Messages in this thread | | | Date | Tue, 26 Jan 2010 06:21:46 -0800 | From | Darren Hart <> | Subject | Re: futex() on vdso makes process unkillable |
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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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