*
         * Set soft dirty bit so we can notice
         * in userspace the ptes were moved.
         */
#ifdef CONFIG_MEM_SOFT_DIRTY
        if (pte_present(pte))
                pte = pte_mksoft_dirty(pte);
        else if (is_swap_pte(pte))
                pte = pte_swp_mksoft_dirty(pte);
#endif
        return pte;
}

static void move_ptes(struct vm_area_struct *vma, pmd_t *old_pmd,
                unsigned long old_addr, unsigned long old_end,
                struct vm_area_struct *new_vma, pmd_t *new_pmd,
                unsigned long new_addr, bool need_rmap_locks)
{
        struct mm_struct *mm = vma->vm_mm;
        pte_t *old_pte, *new_pte, pte;
        spinlock_t *old_ptl, *new_ptl;
        bool force_flush = false;
        unsigned long len = old_end - old_addr;

        /*
         * When need_rmap_locks is true, we take the i_mmap_rwsem and anon_vma
         * locks to ensure that rmap will always observe either the old or the
         * new ptes. This is the easiest way to avoid races with
         * truncate_pagecache(), page migration, etc...
         *
         * When need_rmap_locks is false, we use other ways to avoid
         * such races:
         *
         * - During exec() shift_arg_pages(), we use a specially tagged vma
         *   which rmap call sites look for using vma_is_temporary_stack().
         *
         * - During mremap(), new_vma is often known to be placed after vma
         *   in rmap traversal order. This ensures rmap will always observe
         *   either the old pte, or the new pte, or both (the page table locks
         *   serialize access to individual ptes, but only rmap traversal
         *   order guarantees that we won't miss both the old and new ptes).
         */
        if (need_rmap_locks)
                take_rmap_locks(vma);

        /*
         * We don't have to worry about the ordering of src and dst
         * pte locks because exclusive mmap_lock prevents deadlock.
         */
        old_pte = pte_offset_map_lock(mm, old_pmd, old_addr, &old_ptl);
        new_pte = pte_offset_map(new_pmd, new_addr);
        new_ptl = pte_lockptr(mm, new_pmd);
        if (new_ptl != old_ptl)
                spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);
        flush_tlb_batched_pending(vma->vm_mm);
        arch_enter_lazy_mmu_mode();

        for (; old_addr < old_end; old_pte++, old_addr += PAGE_SIZE,
                                   new_pte++, new_addr += PAGE_SIZE) {
                if (pte_none(*old_pte))
                        continue;

                pte = ptep_get_and_clear(mm, old_addr, old_pte);
                /*
                 * If we are remapping a valid PTE, make sure
                 * to flush TLB before we drop the PTL for the
                 * PTE.
                 *
                 * NOTE! Both old and new PTL matter: the old one
                 * for racing with page_mkclean(), the new one to
                 * make sure the physical page stays valid until
                 * the TLB entry for the old mapping has been
                 * flushed.
                 */
                if (pte_present(pte))
                        force_flush = true;
                pte = move_pte(pte, new_vma->vm_page_prot, old_addr, new_addr);
                pte = move_soft_dirty_pte(pte);
                set_pte_at(mm, new_addr, new_pte, pte);
        }

        arch_leave_lazy_mmu_mode();
        if (force_flush)
                flush_tlb_range(vma, old_end - len, old_end);
        if (new_ptl != old_ptl)
                spin_unlock(new_ptl);
        pte_unmap(new_pte - 1);
        pte_unmap_unlock(old_pte - 1, old_ptl);
        if (need_rmap_locks)
                drop_rmap_locks(vma);
}

#ifdef CONFIG_HAVE_MOVE_PMD
static bool move_normal_pmd(struct vm_area_struct *vma, unsigned long old_addr,
                  unsigned long new_addr, unsigned long old_end,
                  pmd_t *old_pmd, pmd_t *new_pmd)
{
        spinlock_t *old_ptl, *new_ptl;
        struct mm_struct *mm = vma->vm_mm;
        pmd_t pmd;

        if ((old_addr & ~PMD_MASK) || (new_addr & ~PMD_MASK)
            || old_end - old_addr < PMD_SIZE)
                return false;

        /*
         * The destination pmd shouldn't be established, free_pgtables()
         * should have released it.
         *
         * However, there's a case during execve() where we use mremap
         * to move the initial stack, and in that case the target area
         * may overlap the source area (always moving down).
         *
         * If everything is PMD-aligned, that works fine, as moving
         * each pmd down will clear the source pmd. But if we first
         * have a few 4kB-only pages that get moved down, and then
         * hit the "now the rest is PMD-aligned, let's do everything
         * one pmd at a time", we will still have the old (now empty
         * of any 4kB pages, but still there) PMD in the page table
         * tree.
         *
         * Warn on it once - because we really should try to figure
         * out how to do this better - but then say "I won't move
         * this pmd".
         *
         * One alternative might be to just unmap the target pmd at
         * this point, and verify that it really is empty. We'll see.
         */
        if (WARN_ON_ONCE(!pmd_none(*new_pmd)))
                return false;

        /*
         * We don't have to worry about the ordering of src and dst
         * ptlocks because exclusive mmap_lock prevents deadlock.
         */
        old_ptl = pmd_lock(vma->vm_mm, old_pmd);
        new_ptl = pmd_lockptr(mm, new_pmd);
        if (new_ptl != old_ptl)
                spin_lock_nested(new_ptl, SINGLE_DEPTH_NESTING);

        /* Clear the pmd */
        pmd = *old_pmd;
        pmd_clear(old_pmd);

        VM_BUG_ON(!pmd_none(*new_pmd));

        /* Set the new pmd */
        set_pmd_at(mm, new_addr, new_pmd, pmd);
        flush_tlb_range(vma, old_addr, old_addr + PMD_SIZE);
        if (new_ptl != old_ptl)
                spin_unlock(new_ptl);
        spin_unlock(old_ptl);

        return true;
}
#endif

unsigned long move_page_tables(struct vm_area_struct *vma,
                unsigned long old_addr, struct vm_area_struct *new_vma,
                unsigned long new_addr, unsigned long len,
                bool need_rmap_locks)
{
        unsigned long extent, next, old_end;
        struct mmu_notifier_range range;
        pmd_t *old_pmd, *new_pmd;

        old_end = old_addr + len;
        flush_cache_range(vma, old_addr, old_end);

        mmu_notifier_range_init(&range, MMU_NOTIFY_UNMAP, 0, vma, vma->vm_mm,
                                old_addr, old_end);
        mmu_notifier_invalidate_range_start(&range);

        for (; old_addr < old_end; old_addr += extent, new_addr += extent) {
                cond_resched();
                next = (old_addr + PMD_SIZE) & PMD_MASK;
                /* even if next overflowed, extent below will be ok */
                extent = next - old_addr;
                if (extent > old_end - old_addr)
                        extent = old_end - old_addr;
                old_pmd = get_old_pmd(vma->vm_mm, old_addr);
                if (!old_pmd)
                        continue;
                new_pmd = alloc_new_pmd(vma->vm_mm, vma, new_addr);
                if (!new_pmd)
                        break;
                if (is_swap_pmd(*old_pmd) || pmd_trans_huge(*old_pmd) || pmd_devmap(*old_pmd)) {
                        if (extent == HPAGE_PMD_SIZE) {
                                bool moved;
                                /* See comment in move_ptes() */
                                if (need_rmap_locks)
                                        take_rmap_locks(vma);
                                moved = move_huge_pmd(vma, old_addr, new_addr,
                                                    old_end, old_pmd, new_pmd);
                                if (need_rmap_locks)
                                        drop_rmap_locks(vma);
                                if (moved)
                                        continue;
                        }
                        split_huge_pmd(vma, old_pmd, old_addr);
                        if (pmd_trans_unstable(old_pmd))
                                continue;
                } else if (extent == PMD_SIZE) {
#ifdef CONFIG_HAVE_MOVE_PMD
                        /*
                         * If the extent is PMD-sized, try to speed the move by
                         * moving at the PMD level if possible.
                         */
                        bool moved;

                        if (need_rmap_locks)
                                take_rmap_locks(vma);
                        moved = move_normal_pmd(vma, old_addr, new_addr,
                                        old_end, old_pmd, new_pmd);
                        if (need_rmap_locks)
                                drop_rmap_locks(vma);
                        if (moved)
                                continue;
#endif
                }

                if (pte_alloc(new_vma->vm_mm, new_pmd))
                        break;
                next = (new_addr + PMD_SIZE) & PMD_MASK;
                if (extent > next - new_addr)
                        extent = next - new_addr;
                move_ptes(vma, old_pmd, old_addr, old_addr + extent, new_vma,
                          new_pmd, new_addr, need_rmap_locks);
        }

        mmu_notifier_invalidate_range_end(&range);

        return len + old_addr - old_end;        /* how much done */
}

static unsigned long move_vma(struct vm_area_struct *vma,
                unsigned long old_addr, unsigned long old_len,
                unsigned long new_len, unsigned long new_addr,
                bool *locked, unsigned long flags,
                struct vm_userfaultfd_ctx *uf, struct list_head *uf_unmap)
{
        struct mm_struct *mm = vma->vm_mm;
        struct vm_area_struct *new_vma;
        unsigned long vm_flags = vma->vm_flags;
        unsigned long new_pgoff;
        unsigned long moved_len;
        unsigned long excess = 0;
        unsigned long hiwater_vm;
        int split = 0;
        int err;
        bool need_rmap_locks;

        /*
         * We'd prefer to avoid failure later on in do_munmap:
         * which may split one vma into three before unmapping.
         */
        if (mm->map_count >= sysctl_max_map_count - 3)
                return -ENOMEM;

        /*
         * Advise KSM to break any KSM pages in the area to be moved:
         * it would be confusing if they were to turn up at the new
         * location, where they happen to coincide with different KSM
         * pages recently unmapped.  But leave vma->vm_flags as it was,
         * so KSM can come around to merge on vma and new_vma afterwards.
         */
        err = ksm_madvise(vma, old_addr, old_addr + old_len,
                                                MADV_UNMERGEABLE, &vm_flags);
        if (err)
                return err;

        new_pgoff = vma->vm_pgoff + ((old_addr - vma->vm_start) >> PAGE_SHIFT);
        new_vma = copy_vma(&vma, new_addr, new_len, new_pgoff,
                           &need_rmap_locks);
        if (!new_vma)
                return -ENOMEM;

        moved_len = move_page_tables(vma, old_addr, new_vma, new_addr, old_len,
                                     need_rmap_locks);
        if (moved_len < old_len) {
                err = -ENOMEM;
        } else if (vma->vm_ops && vma->vm_ops->mremap) {
                err = vma->vm_ops->mremap(new_vma);
        }

        if (unlikely(err)) {
                /*
                 * On error, move entries back from new area to old,
                 * which will succeed since page tables still there,
                 * and then proceed to unmap new area instead of old.
                 */
                move_page_tables(new_vma, new_addr, vma, old_addr, moved_len,
                                 true);
                vma = new_vma;
                old_len = new_len;
                old_addr = new_addr;
                new_addr = err;
        } else {
                mremap_userfaultfd_prep(new_vma, uf);
                arch_remap(mm, old_addr, old_addr + old_len,
                           new_addr, new_addr + new_len);
        }

        /* Conceal VM_ACCOUNT so old reservation is not undone */
        if (vm_flags & VM_ACCOUNT) {
                vma->vm_flags &= ~VM_ACCOUNT;
                excess = vma->vm_end - vma->vm_start - old_len;
                if (old_addr > vma->vm_start &&
                    old_addr + old_len < vma->vm_end)
                        split = 1;
        }

        /*
         * If we failed to move page tables we still do total_vm increment
         * since do_munmap() will decrement it by old_len == new_len.
         *
         * Since total_vm is about to be raised artificially high for a
         * moment, we need to restore high watermark afterwards: if stats
         * are taken meanwhile, total_vm and hiwater_vm appear too high.
         * If this were a serious issue, we'd add a flag to do_munmap().
         */
        hiwater_vm = mm->hiwater_vm;
        vm_stat_account(mm, vma->vm_flags, new_len >> PAGE_SHIFT);

        /* Tell pfnmap has moved from this vma */
        if (unlikely(vma->vm_flags & VM_PFNMAP))
                untrack_pfn_moved(vma);

        if (unlikely(!err && (flags & MREMAP_DONTUNMAP))) {
                if (vm_flags & VM_ACCOUNT) {
                        /* Always put back VM_ACCOUNT since we won't unmap */
                        vma->vm_flags |= VM_ACCOUNT;

                        vm_acct_memory(new_len >> PAGE_SHIFT);
                }

                /*
                 * VMAs can actually be merged back together in copy_vma
                 * calling merge_vma. This can happen with anonymous vmas
                 * which have not yet been faulted, so if we were to consider
                 * this VMA split we'll end up adding VM_ACCOUNT on the
                 * next VMA, which is completely unrelated if this VMA
                 * was re-merged.
                 */
                if (split && new_vma == vma)
                        split = 0;

                /* We always clear VM_LOCKED[ONFAULT] on the old vma */
                vma->vm_flags &= VM_LOCKED_CLEAR_MASK;

                /* Because we won't unmap we don't need to touch locked_vm */
                goto out;
        }

        if (do_munmap(mm, old_addr, old_len, uf_unmap) < 0) {
                /* OOM: unable to split vma, just get accounts right */
                vm_unacct_memory(excess >> PAGE_SHIFT);
                excess = 0;
        }

        if (vm_flags & VM_LOCKED) {
                mm->locked_vm += new_len >> PAGE_SHIFT;
                *locked = true;
        }
out:
        mm->hiwater_vm = hiwater_vm;

        /* Restore VM_ACCOUNT if one or two pieces of vma left */
        if (excess) {
                vma->vm_flags |= VM_ACCOUNT;
                if (split)
                        vma->vm_next->vm_flags |= VM_ACCOUNT;
        }

        return new_addr;
}

static struct vm_area_struct *vma_to_resize(unsigned long addr,
        unsigned long old_len, unsigned long new_len, unsigned long flags,
        unsigned long *p)
{
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma = find_vma(mm, addr);
        unsigned long pgoff;

        if (!vma || vma->vm_start > addr)
                return ERR_PTR(-EFAULT);

        /*
         * !old_len is a special case where an attempt is made to 'duplicate'
         * a mapping.  This makes no sense for private mappings as it will
         * instead create a fresh/new mapping unrelated to the original.  This
         * is contrary to the basic idea of mremap which creates new mappings
         * based on the original.  There are no known use cases for this
         * behavior.  As a result, fail such attempts.
         */
        if (!old_len && !(vma->vm_flags & (VM_SHARED | VM_MAYSHARE))) {
                pr_warn_once("%s (%d): attempted to duplicate a private mapping with mremap.  This is not supported.\n", current->comm, current->pid);
                return ERR_PTR(-EINVAL);
        }

        if (flags & MREMAP_DONTUNMAP && (!vma_is_anonymous(vma) ||
                        vma->vm_flags & VM_SHARED))
                return ERR_PTR(-EINVAL);

        if (is_vm_hugetlb_page(vma))
                return ERR_PTR(-EINVAL);

        /* We can't remap across vm area boundaries */
        if (old_len > vma->vm_end - addr)
                return ERR_PTR(-EFAULT);

        if (new_len == old_len)
                return vma;

        /* Need to be careful about a growing mapping */
        pgoff = (addr - vma->vm_start) >> PAGE_SHIFT;
        pgoff += vma->vm_pgoff;
        if (pgoff + (new_len >> PAGE_SHIFT) < pgoff)
                return ERR_PTR(-EINVAL);

        if (vma->vm_flags & (VM_DONTEXPAND | VM_PFNMAP))
                return ERR_PTR(-EFAULT);

        if (vma->vm_flags & VM_LOCKED) {
                unsigned long locked, lock_limit;
                locked = mm->locked_vm << PAGE_SHIFT;
                lock_limit = rlimit(RLIMIT_MEMLOCK);
                locked += new_len - old_len;
                if (locked > lock_limit && !capable(CAP_IPC_LOCK))
                        return ERR_PTR(-EAGAIN);
        }

        if (!may_expand_vm(mm, vma->vm_flags,
                                (new_len - old_len) >> PAGE_SHIFT))
                return ERR_PTR(-ENOMEM);

        if (vma->vm_flags & VM_ACCOUNT) {
                unsigned long charged = (new_len - old_len) >> PAGE_SHIFT;
                if (security_vm_enough_memory_mm(mm, charged))
                        return ERR_PTR(-ENOMEM);
                *p = charged;
        }

        return vma;
}

static unsigned long mremap_to(unsigned long addr, unsigned long old_len,
                unsigned long new_addr, unsigned long new_len, bool *locked,
                unsigned long flags, struct vm_userfaultfd_ctx *uf,
                struct list_head *uf_unmap_early,
                struct list_head *uf_unmap)
{
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma;
        unsigned long ret = -EINVAL;
        unsigned long charged = 0;
        unsigned long map_flags = 0;

        if (offset_in_page(new_addr))
                goto out;

        if (new_len > TASK_SIZE || new_addr > TASK_SIZE - new_len)
                goto out;

        /* Ensure the old/new locations do not overlap */
        if (addr + old_len > new_addr && new_addr + new_len > addr)
                goto out;

        /*
         * move_vma() need us to stay 4 maps below the threshold, otherwise
         * it will bail out at the very beginning.
         * That is a problem if we have already unmaped the regions here
         * (new_addr, and old_addr), because userspace will not know the
         * state of the vma's after it gets -ENOMEM.
         * So, to avoid such scenario we can pre-compute if the whole
         * operation has high chances to success map-wise.
         * Worst-scenario case is when both vma's (new_addr and old_addr) get
         * split in 3 before unmaping it.
         * That means 2 more maps (1 for each) to the ones we already hold.
         * Check whether current map count plus 2 still leads us to 4 maps below
         * the threshold, otherwise return -ENOMEM here to be more safe.
         */
        if ((mm->map_count + 2) >= sysctl_max_map_count - 3)
                return -ENOMEM;

        if (flags & MREMAP_FIXED) {
                ret = do_munmap(mm, new_addr, new_len, uf_unmap_early);
                if (ret)
                        goto out;
        }

        if (old_len >= new_len) {
                ret = do_munmap(mm, addr+new_len, old_len - new_len, uf_unmap);
                if (ret && old_len != new_len)
                        goto out;
                old_len = new_len;
        }

        vma = vma_to_resize(addr, old_len, new_len, flags, &charged);
        if (IS_ERR(vma)) {
                ret = PTR_ERR(vma);
                goto out;
        }

        /* MREMAP_DONTUNMAP expands by old_len since old_len == new_len */
        if (flags & MREMAP_DONTUNMAP &&
                !may_expand_vm(mm, vma->vm_flags, old_len >> PAGE_SHIFT)) {
                ret = -ENOMEM;
                goto out;
        }

        if (flags & MREMAP_FIXED)
                map_flags |= MAP_FIXED;

        if (vma->vm_flags & VM_MAYSHARE)
                map_flags |= MAP_SHARED;

        ret = get_unmapped_area(vma->vm_file, new_addr, new_len, vma->vm_pgoff +
                                ((addr - vma->vm_start) >> PAGE_SHIFT),
                                map_flags);
        if (IS_ERR_VALUE(ret))
                goto out1;

        /* We got a new mapping */
        if (!(flags & MREMAP_FIXED))
                new_addr = ret;

        ret = move_vma(vma, addr, old_len, new_len, new_addr, locked, flags, uf,
                       uf_unmap);

        if (!(offset_in_page(ret)))
                goto out;

out1:
        vm_unacct_memory(charged);

out:
        return ret;
}

static int vma_expandable(struct vm_area_struct *vma, unsigned long delta)
{
        unsigned long end = vma->vm_end + delta;
        if (end < vma->vm_end) /* overflow */
                return 0;
        if (vma->vm_next && vma->vm_next->vm_start < end) /* intersection */
                return 0;
        if (get_unmapped_area(NULL, vma->vm_start, end - vma->vm_start,
                              0, MAP_FIXED) & ~PAGE_MASK)
                return 0;
        return 1;
}

/*
 * Expand (or shrink) an existing mapping, potentially moving it at the
 * same time (controlled by the MREMAP_MAYMOVE flag and available VM space)
 *
 * MREMAP_FIXED option added 5-Dec-1999 by Benjamin LaHaise
 * This option implies MREMAP_MAYMOVE.
 */
SYSCALL_DEFINE5(mremap, unsigned long, addr, unsigned long, old_len,
                unsigned long, new_len, unsigned long, flags,
                unsigned long, new_addr)
{
        struct mm_struct *mm = current->mm;
        struct vm_area_struct *vma;
        unsigned long ret = -EINVAL;
        unsigned long charged = 0;
        bool locked = false;
        bool downgraded = false;
        struct vm_userfaultfd_ctx uf = NULL_VM_UFFD_CTX;
        LIST_HEAD(uf_unmap_early);
        LIST_HEAD(uf_unmap);

        /*
         * There is a deliberate asymmetry here: we strip the pointer tag
         * from the old address but leave the new address alone. This is
         * for consistency with mmap(), where we prevent the creation of
         * aliasing mappings in userspace by leaving the tag bits of the
         * mapping address intact. A non-zero tag will cause the subsequent
         * range checks to reject the address as invalid.
         *
         * See Documentation/arm64/tagged-address-abi.rst for more information.
         */
        addr = untagged_addr(addr);

        if (flags & ~(MREMAP_FIXED | MREMAP_MAYMOVE | MREMAP_DONTUNMAP))
                return ret;

        if (flags & MREMAP_FIXED && !(flags & MREMAP_MAYMOVE))
                return ret;

        /*
         * MREMAP_DONTUNMAP is always a move and it does not allow resizing
         * in the process.
         */
        if (flags & MREMAP_DONTUNMAP &&
                        (!(flags & MREMAP_MAYMOVE) || old_len != new_len))
                return ret;


        if (offset_in_page(addr))
                return ret;

        old_len = PAGE_ALIGN(old_len);
        new_len = PAGE_ALIGN(new_len);

        /*
         * We allow a zero old-len as a special case
         * for DOS-emu "duplicate shm area" thing. But
         * a zero new-len is nonsensical.
         */
        if (!new_len)
                return ret;

        if (mmap_write_lock_killable(current->mm))
                return -EINTR;

        if (flags & (MREMAP_FIXED | MREMAP_DONTUNMAP)) {
                ret = mremap_to(addr, old_len, new_addr, new_len,
                                &locked, flags, &uf, &uf_unmap_early,
                                &uf_unmap);
                goto out;
        }

        /*
         * Always allow a shrinking remap: that just unmaps
         * the unnecessary pages..
         * __do_munmap does all the needed commit accounting, and
         * downgrades mmap_lock to read if so directed.
         */
        if (old_len >= new_len) {
                int retval;

                retval = __do_munmap(mm, addr+new_len, old_len - new_len,
                                  &uf_unmap, true);
                if (retval < 0 && old_len != new_len) {
                        ret = retval;
                        goto out;
                /* Returning 1 indicates mmap_lock is downgraded to read. */
                } else if (retval == 1)
                        downgraded = true;
                ret = addr;
                goto out;
        }

        /*
         * Ok, we need to grow..
         */
        vma = vma_to_resize(addr, old_len, new_len, flags, &charged);
        if (IS_ERR(vma)) {
                ret = PTR_ERR(vma);
                goto out;
        }

        /* old_len exactly to the end of the area..
         */
        if (old_len == vma->vm_end - addr) {
                /* can we just expand the current mapping? */
                if (vma_expandable(vma, new_len - old_len)) {
                        int pages = (new_len - old_len) >> PAGE_SHIFT;

                        if (vma_adjust(vma, vma->vm_start, addr + new_len,
                                       vma->vm_pgoff, NULL)) {
                                ret = -ENOMEM;
                                goto out;
                        }

                        vm_stat_account(mm, vma->vm_flags, pages);
                        if (vma->vm_flags & VM_LOCKED) {
                                mm->locked_vm += pages;
                                locked = true;
                                new_addr = addr;
                        }
                        ret = addr;
                        goto out;
                }
        }

        /*
         * We weren't able to just expand or shrink the area,
         * we need to create a new one and move it..
         */
        ret = -ENOMEM;
        if (flags & MREMAP_MAYMOVE) {
                unsigned long map_flags = 0;
                if (vma->vm_flags & VM_MAYSHARE)
                        map_flags |= MAP_SHARED;

                new_addr = get_unmapped_area(vma->vm_file, 0, new_len,
                                        vma->vm_pgoff +
                                        ((addr - vma->vm_start) >> PAGE_SHIFT),
                                        map_flags);
                if (IS_ERR_VALUE(new_addr)) {
                        ret = new_addr;
                        goto out;
                }

                ret = move_vma(vma, addr, old_len, new_len, new_addr,
                               &locked, flags, &uf, &uf_unmap);
        }
out:
        if (offset_in_page(ret)) {
                vm_unacct_memory(charged);
                locked = false;
        }
        if (downgraded)
                mmap_read_unlock(current->mm);
        else
                mmap_write_unlock(current->mm);
        if (locked && new_len > old_len)
                mm_populate(new_addr + old_len, new_len - old_len);
        userfaultfd_unmap_complete(mm, &uf_unmap_early);
        mremap_userfaultfd_complete(&uf, addr, ret, old_len);
        userfaultfd_unmap_complete(mm, &uf_unmap);
        return ret;
}