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Date:   Fri, 14 Oct 2022 17:32:38 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Rick Edgecombe <rick.p.edgecombe@intel.com>
Cc:     x86@kernel.org, "H . Peter Anvin" <hpa@zytor.com>,
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Subject: Re: [PATCH v2 16/39] x86/mm: Update maybe_mkwrite() for shadow stack
Message-ID: <Y0mBFjfOAA+hJcUn@hirez.programming.kicks-ass.net>
References: <20220929222936.14584-1-rick.p.edgecombe@intel.com>
 <20220929222936.14584-17-rick.p.edgecombe@intel.com>
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On Thu, Sep 29, 2022 at 03:29:13PM -0700, Rick Edgecombe wrote:

> diff --git a/include/linux/mm.h b/include/linux/mm.h
> index 8cd413c5a329..fef14ab3abcb 100644
> --- a/include/linux/mm.h
> +++ b/include/linux/mm.h
> @@ -981,13 +981,25 @@ void free_compound_page(struct page *page);
>   * servicing faults for write access.  In the normal case, do always want
>   * pte_mkwrite.  But get_user_pages can cause write faults for mappings
>   * that do not have writing enabled, when used by access_process_vm.
> + *
> + * If a vma is shadow stack (a type of writable memory), mark the pte shadow
> + * stack.
>   */
> +#ifndef maybe_mkwrite
>  static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
>  {
> -	if (likely(vma->vm_flags & VM_WRITE))
> +	if (!(vma->vm_flags & VM_WRITE))
> +		goto out;
> +
> +	if (vma->vm_flags & VM_SHADOW_STACK)
> +		pte = pte_mkwrite_shstk(pte);
> +	else
>  		pte = pte_mkwrite(pte);
> +
> +out:
>  	return pte;
>  }
> +#endif

Why the #ifndef guard? There is no other implementation, nor does this
patch introduce one.

Also, wouldn't it be simpler to write it like:

static inline pte_t maybe_mkwrite(pte_t pte, struct vm_area_struct *vma)
{
	if (!(vma->vm_flags & VM_WRITE))
		return pte;

	if (vma->vm_flags & VM_SHADOW_STACK)
		return pte_mkwrite_shstk(pte);

	return pte_mkwrite(pte);
}

? (idem for the pmd version etc..)