linux-kernel - Re: [PATCH v2 0/2] x86: Relocate the compat vdso per process

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Date:	Wed, 12 Mar 2014 10:32:06 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>
CC:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"H. Peter Anvin" <hpa@...ux.intel.com>, X86 ML <x86@...nel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Dave Jones <davej@...hat.com>,
	Martin Runge <Martin.Runge@...de-schwarz.com>,
	Andi Kleen <andi@...stfloor.org>,
	Stefani Seibold <stefani@...bold.net>,
	Andreas Brief <Andreas.Brief@...de-schwarz.com>
Subject: Re: [PATCH v2 0/2] x86: Relocate the compat vdso per process

In taking about 1 here.  

On March 12, 2014 9:20:50 AM PDT, Andy Lutomirski <luto@...capital.net> wrote:
>On Tue, Mar 11, 2014 at 11:14 PM, H. Peter Anvin <hpa@...or.com> wrote:
>> There is one sensible address: end of address space perhaps minus
>some small offset.  Unlikely to be used by anything specific.
>
>Meh.  The compat vdso logic will end up being either:
>
>1) Try to map at the preferred address.  If it fails, don't map it.
>Force ASLR off for the vdso
>
>2) Try to map at the preferred address.  If it fails, map somewhere
>else and relocate.
>
>3) Try to map at the preferred address unless ASLR is on.  If it fails
>or ASLR is on, map somewhere else and relocate.
>
>since the whole point is to simplify this stuff, I don't really like
>any of these choices.  That's a lot of code to save 4k per process,
>and when it breaks (which I suspect it will the next time anyone
>touches the address space layout) so one will notice that 4k gets
>wasted again.
>
>>
>> On March 11, 2014 11:09:11 PM PDT, Andy Lutomirski
><luto@...capital.net> wrote:
>>>On Mar 11, 2014 10:02 PM, "H. Peter Anvin" <hpa@...or.com> wrote:
>>>>
>>>> On 03/11/2014 03:15 PM, Andy Lutomirski wrote:
>>>>>
>>>>> The meat of this patch series is in patch 1.  Patch 2 is split out
>>>for
>>>>> improved bisectability.
>>>>>
>>>>> Changes from v1: Split into two patches and fixed a comment.
>>>>>
>>>>> Andy Lutomirski (2):
>>>>>    x86: Dynamically relocate the compat vdso
>>>>>    x86_32: Remove user bit from identity map PDE
>>>>>
>>>>>   Documentation/kernel-parameters.txt  |  18 +++-
>>>>>   arch/x86/Kconfig                     |  24 +++--
>>>>>   arch/x86/include/asm/elf.h           |   4 -
>>>>>   arch/x86/include/asm/fixmap.h        |   8 --
>>>>>   arch/x86/include/asm/pgtable_types.h |   7 +-
>>>>>   arch/x86/include/asm/vdso.h          |   5 +-
>>>>>   arch/x86/vdso/vdso-layout.lds.S      |   2 +-
>>>>>   arch/x86/vdso/vdso32-setup.c         | 173
>>>++++++++++++++++-------------------
>>>>>   arch/x86/vdso/vdso32/vdso32.lds.S    |   2 -
>>>>>   9 files changed, 111 insertions(+), 132 deletions(-)
>>>>>
>>>>
>>>> Why per process?  *If* we have compat vdso turned on, can't we just
>>>put it in one place system-wide?
>>>
>>>My machine has ASLR turned on, and the vma really does end up
>>>somewhere different each time.  I suspect that most (?) ASLR users
>>>would happily accept the 4k hit to keep ASLR working fully.
>>>
>>>Another downside of trying to have a common address for all
>processes:
>>>someone needs to figure out what that address is.  What happens if an
>>>ELF binary (or interpreter) wants to load something at the chosen
>>>address?
>>>
>>>--Andy
>>
>> --
>> Sent from my mobile phone.  Please pardon brevity and lack of
>formatting.

-- 
Sent from my mobile phone.  Please pardon brevity and lack of formatting.
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