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

[Oleg Nesterov <oleg@redhat.com>]

On 04/15, Peter Zijlstra wrote:
>
> On Sat, 2012-04-14 at 22:52 +0200, Oleg Nesterov wrote:
> > > >     - can it work or I missed something "in general" ?
> > >
> > > So we insert in the rb-tree before we take mmap_sem, this means we can
> > > hit a non-uprobe int3 and still find a uprobe there, no?
> >
> > Yes, but unless I miss something this is "off-topic", this
> > can happen with or without these changes. If find_uprobe()
> > succeeds we assume that this bp was inserted by uprobe.
>
> OK, but then I completely missed what the point of that
> down_write() stuff is..

To ensure handle_swbp() can't race with unregister + register
and send the wrong SIGTRAP.

handle_swbp() roughly does under down_read(mmap_sem)


	if (find_uprobe(vaddr))
		process_uprobe();
	else
	if (is_swbp_at_addr_fast(vaddr))	// non-uprobe int3
		send_sig(SIGTRAP);
	else
		restart_insn(vaddr);		// raced with unregister


note that is_swbp_at_addr_fast() is used (currently) to detect
the race with upbrobe_unregister() and that is why we can remove
uprobes_srcu.

But if find_uprobe() fails, there is a window before
is_swbp_at_addr_fast() reads the memory. Suppose that the next
uprobe_register() inserts the new uprobe at the same address.
In this case the task will be wrongly killed.

Oleg.

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