Re: inconsistence in mprotect_fixup mlock_fixup madvise_update_vma

[Jeff Xu]

Hello Peter,

Thanks for responding.

On Tue, Jun 13, 2023 at 1:16 PM Peter Xu <peterx@xxxxxxxxxx> wrote:
>
> Hi, Jeff,
>
> On Tue, Jun 13, 2023 at 08:26:26AM -0700, Jeff Xu wrote:
> > + more ppl to the list.
> >
> > On Mon, Jun 12, 2023 at 6:04 PM Jeff Xu <jeffxu@xxxxxxxxxxxx> wrote:
> > >
> > > Hello,
> > >
> > > There seems to be inconsistency in different VMA fixup
> > > implementations, for example:
> > > mlock_fixup will skip VMA that is hugettlb, etc, but those checks do
> > > not exist in mprotect_fixup and madvise_update_vma. Wouldn't this be a
> > > problem? the merge/split skipped by mlock_fixup, might get acted on in
> > > the madvice/mprotect case.
> > >
> > > mlock_fixup currently check for
> > > if (newflags == oldflags || (oldflags & VM_SPECIAL) ||
> > > is_vm_hugetlb_page(vma) || vma == get_gate_vma(current->mm) ||
> > > vma_is_dax(vma) || vma_is_secretmem(vma))
>
> The special handling you mentioned in mlock_fixup mostly makes sense to me.
>
> E.g., I think we can just ignore mlock a hugetlb page if it won't be
> swapped anyway.
>
> Do you encounter any issue with above?
>
> > > Should there be a common function to handle VMA merge/split ?
>
> IMHO vma_merge() and split_vma() are the "common functions".  Copy Lorenzo
> as I think he has plan to look into the interface to make it even easier to
> use.
>
The mprotect_fixup doesn't have the same check as mlock_fixup. When
userspace calls mlock(), two VMAs might not merge or split because of
vma_is_secretmem check, However, when user space calls mprotect() with
the same address range, it will merge/split.  If mlock() is doing the
right thing to merge/split the VMAs, then mprotect() is not ?

Also skipping merge of VMA might be OK, but skipping split doesn't,
wouldn't that cause inconsistent between vma->vm_flags and what is
provisioned in the page ?

Thanks
-Jeff Xu


> --
> Peter Xu
>