linux-kernel - Re: [PATCHv3 05/16] pps: access pps device by direct pointer

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Message-ID: <20100805123147.GA19407@gundam.enneenne.com>
Date:	Thu, 5 Aug 2010 14:31:47 +0200
From:	Rodolfo Giometti <giometti@...eenne.com>
To:	Alexander Gordeev <lasaine@....cs.msu.su>
Cc:	linux-kernel@...r.kernel.org,
	"Nikita V. Youshchenko" <yoush@...msu.su>,
	linuxpps@...enneenne.com, john stultz <johnstul@...ibm.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Tejun Heo <tj@...nel.org>, Joonwoo Park <joonwpark81@...il.com>
Subject: Re: [PATCHv3 05/16] pps: access pps device by direct pointer

On Thu, Aug 05, 2010 at 03:42:31PM +0400, Alexander Gordeev wrote:
> ?? Thu, 5 Aug 2010 11:32:36 +0200
> Rodolfo Giometti <giometti@...eenne.com> ??????????:
> 
> > On Thu, Aug 05, 2010 at 01:06:42AM +0400, Alexander Gordeev wrote:
> > > Using device index as a pointer needs some unnecessary work to be done
> > > every time the pointer is needed (in irq handler for example).
> > > Using a direct pointer is much more easy (and safe as well).
> > > 
> > > Signed-off-by: Alexander Gordeev <lasaine@....cs.msu.su>
> [snip]
> > 
> > If you remove these functions you can't be sure anymore that nobodies
> > may call pps_event() over a non existent device...
> 
> [snip]
> 
> > By dropping pps_get_source you may be here by a call from (i.e.) a
> > serial port driver whose doesn't know if your PPS source is gone or
> > not...
> > 
> > I don't understand how your modifications may resolve this problem.
> 
> Well, this can happen only if PPS client module calls pps_event before
> calling pps_register_source() or after pps_unregister_source(). This
> means that it's broken! If we try to handle/workaround broken clients it

Suppose we are using pps-ldisc client. How can you assure that
nobodies may execute pps_tty_close() while you are into the
pps_event() related to the same serial port?

You can't disable serial interrupts in order to avoid
pps_tty_dcd_change calls...

> affects performance. So we have to choose what is the priority:
> security or performance. My guru told me I shouldn't bother too much
> about broken kernel-space code which my code interacts with. If it's
> broken it should be fixed. Some assertions enabled by DEBUG define are
> enough. For me it makes sense but I don't know what should I check here?

I'm sorry but I disagree with you. Kernel code can't allow userland
programs to corrupt it!

We are not discussing about security or performance but about
reliability.

Ciao,

Rodolfo

-- 

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Linux Device Driver                          giometti@...ux.it
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