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

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Message-ID: <87edfzavof.fsf@oldenburg.str.redhat.com>
Date:   Wed, 06 Dec 2023 22:05:36 +0100
From:   Florian Weimer <fweimer@...hat.com>
To:     Kees Cook <keescook@...omium.org>
Cc:     Alexey Dobriyan <adobriyan@...il.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)

* 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.

>> x86_64 machine with 1 GiB pages:
>> 
>> 	00000030  06 00 00 00 00 00 00 00  00 10 00 00 00 00 00 00
>> 	00000040  1d 00 00 00 00 00 00 00  00 10 20 40 00 00 00 00
>> 
>> x86_64 machine with 2 MiB pages only:
>> 
>> 	00000030  06 00 00 00 00 00 00 00  00 10 00 00 00 00 00 00
>> 	00000040  1d 00 00 00 00 00 00 00  00 10 20 00 00 00 00 00
>> 
>> AT_PAGESZ is always 4096 which is not that interesting.
>
> That's not always true. For example, see arm64:
> arch/arm64/include/asm/elf.h:#define ELF_EXEC_PAGESIZE  PAGE_SIZE

I'm pretty sure the comment refers to the x86-64 situation. 8-)

> I'm not actually sure why x86 forces it to 4096. I'd need to go look
> through the history there.

On x86-64, page size 4096 is architectural.  Likewise on s390x and a few
other architectures.

Thanks,
Florian