Re: [PATCH 1/2] swiotlb: Make DIRECT_MAPPING_ERROR viable

[Robin Murphy]

On 20/11/2018 14:49, Konrad Rzeszutek Wilk wrote:
> On Tue, Nov 20, 2018 at 02:09:52PM +0000, Robin Murphy wrote:
> > With the overflow buffer removed, we no longer have a unique address
> > which is guaranteed not to be a valid DMA target to use as an error
> > token. The DIRECT_MAPPING_ERROR value of 0 tries to at least represent
> > an unlikely DMA target, but unfortunately there are already SWIOTLB
> > users with DMA-able memory at physical address 0 which now gets falsely
> > treated as a mapping failure and leads to all manner of misbehaviour.
> >
> > The best we can do to mitigate that is flip DIRECT_MAPPING_ERROR to the
> > commonly-used all-bits-set value, since the last single byte of memory
> > is by far the least-likely-valid DMA target.
>
> Are all the callers checking for DIRECT_MAPPING_ERROR or is it more of
> a comparison (as in if (!ret)) ?

dma_direct_map_page() and dma_direct_mapping_error() were already doing 
the right thing, and external callers must rely on the latter via 
dma_mapping_error() rather than trying to inspect the actual value 
themselves, since that varies between implementations anyway. AFAICS all 
the new return paths from swiotlb_map_page() are also robust in 
referencing the macro explicitly, so I think we're good.

Thanks,
Robin.

> > Fixes: dff8d6c1ed58 ("swiotlb: remove the overflow buffer")]
> > Reported-by: John Stultz <john.stultz@xxxxxxxxxx>
> > Signed-off-by: Robin Murphy <robin.murphy@xxxxxxx>
> > ---
> >   include/linux/dma-direct.h | 2 +-
> >   1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
> > index bd73e7a91410..9de9c7ab39d6 100644
> > --- a/include/linux/dma-direct.h
> > +++ b/include/linux/dma-direct.h
> > @@ -5,7 +5,7 @@
> >   #include <linux/dma-mapping.h>
> >   #include <linux/mem_encrypt.h>
> >   
> > -#define DIRECT_MAPPING_ERROR		0
> > +#define DIRECT_MAPPING_ERROR		~(dma_addr_t)0
> >   
> >   #ifdef CONFIG_ARCH_HAS_PHYS_TO_DMA
> >   #include <asm/dma-direct.h>
> > --
> > 2.19.1.dirty