linux-kernel - Re: [PATCH 5/8] x86/arch_prctl/64: Remove FSBASE/GSBASE

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Message-ID: <CALCETrU_ipRP6baNTpCjrAXvhhk7=8oWzZzvwTqkJc4JrMBJZA@mail.gmail.com>
Date:	Tue, 26 Apr 2016 15:33:30 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Andy Lutomirski <luto@...nel.org>, X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...en8.de>, Brian Gerst <brgerst@...il.com>
Subject: Re: [PATCH 5/8] x86/arch_prctl/64: Remove FSBASE/GSBASE < 4G optimization

On Tue, Apr 26, 2016 at 1:50 PM, Andi Kleen <andi@...stfloor.org> 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