Messages in this thread | | | Subject | Re: [PATCH v2 7/11] Uprobes Implementation | From | Peter Zijlstra <> | Date | Tue, 11 May 2010 22:43:23 +0200 |
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On Thu, 2010-04-22 at 19:01 +0530, Srikar Dronamraju wrote: > * Oleg Nesterov <oleg@redhat.com> [2010-04-21 18:05:15]:
> > 4. mremap(). What if the application does mremap() and moves the > > memory? After that vaddr of user_bkpt/uprobe no longer matches > > the virtual address of bp. This breaks uprobe_bkpt_notifier(), > > unregister_uprobe(), etc. > > > > Even worse. Say, unregister_uprobe() calls remove_bkpt(). > > mremap()+mmap() can be called after ->read_opcode() verifies vaddr > > points to bkpt_insn, but before write_opcode() changes the page. > > > > I dont think we handle this case now. I think even munmap of the region > where there are probes inserted also can have the same problem. > > Are there ways to handle this. > I think taking a write lock on mmap_sem instead of the read lock could > handle this problem. > > I am copying Mel Gorman and Andrea Arcangeli so that they can provide > their inputs on VM and KSM related issues.
KSM only does anonymous pages, and I thought uprobes was limited to MAP_PRIVATE|PROT_EXEC file maps.
We can't hold mmap_sem (for either read or write -- read would be sufficient to serialize against mmap/mremap/munmap) from atomic uprobe context, what we can do is validate that there is a INT3 on that particular address, a mremap/munmap/munmap+mmap will either end not having a pte entry for the address, or not have the INT3.
That said, you shouldn't be executing code on maps you're changing, much fun can happen if you try, so I don't think we should expend too much effort as long as the race will only result in the app crashing and not the kernel.
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