linux-kernel - Re: [PATCH v2] ELF: supply userspace with available page shifts (AT_PAGE_SHIFT

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Message-ID: <bb7fdb53-6917-44aa-8118-a8b357583c50@p183>
Date:   Thu, 7 Dec 2023 18:04:09 +0300
From:   Alexey Dobriyan <adobriyan@...il.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Florian Weimer <fweimer@...hat.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org, linux-arch@...r.kernel.org,
        linux-api@...r.kernel.org, x86@...nel.org
Subject: Re: [PATCH v2] ELF: supply userspace with available page shifts
 (AT_PAGE_SHIFT_MASK)

On Wed, Dec 06, 2023 at 01:09:01PM -0800, Kees Cook wrote:
> On Wed, Dec 06, 2023 at 10:05:36PM +0100, Florian Weimer wrote:
> > * Kees Cook:
> > 
> > > On Tue, Dec 05, 2023 at 07:01:34PM +0300, Alexey Dobriyan wrote:
> > >> Report available page shifts in arch independent manner, so that
> > >> userspace developers won't have to parse /proc/cpuinfo hunting
> > >> for arch specific strings:
> > >> 
> > >> Note!
> > >> 
> > >> This is strictly for userspace, if some page size is shutdown due
> > >> to kernel command line option or CPU bug workaround, than is must not
> > >> be reported in aux vector!
> > >
> > > Given Florian in CC, I assume this is something glibc would like to be
> > > using? Please mention this in the commit log.
> > 
> > Nope, I just wrote a random drive-by comment on the first version.
> 
> Ah, okay. Then Alexey, who do you expect to be the consumer of this new
> AT value?

libhugetlbfs and everyone who is using 2 MiB pages.

New code should look like this:

	#ifndef AT_PAGE_SHIFT_MASK
	#define AT_PAGE_SHIFT_MASK 29
	#endif
	unsigned long val = getauxval(AT_PAGE_SHIFT_MASK);
	if (val) {
		g_page_size_2mib = val & (1UL << 21);
		return;
	}
	// old 2 MiB page detection code

It is few lines of fast code before code they're already using.