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

[Konrad Rzeszutek Wilk <konrad.wilk-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>]

On Tue, Nov 20, 2018 at 03:01:33PM +0000, Robin Murphy wrote:
> 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.

Cool! Thank you for checking.

Acked-by: Konrad Rzeszutek Wilk <konrad.wilk-QHcLZuEGTsvQT0dZR+AlfA@public.gmane.org>

Thank you!
> 
> Thanks,
> Robin.
> 
> > > Fixes: dff8d6c1ed58 ("swiotlb: remove the overflow buffer")]
> > > Reported-by: John Stultz <john.stultz-QSEj5FYQhm4dnm+yROfE0A@public.gmane.org>
> > > Signed-off-by: Robin Murphy <robin.murphy-5wv7dgnIgG8@public.gmane.org>
> > > ---
> > >   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
> > >