Re: [PATCH] swiotlb: fix phys_addr_t overflow warning

From: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
To: Stefano Stabellini <sstabellini@kernel.org>
Cc: Juergen Gross <jgross@suse.com>, Joerg Roedel <jroedel@suse.de>,
	Andrew Morton <akpm@linux-foundation.org>,
	Arnd Bergmann <arnd@arndb.de>,
	linux-kernel@vger.kernel.org, Mike Rapoport <rppt@linux.ibm.com>,
	iommu@lists.linux-foundation.org,
	Jesper Dangaard Brouer <brouer@redhat.com>,
	xen-devel@lists.xenproject.org,
	Boris Ostrovsky <boris.ostrovsky@oracle.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Christoph Hellwig <hch@lst.de>
Subject: Re: [PATCH] swiotlb: fix phys_addr_t overflow warning
Date: Wed, 19 Jun 2019 10:28:12 -0400	[thread overview]
Message-ID: <20190619142812.GM10432@char.us.oracle.com> (raw)
In-Reply-To: <alpine.DEB.2.21.1906170913080.2072@sstabellini-ThinkPad-T480s>

On Mon, Jun 17, 2019 at 09:13:16AM -0700, Stefano Stabellini wrote:
> On Mon, 17 Jun 2019, Arnd Bergmann wrote:
> > On architectures that have a larger dma_addr_t than phys_addr_t,
> > the swiotlb_tbl_map_single() function truncates its return code
> > in the failure path, making it impossible to identify the error
> > later, as we compare to the original value:
> > 
> > kernel/dma/swiotlb.c:551:9: error: implicit conversion from 'dma_addr_t' (aka 'unsigned long long') to 'phys_addr_t' (aka 'unsigned int') changes value from 18446744073709551615 to 4294967295 [-Werror,-Wconstant-conversion]
> >         return DMA_MAPPING_ERROR;
> > 
> > Use an explicit typecast here to convert it to the narrower type,
> > and use the same expression in the error handling later.
> > 
> > Fixes: b907e20508d0 ("swiotlb: remove SWIOTLB_MAP_ERROR")
> > Signed-off-by: Arnd Bergmann <arnd@arndb.de>
> 
> Acked-by: Stefano Stabellini <sstabellini@kernel.org>

queued.
> 
> 
> > ---
> > I still think that reverting the original commit would have
> > provided clearer semantics for this corner case, but at least
> > this patch restores the correct behavior.
> > ---
> >  drivers/xen/swiotlb-xen.c | 2 +-
> >  kernel/dma/swiotlb.c      | 4 ++--
> >  2 files changed, 3 insertions(+), 3 deletions(-)
> > 
> > diff --git a/drivers/xen/swiotlb-xen.c b/drivers/xen/swiotlb-xen.c
> > index d53f3493a6b9..cfbe46785a3b 100644
> > --- a/drivers/xen/swiotlb-xen.c
> > +++ b/drivers/xen/swiotlb-xen.c
> > @@ -402,7 +402,7 @@ static dma_addr_t xen_swiotlb_map_page(struct device *dev, struct page *page,
> >  
> >  	map = swiotlb_tbl_map_single(dev, start_dma_addr, phys, size, dir,
> >  				     attrs);
> > -	if (map == DMA_MAPPING_ERROR)
> > +	if (map == (phys_addr_t)DMA_MAPPING_ERROR)
> >  		return DMA_MAPPING_ERROR;
> >  
> >  	dev_addr = xen_phys_to_bus(map);
> > diff --git a/kernel/dma/swiotlb.c b/kernel/dma/swiotlb.c
> > index e906ef2e6315..a3be651973ad 100644
> > --- a/kernel/dma/swiotlb.c
> > +++ b/kernel/dma/swiotlb.c
> > @@ -548,7 +548,7 @@ phys_addr_t swiotlb_tbl_map_single(struct device *hwdev,
> >  	if (!(attrs & DMA_ATTR_NO_WARN) && printk_ratelimit())
> >  		dev_warn(hwdev, "swiotlb buffer is full (sz: %zd bytes), total %lu (slots), used %lu (slots)\n",
> >  			 size, io_tlb_nslabs, tmp_io_tlb_used);
> > -	return DMA_MAPPING_ERROR;
> > +	return (phys_addr_t)DMA_MAPPING_ERROR;
> >  found:
> >  	io_tlb_used += nslots;
> >  	spin_unlock_irqrestore(&io_tlb_lock, flags);
> > @@ -666,7 +666,7 @@ bool swiotlb_map(struct device *dev, phys_addr_t *phys, dma_addr_t *dma_addr,
> >  	/* Oh well, have to allocate and map a bounce buffer. */
> >  	*phys = swiotlb_tbl_map_single(dev, __phys_to_dma(dev, io_tlb_start),
> >  			*phys, size, dir, attrs);
> > -	if (*phys == DMA_MAPPING_ERROR)
> > +	if (*phys == (phys_addr_t)DMA_MAPPING_ERROR)
> >  		return false;
> >  
> >  	/* Ensure that the address returned is DMA'ble */
> > -- 
> > 2.20.0
> > 
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