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SubjectRe: [PATCH v3 2/3] userfaultfd: UFFDIO_MOVE uABI
From
Focusing on validate_remap_areas():

> +
> +static int validate_remap_areas(struct vm_area_struct *src_vma,
> + struct vm_area_struct *dst_vma)
> +{
> + /* Only allow remapping if both have the same access and protection */
> + if ((src_vma->vm_flags & VM_ACCESS_FLAGS) != (dst_vma->vm_flags & VM_ACCESS_FLAGS) ||
> + pgprot_val(src_vma->vm_page_prot) != pgprot_val(dst_vma->vm_page_prot))
> + return -EINVAL;

Makes sense. I do wonder about pkey and friends and if we even have to
so anything special.

> +
> + /* Only allow remapping if both are mlocked or both aren't */
> + if ((src_vma->vm_flags & VM_LOCKED) != (dst_vma->vm_flags & VM_LOCKED))
> + return -EINVAL;
> +
> + if (!(src_vma->vm_flags & VM_WRITE) || !(dst_vma->vm_flags & VM_WRITE))
> + return -EINVAL;

Why does one of both need VM_WRITE? If one really needs it, then the
destination (where we're moving stuff to).

> +
> + /*
> + * Be strict and only allow remap_pages if either the src or
> + * dst range is registered in the userfaultfd to prevent
> + * userland errors going unnoticed. As far as the VM
> + * consistency is concerned, it would be perfectly safe to
> + * remove this check, but there's no useful usage for
> + * remap_pages ouside of userfaultfd registered ranges. This
> + * is after all why it is an ioctl belonging to the
> + * userfaultfd and not a syscall.

I think the last sentence is the important bit and the comment can
likely be reduced.

> + *
> + * Allow both vmas to be registered in the userfaultfd, just
> + * in case somebody finds a way to make such a case useful.
> + * Normally only one of the two vmas would be registered in
> + * the userfaultfd.

Should we just check the destination? That makes most sense to me,
because with uffd we are resolving uffd-events. And just like
copy/zeropage we want to resolve a page fault ("userfault") of a
non-present page on the destination.


> + */
> + if (!dst_vma->vm_userfaultfd_ctx.ctx &&
> + !src_vma->vm_userfaultfd_ctx.ctx)
> + return -EINVAL;



> +
> + /*
> + * FIXME: only allow remapping across anonymous vmas,
> + * tmpfs should be added.
> + */
> + if (!vma_is_anonymous(src_vma) || !vma_is_anonymous(dst_vma))
> + return -EINVAL;

Why a FIXME here? Just drop the comment completely or replace it with
"We only allow to remap anonymous folios accross anonymous VMAs".

> +
> + /*
> + * Ensure the dst_vma has a anon_vma or this page
> + * would get a NULL anon_vma when moved in the
> + * dst_vma.
> + */
> + if (unlikely(anon_vma_prepare(dst_vma)))
> + return -ENOMEM;

Makes sense.

> +
> + return 0;
> +}


--
Cheers,

David / dhildenb

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