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    SubjectRe: [RFC PATCH v2 14/27] mm: Handle THP/HugeTLB shadow stack page fault
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    On Fri, 2018-07-20 at 07:20 -0700, Dave Hansen wrote:
    > On 07/10/2018 03:26 PM, Yu-cheng Yu wrote:
    > >
    > > @@ -1193,6 +1195,8 @@ static int
    > > do_huge_pmd_wp_page_fallback(struct vm_fault *vmf, pmd_t orig_pmd,
    > >   pte_t entry;
    > >   entry = mk_pte(pages[i], vma->vm_page_prot);
    > >   entry = maybe_mkwrite(pte_mkdirty(entry), vma);
    > > + if (is_shstk_mapping(vma->vm_flags))
    > > + entry = pte_mkdirty_shstk(entry);
    > Peter Z was pointing out that we should get rid of all this generic
    > code
    > manipulation.  We might not easily be able to do it *all*, but we
    > can do
    > better than what we've got here.
    >
    > Basically, if you have code outside of arch/x86 in your patch set
    > that
    > refers to shadow stacks, you should consider it a bug (for now),
    > especially if you have to hack .c files.
    >
    > For instance, in the code above, you could move the
    > is_shstk_mapping() into:
    >
    > static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct
    > *vma)
    > {
    >         if (likely(vma->vm_flags & VM_WRITE))
    >                 pte = pte_mkwrite(pte);
    >
    > + pte = arch_pte_mkwrite(pte, vma);
    > +
    >         return pte;
    > }
    >
    > ... and add an arch callback that does:
    >
    > static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct
    > *vma)
    > {
    > if (!is_shstk_mapping(vma->vm_flags))
    > return pte;
    >
    > WARN_ON(... pte bits incompatible with shadow stacks?);
    >
    > /* Lots of comments of course */
    > entry = pte_mkdirty_shstk(entry);
    > }
    >
    > This is just one example.  You are probably going to need a couple
    > of
    > similar things.  Just remember: the bar is very high to make changes
    > to
    > .c files outside of arch/x86.  You can do a _bit_ more in non-x86
    > headers, but you have the most freedom to patch what you want as
    > long as
    > it's in arch/x86.

    Ok, I will work on that.  Thanks!

    Yu-cheng

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