Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu_id

From: Peter Zijlstra
Date: Mon Apr 23 2012 - 03:41:12 EST

On Mon, 2012-04-23 at 12:54 +0530, Srikar Dronamraju wrote:
> * Peter Zijlstra <peterz@xxxxxxxxxxxxx> [2012-04-23 09:14:00]:
> > On Fri, 2012-04-20 at 20:37 +0200, Oleg Nesterov wrote:
> > > Say, a user wants to probe /sbin/init only. What if init forks?
> > > We should remove breakpoints from child->mm somehow.
> >
> > How is that hard? dup_mmap() only copies the VMAs, this doesn't actually
> > copy the breakpoint. So the child doesn't have a breakpoint to be
> > removed.
> >
> Because the pages are COWED, the breakpoint gets copied over to the
> child. If we dont want the breakpoints to be not visible to the child,
> then we would have to remove them explicitly based on the filter (i.e if
> and if we had inserted breakpoints conditionally based on filter).

I thought we didn't COW shared maps since the fault handler will fill in
the pages right and only anon stuff gets copied.

> Once we add the conditional breakpoint insertion (which is tricky),

How so?

> we have
> to support conditional breakpoint removal in the dup_mmap() thro the
> uprobe_mmap hook (which I think is not that hard). Conditional removal
> of breakpoints in fork path would just be an extension of the
> conditional breakpoint insertion.

Right, I don't think that removal is particularly hard if needed.
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