Re: futex() on vdso makes process unkillable

[Darren Hart]

KOSAKI Motohiro wrote:
> > Peter Zijlstra wrote:
> > > On Mon, 2010-01-25 at 16:27 +0900, KOSAKI Motohiro wrote:
> > <snip>
> > > > Futex should work both file anon anon. however I personally think 
> > > > vdso is not file nor anon. it is special mappings. nobody defined
> > > > futex spec on special mappings. (yes, undefined).
> > > >
> > > > Personally, I think EINVAL or EFAULT are best result of vdso futexing, like as
> > > > futexing againt kernel address. but I guess another person have another thinking.
> > > >
> > > > I'd like to hear futex folks's opinion.
> > > Well, my opinion is we should remove the vdso, its ugly as hell :-)
> > >
> > > But I think it would make most sense to extend its definition in the
> > > direction of it being a file (for all intents and purposes its a special
> > > DSO -- which unfortunately isn't present in any filesystem).
> > >
> > > [ For all intents and purposes processes can already communicate through
> > > futexes on the libc space, so being able to do so through the vsdo
> > > really doesn't add anything ]
> > >
> > > So the problem is that the VDSO pages do not have a page->mapping
> > > because they lack the actual filesystem part of files, so even if (with
> > > the recent zero-page patch from Kosaki-san) you make private COWs of the
> > > VDSO, you'll get stuck in that loop.
> > >
> > > So the prettiest solution is to simply place the vdso in an actual
> > > filesystem and slowly migrate towards letting userspace map it as a
> > > regular DSO -- /sys/lib{32,64}/libkernel.so like.
> > >
> > > [ that has the bonus of getting rid of install_special_mapping() ]
> > >
> > > The ugly solution is special casing the vdso in get_futex_key().
> > I like the creating-a-real-file solution. However, for now (and for 
> > stable), I think Kosaki's suggestion of EINVAL or EFAULT is a good 
> > stop-gap. EINVAL might play the best with existing glibc implementations.
>
> May I confirm your mention?
>
> If we can accept EFAULT, we don't need any change. my previous futex patch
> already did. because 1) VDSO is alwasys read-only mapped 2) write mode
> get_user_pages_fast() against read-only pte/vma return EFAULT.
>
> Current linus and stable tree don't cause Mark's original problem. instead, just
> return EFAULT. (Well, I'm sorry. my previous mail was unclear. I wrote v2.6.31 test
> result)
>
> If you can't accept EFAULT, we need to add vdso specific logic into get_futex_key().
> Is this your intention?

That was my intention, but after looking at the glibc source, I don't 
see any reason for EINVAL over EFAULT. I apparently mis-remembered 
something there. EFAULT is fine.

--
Darren Hart
IBM Linux Technology Center
Real-Time Linux Team
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