On 10/23/24 22:38, Lorenzo Stoakes wrote:
> We have seen bugs and resource leaks arise from the complexity of the
> __mmap_region() function. This, and the generally deeply fragile error
> handling logic and complexity which makes understanding the function
> difficult make it highly desirable to refactor it into something readable.
>
> Achieve this by separating the function into smaller logical parts which
> are easier to understand and follow, and which importantly very
> significantly simplify the error handling.
>
> Note that we now call vms_abort_munmap_vmas() in more error paths than we
> used to, however in cases where no abort need occur, vms->nr_pages will be
> equal to zero and we simply exit this function without doing more than we
> would have done previously.
>
> Importantly, the invocation of the driver mmap hook via mmap_file() now has
> very simple and obvious handling (this was previously the most problematic
> part of the mmap() operation).
>
> Signed-off-by: Lorenzo Stoakes <lorenzo.stoakes@xxxxxxxxxx>
Reviewed-by: Vlastimil Babka <vbabka@xxxxxxx>
Some nits:
> +static int __mmap_prepare(struct mmap_state *map, struct vma_merge_struct *vmg,
> + struct list_head *uf)
> {
> - struct mm_struct *mm = current->mm;
> - struct vm_area_struct *vma = NULL;
> - pgoff_t pglen = PHYS_PFN(len);
> - unsigned long charged = 0;
> - struct vma_munmap_struct vms;
> - struct ma_state mas_detach;
> - struct maple_tree mt_detach;
> - unsigned long end = addr + len;
> int error;
> - VMA_ITERATOR(vmi, mm, addr);
> - VMG_STATE(vmg, mm, &vmi, addr, end, vm_flags, pgoff);
> -
> - vmg.file = file;
> - /* Find the first overlapping VMA */
> - vma = vma_find(&vmi, end);
> - init_vma_munmap(&vms, &vmi, vma, addr, end, uf, /* unlock = */ false);
> - if (vma) {
> - mt_init_flags(&mt_detach, vmi.mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
> - mt_on_stack(mt_detach);
> - mas_init(&mas_detach, &mt_detach, /* addr = */ 0);
> + struct vma_iterator *vmi = map->vmi;
> + struct vma_munmap_struct *vms = &map->vms;
> +
> + /* Find the first overlapping VMA and initialise unmap state. */
> + vms->vma = vma_find(vmi, map->end);
> + init_vma_munmap(vms, vmi, vms->vma, map->addr, map->end, uf,
> + /* unlock = */ false);
> +
> + /* OK, we have overlapping VMAs - prepare to unmap them. */
> + if (vms->vma) {
> + mt_init_flags(&map->mt_detach,
> + vmi->mas.tree->ma_flags & MT_FLAGS_LOCK_MASK);
> + mt_on_stack(map->mt_detach);
> + mas_init(&map->mas_detach, &map->mt_detach, /* addr = */ 0);
> /* Prepare to unmap any existing mapping in the area */
> - error = vms_gather_munmap_vmas(&vms, &mas_detach);
> - if (error)
> - goto gather_failed;
> + error = vms_gather_munmap_vmas(vms, &map->mas_detach);
> + if (error) {
> + /* On error VMAs will already have been reattached. */
> + vms->nr_pages = 0;
> + return error;
> + }
>
> - vmg.next = vms.next;
> - vmg.prev = vms.prev;
> - vma = NULL;
> + map->next = vms->next;
> + map->prev = vms->prev;
> } else {
> - vmg.next = vma_iter_next_rewind(&vmi, &vmg.prev);
> + map->next = vma_iter_next_rewind(vmi, &map->prev);
> }
>
> + /* Set up vmg for merge attempt. */
> + vmg->file = map->file;
> + vmg->prev = map->prev;
> + vmg->next = map->next;
It seems arbitrary to do this here. vmg->flags are set in __mmap_region().
Why not all of this? We wouldn't need to pass vmg to __mmap_prepare() at
all? Do any of the values chenge between here and returning and vmg needs to
capture them as they are now? AFAICS not.
> +
> /* Check against address space limit. */
> - if (!may_expand_vm(mm, vm_flags, pglen - vms.nr_pages)) {
> - error = -ENOMEM;
> - goto abort_munmap;
> - }
> + if (!may_expand_vm(map->mm, map->flags, map->pglen - vms->nr_pages))
> + return -ENOMEM;
>
> - /*
> - * Private writable mapping: check memory availability
> - */
> - if (accountable_mapping(file, vm_flags)) {
> - charged = pglen;
> - charged -= vms.nr_accounted;
> - if (charged) {
> - error = security_vm_enough_memory_mm(mm, charged);
> + /* Private writable mapping: check memory availability. */
> + if (accountable_mapping(map->file, map->flags)) {
> + map->charged = map->pglen;
> + map->charged -= vms->nr_accounted;
> + if (map->charged) {
> + error = security_vm_enough_memory_mm(map->mm, map->charged);
> if (error)
> - goto abort_munmap;
> + return error;
> }
>
> - vms.nr_accounted = 0;
> - vm_flags |= VM_ACCOUNT;
> - vmg.flags = vm_flags;
> + vms->nr_accounted = 0;
> + map->flags |= VM_ACCOUNT;
> }
>
> /*
> - * clear PTEs while the vma is still in the tree so that rmap
> + * Clear PTEs while the vma is still in the tree so that rmap
> * cannot race with the freeing later in the truncate scenario.
> * This is also needed for mmap_file(), which is why vm_ops
> * close function is called.
> */
> - vms_clean_up_area(&vms, &mas_detach);
> - vma = vma_merge_new_range(&vmg);
> - if (vma)
> - goto expanded;
> + vms_clean_up_area(vms, &map->mas_detach);
> +
> + return 0;
> +}
> +
<snip>
> +static int __mmap_new_vma(struct mmap_state *map, struct vma_merge_struct *vmg,
> + struct vm_area_struct **vmap)
> +{
> + struct vma_iterator *vmi = map->vmi;
> + int error = 0;
> + bool merged = false;
> + struct vm_area_struct *vma;
> +
> /*
> * Determine the object being mapped and call the appropriate
> * specific mapper. the address has already been validated, but
> * not unmapped, but the maps are removed from the list.
> */
> - vma = vm_area_alloc(mm);
> - if (!vma) {
> - error = -ENOMEM;
> - goto unacct_error;
> - }
> + vma = vm_area_alloc(map->mm);
> + if (!vma)
> + return -ENOMEM;
>
> - vma_iter_config(&vmi, addr, end);
> - vma_set_range(vma, addr, end, pgoff);
> - vm_flags_init(vma, vm_flags);
> - vma->vm_page_prot = vm_get_page_prot(vm_flags);
> + vma_iter_config(vmi, map->addr, map->end);
> + vma_set_range(vma, map->addr, map->end, map->pgoff);
> + vm_flags_init(vma, map->flags);
> + vma->vm_page_prot = vm_get_page_prot(map->flags);
>
> - if (vma_iter_prealloc(&vmi, vma)) {
> + if (vma_iter_prealloc(vmi, vma)) {
> error = -ENOMEM;
> goto free_vma;
> }
>
> - if (file) {
> - vma->vm_file = get_file(file);
> - error = mmap_file(file, vma);
> - if (error)
> - goto unmap_and_free_file_vma;
> -
> - /* Drivers cannot alter the address of the VMA. */
> - WARN_ON_ONCE(addr != vma->vm_start);
> - /*
> - * Drivers should not permit writability when previously it was
> - * disallowed.
> - */
> - VM_WARN_ON_ONCE(vm_flags != vma->vm_flags &&
> - !(vm_flags & VM_MAYWRITE) &&
> - (vma->vm_flags & VM_MAYWRITE));
> -
> - vma_iter_config(&vmi, addr, end);
> - /*
> - * If vm_flags changed after mmap_file(), we should try merge
> - * vma again as we may succeed this time.
> - */
> - if (unlikely(vm_flags != vma->vm_flags && vmg.prev)) {
> - struct vm_area_struct *merge;
> -
> - vmg.flags = vma->vm_flags;
> - /* If this fails, state is reset ready for a reattempt. */
> - merge = vma_merge_new_range(&vmg);
> -
> - if (merge) {
> - /*
> - * ->mmap() can change vma->vm_file and fput
> - * the original file. So fput the vma->vm_file
> - * here or we would add an extra fput for file
> - * and cause general protection fault
> - * ultimately.
> - */
> - fput(vma->vm_file);
> - vm_area_free(vma);
> - vma = merge;
> - /* Update vm_flags to pick up the change. */
> - vm_flags = vma->vm_flags;
> - goto file_expanded;
> - }
> - vma_iter_config(&vmi, addr, end);
> - }
> -
> - vm_flags = vma->vm_flags;
> - } else if (vm_flags & VM_SHARED) {
> + if (map->file)
> + error = __mmap_new_file_vma(map, vmg, &vma, &merged);
> + else if (map->flags & VM_SHARED)
> error = shmem_zero_setup(vma);
> - if (error)
> - goto free_iter_vma;
> - } else {
> + else
> vma_set_anonymous(vma);
> - }
> +
> + if (error)
> + goto free_iter_vma;
> +
> + if (merged)
> + goto file_expanded;
>
> #ifdef CONFIG_SPARC64
> /* TODO: Fix SPARC ADI! */
> - WARN_ON_ONCE(!arch_validate_flags(vm_flags));
> + WARN_ON_ONCE(!arch_validate_flags(map->flags));
> #endif
>
> /* Lock the VMA since it is modified after insertion into VMA tree */
> vma_start_write(vma);
> - vma_iter_store(&vmi, vma);
> - mm->map_count++;
> + vma_iter_store(vmi, vma);
> + map->mm->map_count++;
> vma_link_file(vma);
>
> /*
> * vma_merge_new_range() calls khugepaged_enter_vma() too, the below
> * call covers the non-merge case.
> */
> - khugepaged_enter_vma(vma, vma->vm_flags);
> + khugepaged_enter_vma(vma, map->flags);
>
> file_expanded:
> - file = vma->vm_file;
> ksm_add_vma(vma);
I'm wondering if this could go to the "non file expanded" region above. If
we expanded a vma, it was either in ksm and stays in ksm, or it was not
eligible and that couldn't have changed by expanding?
> -expanded:
> + *vmap = vma;
> + return 0;
> +
> +free_iter_vma:
> + vma_iter_free(vmi);
> +free_vma:
> + vm_area_free(vma);
> + return error;
> +}
> +
