linux-kernel - Re: [PATCH v6 1/3] x86/mm: PUD VA support for physical mapping (x86

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Date:	Tue, 21 Jun 2016 10:18:18 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Thomas Garnier <thgarnie@...gle.com>
Cc:	Kees Cook <keescook@...omium.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
	Borislav Petkov <bp@...e.de>, Juergen Gross <jgross@...e.com>,
	Matt Fleming <matt@...eblueprint.co.uk>,
	Toshi Kani <toshi.kani@....com>, Baoquan He <bhe@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Dan Williams <dan.j.williams@...el.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	"Aneesh Kumar K.V" <aneesh.kumar@...ux.vnet.ibm.com>,
	"Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
	Martin Schwidefsky <schwidefsky@...ibm.com>,
	Andy Lutomirski <luto@...nel.org>,
	Alexander Kuleshov <kuleshovmail@...il.com>,
	Alexander Popov <alpopov@...ecurity.com>,
	Joerg Roedel <jroedel@...e.de>, Dave Young <dyoung@...hat.com>,
	Lv Zheng <lv.zheng@...el.com>,
	Mark Salter <msalter@...hat.com>,
	Stephen Smalley <sds@...ho.nsa.gov>,
	Dmitry Vyukov <dvyukov@...gle.com>,
	Boris Ostrovsky <boris.ostrovsky@...cle.com>,
	David Rientjes <rientjes@...gle.com>,
	Christian Borntraeger <borntraeger@...ibm.com>,
	Jan Beulich <JBeulich@...e.com>,
	Kefeng Wang <wangkefeng.wang@...wei.com>,
	Seth Jennings <sjennings@...iantweb.net>,
	Yinghai Lu <yinghai@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v6 1/3] x86/mm: PUD VA support for physical mapping
 (x86_64)


* Thomas Garnier <thgarnie@...gle.com> wrote:

> > Secondly, and that's a new problem introduced by this patch:
> >
> >> +     int i = pud_index((unsigned long)__va(addr));
> >> +             pud_t *pud = pud_page + pud_index((unsigned long)__va(addr));
> >
> > ... beyond the repetition, using type casts is fragile. Type casts should be a red
> > flag to anyone involved in low level, security relevant code! So I'm pretty
> > unhappy about seeing such a problem in such a patch.
> >
> > This code should be doing something like:
> >
> >         unsigned long vaddr_start = __va(paddr_start);
> >
> > ... which gets rid of the type cast, the repetition and documents the code much
> > better as well.
> 
> Unfortunately, we can't do that because __va return a void*. We will
> get this warning on compile:
> 
> arch/x86/mm/init_64.c:537:8: warning: assignment makes integer from
> pointer without a cast [enabled by default]
>   vaddr = __va(paddr_start);
> 
> If we used void*, we would need to type cast even more places. What do
> you think?

Hm, indeed, you are right - so I guess the type cast is OK.

Thanks,

	Ingo