Re: [PATCH 5/8] x86/arch_prctl/64: Remove FSBASE/GSBASE < 4G optimization

[Andy Lutomirski]

On Tue, Apr 26, 2016 at 1:50 PM, Andi Kleen <andi@xxxxxxxxxxxxxx> wrote:
> On Tue, Apr 26, 2016 at 12:23:28PM -0700, Andy Lutomirski wrote:
>> As far as I know, the optimization doesn't work on any modern distro
>> because modern distros use high addresses for ASLR.  Remove it.
>
> I disagree with this patch. For example it will be a regression
> for static executables.  And randomly making old systems slower is a bad
> idea.

That's odd.  statically linked glibc uses low addresses, even in PIE
mode.  I wonder why.

In any event, this isn't actually much of a performance regression, it
has no effect on normal dynamically linked programs, and it's a
considerably simplification, so I still think it's a good idea.  It
also removes some nasty special cases from code that is already way
too full of special cases for comfort.

--Andy