linux-kernel - Re: [PATCH v7 3.2-rc2 4/30] uprobes: Define hooks for mmap/munmap.

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Date:	Wed, 30 Nov 2011 11:00:44 +0530
From:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Linus Torvalds <torvalds@...ux-foundation.org>,
	Oleg Nesterov <oleg@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-mm <linux-mm@...ck.org>, Ingo Molnar <mingo@...e.hu>,
	Andi Kleen <andi@...stfloor.org>,
	Christoph Hellwig <hch@...radead.org>,
	Steven Rostedt <rostedt@...dmis.org>,
	Roland McGrath <roland@...k.frob.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Masami Hiramatsu <masami.hiramatsu.pt@...achi.com>,
	Arnaldo Carvalho de Melo <acme@...radead.org>,
	Anton Arapov <anton@...hat.com>,
	Ananth N Mavinakayanahalli <ananth@...ibm.com>,
	Jim Keniston <jkenisto@...ux.vnet.ibm.com>,
	Stephen Wilson <wilsons@...rt.ca>, tulasidhard@...il.com
Subject: Re: [PATCH v7 3.2-rc2 4/30] uprobes: Define hooks for mmap/munmap.

> 
> int mmap_uprobe(...) {
> ....
> 	       ret = install_breakpoint(vma->vm_mm, uprobe);
> 	       if (ret == -EEXIST) {
> 			if (!read_opcode(vma->vm_mm, vaddr, &opcode) &&
> 					(opcode == UPROBES_BKPT_INSN))
> 			       atomic_inc(&vma->vm_mm->mm_uprobes_count);
> 		       ret = 0;
> 	       } 
> ....
> }
> 

Infact the check for EEXIST and read_opcode in mmap_uprobe() is needed
for another reason too.

Lets say while unregister_uprobe was around, a thread thats being
probed, just forked a child and the child called mmap_uprobe.

Now mmap_uprobe might find that the breakpoint is already inserted
since the pages are shared with the parent. But before
unregister_uprobe can come around and cleanup, the child can run and hit
the breakpoint. Since the breakpoint count is 0 for the child, we dont
expect the child to have hit a breakpoint placed by uprobes, and the
child gets a SIGTRAP.

With this check for read_opcode on EEXIST from install_breakpoint, we
will know that there is a valid breakpoint underneath and increment
the count. So on a breakpoint hit, the uprobes notifier does the right
thing.

If the unregister_uprobe() had already cleanup the breakpoint in the
parent, the child's copy would also be clean so read_opcode wont find
the breakpoint and hence we wont increment the breakpoint.

-- 
Thanks and Regards
Srikar

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