Re: [PATCH v2 7/11] Uprobes Implementation

[Srikar Dronamraju <srikar@linux.vnet.ibm.com>]

* Peter Zijlstra <peterz@infradead.org> [2010-05-11 22:43:23]:

> 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.

Did you mean "We can hold mmap_sem?" Else I am not sure if we can
traverse the vma. Infact alloc_page_vma() needs mmap_sem to be acquired.
Please clarify?

> 
> 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.
> 

Okay.

--
Thanks and Regards
Srikar