On Tue, 2013-06-11 at 13:02 +0200, Marek Szyprowski wrote: > Hello, > > 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. Really, no, it's not a good idea at all. It invites tons of patches littering the code with BUG_ONs where we might possibly get a NULL dereference. All it does is add extra instructions to a code path for no actual benefit. If you can answer the question: what more information does the BUG_ON give you than the NULL deref Oops would not? then it might be reasonable. James -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
