Re: [PATCH v2 17/39] mm: Fixup places that call pte_mkwrite() directly

[Edgecombe, Rick P]

On Fri, 2022-10-14 at 17:52 +0200, Peter Zijlstra wrote:
> On Thu, Sep 29, 2022 at 03:29:14PM -0700, Rick Edgecombe wrote:
> > diff --git a/mm/userfaultfd.c b/mm/userfaultfd.c
> > index 7327b2573f7c..b49372c7de41 100644
> > --- a/mm/userfaultfd.c
> > +++ b/mm/userfaultfd.c
> > @@ -63,6 +63,7 @@ int mfill_atomic_install_pte(struct mm_struct
> > *dst_mm, pmd_t *dst_pmd,
> >        int ret;
> >        pte_t _dst_pte, *dst_pte;
> >        bool writable = dst_vma->vm_flags & VM_WRITE;
> > +     bool shstk = dst_vma->vm_flags & VM_SHADOW_STACK;
> >        bool vm_shared = dst_vma->vm_flags & VM_SHARED;
> >        bool page_in_cache = page->mapping;
> >        spinlock_t *ptl;
> > @@ -83,9 +84,12 @@ int mfill_atomic_install_pte(struct mm_struct
> > *dst_mm, pmd_t *dst_pmd,
> >                writable = false;
> >        }
> >   
> > -     if (writable)
> > -             _dst_pte = pte_mkwrite(_dst_pte);
> > -     else
> > +     if (writable) {
> > +             if (shstk)
> > +                     _dst_pte = pte_mkwrite_shstk(_dst_pte);
> > +             else
> > +                     _dst_pte = pte_mkwrite(_dst_pte);
> > +     } else
> >                /*
> >                 * We need this to make sure write bit removed; as
> > mk_pte()
> >                 * could return a pte with write bit set.
> 
> Urgh.. that's unfortunate. But yeah, I don't see a way to make that
> pretty either.

Nadav pointed out that:
entry = maybe_mkwrite(pte_mkdirty(entry), vma);

and:
if (vma->vm_flags & VM_WRITE)
	entry = pte_mkwrite(pte_mkdirty(entry));

Are not actually the same, because in the former the non-writable PTE
gets marked dirty. So I was actually going to add two more cases like
the ugly case.