linux-kernel - Re: Fwd: uprobes: register/unregister probes.

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Date:	Thu, 24 Nov 2011 10:49:59 +0100
From:	Peter Zijlstra <peterz@...radead.org>
To:	Srikar Dronamraju <srikar@...ux.vnet.ibm.com>
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>,
	tulasidhard@...il.com
Subject: Re: Fwd: uprobes: register/unregister probes.

On Thu, 2011-11-24 at 12:33 +0530, Srikar Dronamraju wrote:
> > On Fri, 2011-11-18 at 16:37 +0530, Srikar Dronamraju wrote:
> > > +int register_uprobe(struct inode *inode, loff_t offset,
> > > +                               struct uprobe_consumer *consumer)
> > > +{
> > > +       struct uprobe *uprobe;
> > > +       int ret = -EINVAL;
> > > +
> > > +       if (!consumer || consumer->next)
> > > +               return ret;
> > > +
> > > +       inode = igrab(inode);
> > 
> > So why are you dealing with !consumer but not with !inode? and why
> > does
> > it make sense to allow !consumer at all?
> > 
> 
> 
> I am not sure if I got your comment correctly.
> 
> I do check for inode just after the igrab.

No you don't, you check the return value of igrab(), but you crash hard
when someone calls register_uprobe(.inode=NULL).

> I am actually not dealing with !consumer.
> If the consumer is NULL, then we dont have any handler to run so why
> would we want to register such a probe?

Why allow someone calling register_uprobe(.consumer=NULL) to begin with?
That doesn't make any sense.

> Also if consumer->next is Non-NULL, that means that this consumer was
> already used.  Reusing the consumer, can result in consumers list getting
> broken into two.

Yeah, although at that point why be nice about it? Just but a WARN_ON()
in or so.

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