On 13/08/18 05:48 PM, Kit Chow wrote: > On 08/13/2018 04:39 PM, Logan Gunthorpe wrote: >> >> On 13/08/18 05:30 PM, Kit Chow wrote: >>> In arch/x86/include/asm/page.h, there is the following comment in >>> regards to validating the virtual address. >>> >>> /* >>> * virt_to_page(kaddr) returns a valid pointer if and only if >>> * virt_addr_valid(kaddr) returns true. >>> */ >>> #define virt_to_page(kaddr) pfn_to_page(__pa(kaddr) >> PAGE_SHIFT) >>> >>> So it looks like the validation by virt_addr_valid was somehow dropped >>> from the virt_to_page code path. Does anyone have any ideas what >>> happended to it? >> I don't think it was ever validated (though I haven't been around long >> enough to say for certain). What the comment is saying is that you >> shouldn't rely on virt_to_page() unless you know virt_addr_valid() is >> true (which in most cases you can without the extra check). virt_to_page >> is meant to be really fast so adding an extra validation would probably >> be a significant performance regression for the entire kernel. >> >> The fact that this can happen through dma_map_single() is non-ideal at >> best. It assumes the caller is mapping regular memory and doesn't check >> this at all. It may make sense to fix that but I think people expect >> dma_map_single() to be as fast as possible as well... >> > Perhaps include the validation with some debug turned on? The problem is how often do you develop code with any of the debug config options turned on? There's already a couple of BUG_ONs in dma_map_single so maybe another one with virt_addr_valid wouldn't be so bad. Logan
