[PATCH 3/4] powerpc: Reduce ELF_ET_DYN_BASE

[Michael Ellerman]

Kees Cook <keescook at chromium.org> writes:

> On Fri, Jun 23, 2017 at 12:01 AM, Michael Ellerman <mpe at ellerman.id.au> wrote:
>> Kees Cook <keescook at 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