Re: [PATCH] dma-mapping: Add BUG_ON for uninitialized dma_ops

[James Bottomley]

On Wed, 2013-06-12 at 17:06 +0200, Arnd Bergmann wrote:
> On Tuesday 11 June 2013, James Bottomley wrote:
> > Really, no, it's not a good idea at all.  It invites tons of patches
> > littering the code with BUG_ONs where we might possibly get a NULL
> > dereference.  All it does is add extra instructions to a code path for
> > no actual benefit.
> > 
> > If you can answer the question: what more information does the BUG_ON
> > give you than the NULL deref Oops would not? then it might be
> > reasonable.
> 
> The question is if a user can trigger the NULL dereference intentionally,
> in which case they might get the kernel to jump into a  user-provided
> buffer.

Can you elaborate on how they could do this?  If you're thinking they
could alter the pointer and trigger the jump, then yes, but a BUG_ON
won't prevent that because the altered pointer won't be NULL.

James



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