Messages in this thread | | | Subject | Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu_id | From | Peter Zijlstra <> | Date | Mon, 23 Apr 2012 21:18:10 +0200 |
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On Mon, 2012-04-23 at 19:29 +0200, Oleg Nesterov wrote: > On 04/23, Peter Zijlstra wrote: > > > > On Mon, 2012-04-23 at 12:54 +0530, Srikar Dronamraju wrote: > > > * Peter Zijlstra <peterz@infradead.org> [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. > > Confused... > > Do you mean the "Don't copy ptes where a page fault will fill them correctly" > check in copy_page_range() ? Yes, but this vma should have ->anon_vma != NULL > if it has the breakpoint installed by uprobes.
Oh, argh yeah, we add an anon_vma there..
> > > Once we add the conditional breakpoint insertion (which is tricky), > > > > How so? > > I agree with Srikar this doesn't look simple to me. First of all, > currently it is not easy to find the tasks which use this ->mm. > OK, we can simply do for_each_process() under tasklist, but this is > not very nice. > > But again, to me this is not the main problem.
CLONE_VM without CLONE_THREAD is the problem, right?
Can we get away with not supporting that, at least initially?
> > > 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. > > I agree that remove_breakpoint() itself is not that hard, probably. > > But the whole idea of filtering is not clear to me. I mean, when/how > we should call the filter, and what should be the argument. > task_struct? Probably, but I am not sure.
Well, the idea is really very simple: if for a probe an {mm,tasks} set has all negative filters we do not install the probe on that mm.
The filters already take a uprobe_consumer and task_struct as argument.
> And btw fork()->dup_mmap() should call the filter too. Suppose that > uprobe_consumer wants to trace the task T and its children, this looks > very natural.
Agreed.
> And we need to rework uprobe_register(). It can't simply return if > this (inode, offset) already has the consumer.
Not quite sure what you mean. uprobe_register() doesn't have such a return value. It returns 0 on success and an error otherwise. Do you mean __uprobe_register() ? That calls register_for_each_vma() and that can simply call ->filter() for each vma it iterates. In fact, it can get away with only calling the filter for the new consumer.
> So far I think this needs more thinking. And imho we should merge the > working code Srikar already has, then try to add this (agreed, very > important) optimization.
Sure..
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