Re: [PATCH] swiotlb: fix phys_addr_t overflow warning

[Stefano Stabellini]

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@xxxxxxxx>

Acked-by: Stefano Stabellini <sstabellini@xxxxxxxxxx>


> ---
> 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
>