Re: [PATCH 3/4] powerpc: Reduce ELF_ET_DYN_BASE

From	Michael Ellerman <mpe@ellerman.id.au>
Newsgroups	linux.kernel
Subject	Re: [PATCH 3/4] powerpc: Reduce ELF_ET_DYN_BASE
Date	2017-06-26 15:10 +0200
Message-ID	<tWBWa-2LF-5@gated-at.bofh.it> (permalink)
References	<tVeIi-8fG-11@gated-at.bofh.it> <tVeIi-8fG-9@gated-at.bofh.it> <tVqT8-7uy-7@gated-at.bofh.it> <tVD3X-6I5-11@gated-at.bofh.it>
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Kees Cook <keescook@chromium.org> writes:

> On Fri, Jun 23, 2017 at 12:01 AM, Michael Ellerman <mpe@ellerman.id.au> wrote:
>> Kees Cook <keescook@chromium.org> writes:
>>
>>> Now that explicitly executed loaders are loaded in the mmap region,
>>> position PIE binaries lower in the address space to avoid possible
>>> collisions with mmap or stack regions. For 64-bit, align to 4GB to
>>> allow runtimes to use the entire 32-bit address space for 32-bit
>>> pointers.
>>
>> The change log and subject are a bit out of whack with the actual patch
>> because previously we used 512MB.
>>
>> How about?
>>
>>   powerpc: Move ELF_ET_DYN_BASE to 4GB / 4MB
>>
>>   Now that explicitly executed loaders are loaded in the mmap region,
>>   we have more freedom to decide where we position PIE binaries in the
>>   address space to avoid possible collisions with mmap or stack regions.
>>
>>   For 64-bit, align to 4GB to allow runtimes to use the entire 32-bit
>>   address space for 32-bit pointers. On 32-bit use 4MB.
>
> Good idea, thanks. I'll resend the series with the commit logs updated.
>
>> Is there any particular reasoning behind the 4MB value on 32-bit?
>
> So, I've dug around a bit on this, and I *think* the rationale is to
> avoid mapping a possible 4MB page table entry when it won't be using
> at least a portion near the lower end (NULL address area covered
> blocked by mmap_min_addr). It seems to be mainly tradition, though.

OK, that is obscure, especially for CPUs that don't have a 4MB page
size. But consistency across arches is probably best regardless.

cheers

Thread

[PATCH 3/4] powerpc: Reduce ELF_ET_DYN_BASE Kees Cook <keescook@chromium.org> - 2017-06-22 20:10 +0200
  Re: [PATCH 3/4] powerpc: Reduce ELF_ET_DYN_BASE Michael Ellerman <mpe@ellerman.id.au> - 2017-06-23 09:10 +0200
    Re: [PATCH 3/4] powerpc: Reduce ELF_ET_DYN_BASE Kees Cook <keescook@chromium.org> - 2017-06-23 22:10 +0200
      Re: [PATCH 3/4] powerpc: Reduce ELF_ET_DYN_BASE Michael Ellerman <mpe@ellerman.id.au> - 2017-06-26 15:10 +0200
        Re: [PATCH 3/4] powerpc: Reduce ELF_ET_DYN_BASE Kees Cook <keescook@chromium.org> - 2017-06-26 20:30 +0200

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