Re: [PATCH] dma-mapping: Add BUG_ON for uninitialized dma_ops

From: Marek Szyprowski
Date: Tue Jun 11 2013 - 07:02:57 EST


On 6/11/2013 4:34 AM, Bjorn Helgaas wrote:
[+cc Marek]

On Mon, Jun 3, 2013 at 6:44 AM, Michal Simek <michal.simek@xxxxxxxxxx> wrote:
> Check that dma_ops are initialized correctly.
> Signed-off-by: Michal Simek <michal.simek@xxxxxxxxxx>
> ---
> Functions dma_mmap_attrs(), dma_get_sgtable_attrs()
> already have this checking.
> ---
> include/asm-generic/dma-mapping-common.h | 12 ++++++++++++
> 1 file changed, 12 insertions(+)
> diff --git a/include/asm-generic/dma-mapping-common.h b/include/asm-generic/dma-mapping-common.h
> index de8bf89..d430cab 100644
> --- a/include/asm-generic/dma-mapping-common.h
> +++ b/include/asm-generic/dma-mapping-common.h
> @@ -16,6 +16,7 @@ static inline dma_addr_t dma_map_single_attrs(struct device *dev, void *ptr,
> dma_addr_t addr;
> kmemcheck_mark_initialized(ptr, size);
> + BUG_ON(!ops);

Does this actually help anything? I expected that if "ops" is NULL,
we would just oops anyway when we attempted to call "ops->map_page()"
because we already trap null pointer dereferences. At least, when I
tried leaving a pci_bus.ops pointer NULL, I got a nice panic and
backtrace even without adding an explicit BUG_ON().

I cc'd Marek, who added the similar BUG_ON()s in dma_mmap_attrs() and
dma_get_sgtable_attrs() with d2b7428eb0 and 64ccc9c033.

I think that I've copied it from dma_alloc_coherent() in microblaze. You are right that lack
of this check will also trigger oops in ops==NULL case, but I think that adding explicit check
in all functions, which use it, is a good idea. It serves as a kind of documentation and
emphasizes that missing ops is really an issue.

Best regards
Marek Szyprowski
Samsung R&D Institute Poland

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