Messages in this thread | | | Subject | Re: [PATCH v4 3.0-rc2-tip 2/22] 2: uprobes: Breakground page replacement. | From | Peter Zijlstra <> | Date | Tue, 14 Jun 2011 17:07:33 +0200 |
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On Tue, 2011-06-14 at 16:27 +0200, Oleg Nesterov wrote: > On 06/14, Peter Zijlstra wrote: > > > > On Mon, 2011-06-13 at 19:00 +0200, Oleg Nesterov wrote: > > > > > > Also. This is called under down_read(mmap_sem), can't we race with > > > access_process_vm() modifying the same memory? > > > > Shouldn't matter COW and similar things are serialized using the pte > > lock. > > Yes, but afaics this doesn't matter. Suppose that write_opcode() is > called when access_process_vm() does copy_to_user_page(). We can cow > the page before memcpy() completes.
access_process_vm() will end up doing a FOLL_WRITE itself when copy_to_user_page() is called since write=1 in that case.
At that point we have a COW-race, someone wins, but the other will then return the same page.
At this point further PTRACE pokes can indeed race with the memcpy in write_opcode(). A possible fix would be to lock_page() around copy_to_user_page() (its already done in set_page_dirty_lock(), so pulling it out shouldn't matter much).
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