On Tue, Sep 15, 2020 at 10:11:51AM -0400, Thomas Tai wrote:
>
>
> On 2020-09-15 10:07 a.m., Christoph Hellwig wrote:
>> On Tue, Sep 15, 2020 at 08:03:14AM -0600, Thomas Tai wrote:
>>> When booting the kernel v5.9-rc4 on a VM, the kernel would panic when
>>> printing a warning message in swiotlb_map(). It is because dev->dma_mask
>>> can potentially be a null pointer. Using the dma_get_mask() macro can
>>> avoid the NULL pointer dereference.
>>
>> dma_mask must not be zero.  This means drm is calling DMA API functions
>> on something weird.  This needs to be fixed in the caller.
>>
>
> Thanks, Christoph for your comment. The caller already fixed the null 
> pointer in the latest v5.9-rc5. I am thinking that if we had used the 
> dma_get_mask(), the kernel couldn't panic and could properly print out the 
> warning message.

If we want to solve this something like this patch is probably the
right way:



diff --git a/include/linux/dma-direct.h b/include/linux/dma-direct.h
index 6e87225600ae35..064870844f06c1 100644
--- a/include/linux/dma-direct.h
+++ b/include/linux/dma-direct.h
@@ -62,9 +62,6 @@ static inline bool dma_capable(struct device *dev, dma_addr_t 
addr, size_t size,
 {
        dma_addr_t end = addr + size - 1;
 
-       if (!dev->dma_mask)
-               return false;
-
        if (is_ram && !IS_ENABLED(CONFIG_ARCH_DMA_ADDR_T_64BIT) &&
            min(addr, end) < phys_to_dma(dev, PFN_PHYS(min_low_pfn)))
                return false;
diff --git a/kernel/dma/mapping.c b/kernel/dma/mapping.c
index 0d129421e75fc8..2b01d8f7baf160 100644
--- a/kernel/dma/mapping.c
+++ b/kernel/dma/mapping.c
@@ -144,6 +144,10 @@ dma_addr_t dma_map_page_attrs(struct device *dev, struct 
page *page,
        dma_addr_t addr;
 
        BUG_ON(!valid_dma_direction(dir));
+
+       if (WARN_ON_ONCE(!dev->dma_mask))
+               return DMA_MAPPING_ERROR;
+
        if (dma_map_direct(dev, ops))
                addr = dma_direct_map_page(dev, page, offset, size, dir, attrs);
        else
@@ -179,6 +183,10 @@ int dma_map_sg_attrs(struct device *dev, struct 
scatterlist *sg, int nents,
        int ents;
 
        BUG_ON(!valid_dma_direction(dir));
+
+       if (WARN_ON_ONCE(!dev->dma_mask))
+               return 0;
+
        if (dma_map_direct(dev, ops))
                ents = dma_direct_map_sg(dev, sg, nents, dir, attrs);
        else
@@ -217,6 +225,9 @@ dma_addr_t dma_map_resource(struct device *dev, phys_addr_t 
phys_addr,
        if (WARN_ON_ONCE(pfn_valid(PHYS_PFN(phys_addr))))
                return DMA_MAPPING_ERROR;
 
+       if (WARN_ON_ONCE(!dev->dma_mask))
+               return DMA_MAPPING_ERROR;
+
        if (dma_map_direct(dev, ops))
                addr = dma_direct_map_resource(dev, phys_addr, size, dir, 
attrs);
        else if (ops->map_resource)
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