linux-kernel - Re: Generic DMA

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Message-Id: <201001202003.13353.arnd@arndb.de>
Date:	Wed, 20 Jan 2010 20:03:12 +0100
From:	Arnd Bergmann <arnd@...db.de>
To:	monstr@...str.eu
Cc:	Joerg Roedel <joerg.roedel@....com>, linux-kernel@...r.kernel.org,
	michal.simek@...alogix.com, linux-arch@...r.kernel.org,
	fujita.tomonori@....ntt.co.jp, akpm@...ux-foundation.org,
	mingo@...e.hu
Subject: Re: Generic DMA - BUG_ON

On Wednesday 20 January 2010, Michal Simek wrote:
> Joerg Roedel wrote:
> > On Wed, Jan 20, 2010 at 10:53:50AM +0000, Russell King wrote:
> >> and ops is NULL, then this code will oops; you will get a full register
> >> dump and backtrace.  You can use this information along with markup_oops.pl
> >> to find out where the problem is.
> > 
> > You can't rely on the oops if the code runs in process context. The
> > process may have address 0 mapped which would result in a security hole.
> > We had two of these bugs last year.
> 
> That's the same problem which I had some days ago and Microblaze misses 
> valuable backtrace (because we don't have FP or constant frame size).

You can do what x86 does and just print anything in the stack that looks
like part of a kernel function.

> > But I don't see any point in checking for dma_ops != NULL too. Any
> > developer would mention such a bug long before init is started.
> 
> I agree that checking adds extra cycles to every dma-api call.
> 
> I like as wrote Russel to check if ops exists or not.

If you are worried about the overhead, you could just add the BUG_ON
to the map calls but not to unmap and sync, which are rather unlikely
(and incorrect) to be called before a map.

	Arnd
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