linux-kernel - Re: [PATCH 19/35] mm/mmap: Add shadow stack pages to memory accounting

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Date:   Wed, 9 Feb 2022 14:27:53 -0800
From:   Dave Hansen <dave.hansen@...el.com>
To:     Rick Edgecombe <rick.p.edgecombe@...el.com>, x86@...nel.org,
        "H . Peter Anvin" <hpa@...or.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Andy Lutomirski <luto@...nel.org>,
        Balbir Singh <bsingharora@...il.com>,
        Borislav Petkov <bp@...en8.de>,
        Cyrill Gorcunov <gorcunov@...il.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Eugene Syromiatnikov <esyr@...hat.com>,
        Florian Weimer <fweimer@...hat.com>,
        "H . J . Lu" <hjl.tools@...il.com>, Jann Horn <jannh@...gle.com>,
        Jonathan Corbet <corbet@....net>,
        Kees Cook <keescook@...omium.org>,
        Mike Kravetz <mike.kravetz@...cle.com>,
        Nadav Amit <nadav.amit@...il.com>,
        Oleg Nesterov <oleg@...hat.com>, Pavel Machek <pavel@....cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        "Ravi V . Shankar" <ravi.v.shankar@...el.com>,
        Dave Martin <Dave.Martin@....com>,
        Weijiang Yang <weijiang.yang@...el.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        joao.moreira@...el.com, John Allen <john.allen@....com>,
        kcc@...gle.com, eranian@...gle.com
Cc:     Yu-cheng Yu <yu-cheng.yu@...el.com>
Subject: Re: [PATCH 19/35] mm/mmap: Add shadow stack pages to memory
 accounting

On 1/30/22 13:18, Rick Edgecombe wrote:
> +bool is_shadow_stack_mapping(vm_flags_t vm_flags)
> +{
> +	return vm_flags & VM_SHADOW_STACK;
> +}
> diff --git a/include/linux/pgtable.h b/include/linux/pgtable.h
> index bc8713a76e03..21fdb1273571 100644
> --- a/include/linux/pgtable.h
> +++ b/include/linux/pgtable.h
> @@ -911,6 +911,14 @@ static inline void ptep_modify_prot_commit(struct vm_area_struct *vma,
>  	__ptep_modify_prot_commit(vma, addr, ptep, pte);
>  }
>  #endif /* __HAVE_ARCH_PTEP_MODIFY_PROT_TRANSACTION */
> +
> +#ifndef is_shadow_stack_mapping
> +static inline bool is_shadow_stack_mapping(vm_flags_t vm_flags)
> +{
> +	return false;
> +}
> +#endif

Hold your horses there.  Remember:

+#ifdef CONFIG_X86_SHADOW_STACK
+# define VM_SHADOW_STACK       VM_HIGH_ARCH_5
+#else
+# define VM_SHADOW_STACK       VM_NONE
+#endif

Plus:

#define VM_NONE         0x00000000

That means the arch-generic version, when CONFIG_X86_SHADOW_STACK is off
compiles down to:

bool is_shadow_stack_mapping(vm_flags_t vm_flags)
{
	return vm_flags & 0x00000000;
}

I _suspect_ the compiler *might* compile that down to the same thing as:

	return false;

So, why not just have one version, no additional #ifdefs, and be done
with it?  Heck, why have the helper in the first place?  Just check
VM_SHADOW_STACK directly.