Messages in this thread | | | Subject | Re: [PATCH v3] futex: Fix regression with read only mappings | From | Peter Zijlstra <> | Date | Tue, 28 Jun 2011 12:54:35 +0200 |
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On Mon, 2011-06-27 at 17:22 -0500, Shawn Bohrer wrote: > commit 7485d0d3758e8e6491a5c9468114e74dc050785d (futexes: Remove rw > parameter from get_futex_key()) in 2.6.33 introduced a user-mode > regression in that it additionally prevented futex operations on a > region within a read only memory mapped file. For example this breaks > workloads that have one or more reader processes doing a FUTEX_WAIT on a > futex within a read only shared mapping, and a writer processes that has > a writable mapping issuing the FUTEX_WAKE. > > This fixes the regression for futex operations that should be valid on > RO mappings by trying a RO get_user_pages_fast() when the RW > get_user_pages_fast() fails so as not to slow down the common path of > writable anonymous maps and bailing when we used the RO path on > anonymous memory. > > While fixing the regression this patch opens up two possible bad > scenarios as identified by KOSAKI Motohiro: > > 1) This patch also allows FUTEX_WAIT on RO private mappings which have > the following corner case. > > Thread-A: call futex(FUTEX_WAIT, memory-region-A). > get_futex_key() return inode based key. > sleep on the key > Thread-B: call mprotect(PROT_READ|PROT_WRITE, memory-region-A) > Thread-B: write memory-region-A. > COW happen. This process's memory-region-A become related > to new COWed private (ie PageAnon=1) page. > Thread-B: call futex(FUETX_WAKE, memory-region-A). > get_futex_key() return mm based key. > IOW, we fail to wake up Thread-A. > > 2) Current futex code doesn't handle zero page properly. > > Read mode get_user_pages() can return zero page, but current futex > code doesn't handle it at all. Then, zero page makes infinite loop > internally. > > This Patch is based on Peter Zijlstra's initial patch with modifications to > only allow RO mappings for futex operations that need VERIFY_READ access. > > Reported-by: David Oliver <david@rgmadvisors.com> > Signed-off-by: Shawn Bohrer <sbohrer@rgmadvisors.com>
Acked-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
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