linux-kernel - Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu

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Date:	Sun, 15 Apr 2012 21:53:51 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Ingo Molnar <mingo@...e.hu>,
	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Anton Arapov <anton@...hat.com>
Subject: Re: [RFC 0/6] uprobes: kill uprobes_srcu/uprobe_srcu_id

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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