Messages in this thread | | | Date | Tue, 10 Dec 2013 23:57:33 +0100 (CET) | From | Thomas Gleixner <> | Subject | Re: process 'stuck' at exit. |
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On Tue, 10 Dec 2013, Linus Torvalds wrote:
> On Tue, Dec 10, 2013 at 2:19 PM, Linus Torvalds > <torvalds@linux-foundation.org> wrote: > > > > Shouldn't we do something like the attached? > > So I think that kernel/futex.c part of the patch might be a good idea, > but on x86-64 (which is what Dave is running), the > > if (end >> __VIRTUAL_MASK_SHIFT) > > test in get_user_pages_fast() should have been equivalent. And even on > 32-bit, we do check the _PAGE_USER bits in the page tables, so I guess > it's all good on a get_user_pages_fast() side. > > So never mind. It's not the address checking. > > And I think I see what's up. > > I think what happens is: > - get_user_pages_fast(address, 1, 1, &page) fails (because it's read-only) > - get_user_pages_fast(address, 1, 0, &page) succeeds and gets a large-page > - __get_user_pages_fast(address, 1, 1, &page) fails (because it's read-only). > > so what triggers this is likely that Dave now does large-pages, and > one of them is a read-only mapping. > > So I would suggest replacing the second "1" in the > __get_user_pages_fast() call with a "!ro" instead. So how about this > second patch instead (the access_ok() move remains).
Yes, that's what I decoded as well.
But how does the access_ok() move do anything helpful here?
We really need it for the fastpath !fshared case, but for the fshared case you actively break working code, because you force a VERIFY_WRITE check into it. The VERIFY_WRITE is necessary for !fshared, because there is no way that one thread can map the futex RO and the other RW, right?
But for fshared it's legitimate to have a RO mapping if you just wait for the futex. Note, that futexes are (ab)used as user space waitqueues, so RO makes sense. And your move would break them.
If [__]get_user_pages_fast() does not do the right checks, then we need to fix that and not add random access_ok() checks into the call sites.
Thanks,
tglx
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