Re: [PATCH v2 7/11] Uprobes Implementation

[Peter Zijlstra <peterz@infradead.org>]

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.