linux-kernel - Re: [PATCH v4] ELF: AT_PAGE_SHIFT_MASK -- supply userspace with available page shifts

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Message-ID: <202402131138.2CFC8CD5E@keescook>
Date: Tue, 13 Feb 2024 11:39:30 -0800
From: Kees Cook <keescook@...omium.org>
To: Alexey Dobriyan <adobriyan@...il.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Florian Weimer <fweimer@...hat.com>, linux-kernel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-api@...r.kernel.org,
	x86@...nel.org, Eric Biederman <ebiederm@...ssion.com>,
	linux-mm@...ck.org
Subject: Re: [PATCH v4] ELF: AT_PAGE_SHIFT_MASK -- supply userspace with
 available page shifts

On Tue, Feb 13, 2024 at 09:51:01PM +0300, Alexey Dobriyan wrote:
> On Fri, Feb 09, 2024 at 04:41:36PM -0800, Kees Cook wrote:
> > On Fri, Feb 09, 2024 at 03:30:37PM +0300, Alexey Dobriyan wrote:
> > > On Mon, Feb 05, 2024 at 04:48:08AM -0800, Kees Cook wrote:
> > > > On Mon, Feb 05, 2024 at 12:51:43PM +0300, Alexey Dobriyan wrote:
> > > > > +#define ARCH_AT_PAGE_SHIFT_MASK					\
> > > > > +	do {							\
> > > > > +		u32 val = 1 << 12;				\
> > > > > +		if (boot_cpu_has(X86_FEATURE_PSE)) {		\
> > > > > +			val |= 1 << 21;				\
> > > > > +		}						\
> > > > > +		if (boot_cpu_has(X86_FEATURE_GBPAGES)) {	\
> > > > > +			val |= 1 << 30;				\
> > > > > +		}						\
> > > > 
> > > > Can we use something besides literal "12", "21", and "30" values here?
> > > 
> > > Ehh, no, why? Inside x86_64 the page shifts are very specific numbers,
> > > they won't change.
> > 
> > Well, it's nicer to have meaningful words to describe these things.
> 
> Not really. Inside specific arch page shifts are fixed, so using names
> is just more macros one need to remember.
> 
> If I were to invent names (which I wouldn't), the best names are
> 
> 	PAGE_SHIFT
> 	PAGE_SHIFT2
> 	PAGE_SHIFT3
> 	...
> 
> with PAGE_SHIFT2, PAGE_SHIFT3 being optional macros if arch doesn't support
> multiple page sizes.
> 
> > In fact, PAGE_SHIFT already exists for 12, and HPAGE_SHIFT already exists
> > for 21. Please use those, and add another, perhaps GBPAGE_SHIFT, for 30.
> 
> HPAGE_SHIFT is bad name, H doesn't describe anything unless arch is
> known. Hugepages is marketing name. If GBPAGE_SHIFT is good name,
> then HPAGE_SHIFT is bad name, it should've been MBPAGE_SHIFT, which
> wrong because it is 2 MiB not 1 MiB.
> 
> BTW parisc has REAL_HPAGE_SHIFT !

Sure, I mean, we've got an x86-specific function here, so let's use the
x86-specific macros we already have for 12 and 21, and then add the
missing one for 30.

-- 
Kees Cook