Re: [PATCH] x86/vdso: Add prctl to set per-process VDSO load

From: Andy Lutomirski
Date: Fri Sep 19 2014 - 15:28:13 EST

On Wed, Sep 17, 2014 at 7:28 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> On Sep 17, 2014 1:46 AM, "H. Peter Anvin" <hpa@xxxxxxxxx> wrote:
>> On 09/16/2014 11:21 PM, Filipe Brandenburger wrote:
>> > Hi Andy,
>> >
>> > On Tue, Sep 16, 2014 at 10:00 PM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>> >> I think that the patch should instead tweak the vvar mapping to tell
>> >> the vdso not to use rdtsc. It should be based on this:
>> >
>> > I've been working on this approach which extends the vvar from 2 to 3
>> > pages. The third page would initially be mapped to a zero page but
>> > then through a prctl a task could replace it with a real page that
>> > could then be inherited through fork and exec.
>> >
>> > That would make it possible to have per-task vvar contents.
>> >
>> > We could use some of those values as flags to indicate whether vdso
>> > routines may use RDTSC or not.
>> >
>> > In the future, we're planning to also use that to store clock offsets
>> > so that we can ensure CLOCK_MONOTONIC works after CRIU migration
>> > without having to turn off the VDSO or have to always fallback to full
>> > syscalls on every case.
>> >
>> > Do you think that would be a reasonable way to accomplish that?
>> >
>> Why would we need/want per process vvar contents? It seems better to
>> have the code swapped out.
> That seems messier from a build perspective. Also, if we ever want to
> switch this dynamically, swapping out data is much easier than
> swapping out code. I think we should be able to replace the vvar page
> with the zero page, though.
> One tricky bit: currently we can only easily do this on exec, but we
> should be able to do it immediately if we start tracking mremap of the
> vdso. Should we make that a prerequisite? I don't really want this
> to end up being permanently weird.

I have this (special mapping tracking) 3/4 implemented. I'm planning
on making it fully functional for 64-bit programs and almost correct
for 32-bit. (You'll still crash if you have multiple threads, you use
sysenter, and you remap the vdso, but I think that this is essentially
unavoidable until someone lets mremap work on multiple vmas at once.)

> As for an actual post-migration offset, I'd rather add support for
> per-mm forced syscall fallback and then get something into the code
> timing code before even thinking about an x86 vdso fast path. I don't
> think that a feature like per-mm timing offsets should happen as an
> arch-specific thing first.
> --Andy
>> -hpa

Andy Lutomirski
AMA Capital Management, LLC
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