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

On Fri, Sep 19, 2014 at 8:10 PM, Filipe Brandenburger
[off-list ref] wrote:

Hi Andy,

On Fri, Sep 19, 2014 at 3:31 PM, Andy Lutomirski [off-list ref] wrote:

quoted

quoted

Sorry if I wasn't clear... That's the exact point of my patch, to make
vm_special_mapping local to the task.

I started with an approach of keeping a struct vm_special_mapping + a
struct page * array per mm_struct.

I was also looking at keeping two static vm_special_mapping structs,
one with the actual vvar page and the other with the zero page and
then swapping vma->vm_private_data to point to the appropriate one.

This sounds like it may be more complicated than necessary.  Is there
any reason that just doing remap_pfn_range on the vvar page isn't
enough?

I thought of doing that from the prctl but AFAICT remap_pfn_range
requires that it's unmapped before the call (remap_pte_range has
BUG_ON(!pte_none(*pte));) and doing a zap_page_range followed by
remap_pfn_range might incur a race condition if another thread of the
same process is accessing the vvar page at that time... Am I wrong
about that race?

No, you're right.  Ugh.

It might pay to add an explicit .fault callback to vm_special_mapping,
but your approach will work, too.  The main downside is more memory
overhead per mm.

quoted

Changing out the text is a whole can of worms involving self-modifying
code, although it may be completely safe if done through the page
tables.  But I don't think you can't use remap_pfn_range for that.

No, I'm not planning to change the text, just replacing the vvar page
swapping the one where the vsyscall_gtod_data lives with a zero page
(and back depending on the parameter of the prctl.)

Hmm.  This will break the _COARSE timing functions as well as
__vdso_time.  That'll need fixing, either by swapping out the code
(yuck!) or by adding a branch to all of those code paths.

Maybe there's a non-branchy way, though.  Let me think.

quoted

Maybe better: what exactly are you trying to do?

Just replace the page mapped to the vvar address.

But nevermind my ramblings here, looks like you're almost there so go
ahead and when you have a working patch I'll take a look at it.

See:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=vdso/vma_tracking

My offer to help still stands, so let me know if you'd like to see
some of my code (unfinished as it is now or if you'd like me to show
you something that works in a few days.)

Cheers,
Filipe

-- 
Andy Lutomirski
AMA Capital Management, LLC