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SubjectRe: [RFC PATCH v2 16/27] mm: Modify can_follow_write_pte/pmd for shadow stack
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On 07/10/2018 03:26 PM, Yu-cheng Yu wrote:
> There are three possible shadow stack PTE settings:
>
> Normal SHSTK PTE: (R/O + DIRTY_HW)
> SHSTK PTE COW'ed: (R/O + DIRTY_HW)
> SHSTK PTE shared as R/O data: (R/O + DIRTY_SW)
>
> Update can_follow_write_pte/pmd for the shadow stack.

First of all, thanks for the excellent patch headers. It's nice to have
that reference every time even though it's repeated.

> -static inline bool can_follow_write_pte(pte_t pte, unsigned int flags)
> +static inline bool can_follow_write_pte(pte_t pte, unsigned int flags,
> + bool shstk)
> {
> + bool pte_cowed = shstk ? is_shstk_pte(pte):pte_dirty(pte);
> +
> return pte_write(pte) ||
> - ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_dirty(pte));
> + ((flags & FOLL_FORCE) && (flags & FOLL_COW) && pte_cowed);
> }

Can we just pass the VMA in here? This use is OK-ish, but I generally
detest true/false function arguments because you can't tell what they
are when they show up without a named variable.

But... Why does this even matter? Your own example showed that all
shadowstack PTEs have either DIRTY_HW or DIRTY_SW set, and pte_dirty()
checks both.

That makes this check seem a bit superfluous.

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