[Xen-devel] [PATCH] swiotlb: fix phys_addr_t overflow warning

[Arnd Bergmann <arnd@xxxxxxxx>]

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>
---
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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