* Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx> wrote: > > > On Wed, 7 Jan 2009, Matthew Wilcox wrote: > > > > I appreciate this is sample code, but using __get_user() on > > non-userspace pointers messes up architectures which have separate > > user/kernel spaces (eg the old 4G/4G split for x86-32). Do we have an > > appropriate function for kernel space pointers? Is this a good reason > > to add one? > > Yes, you're right. > > We could do the whole "oldfs = get_fs(); set_fs(KERNEL_DS); .. > set_fs(oldfs);" crud, but it would probably be better to just add an > architected accessor. Especially since it's going to generally just be a > > #define get_kernel_careful(val,p) __get_user(val,p) > > for most architectures. > > We've needed that before (and yes, we've simply mis-used __get_user() on > x86 before rather than add it). for the oldfs stuff we already have probe_kernel_read(). OTOH, that involves pagefault_disable() which is an atomic op, so __get_user_careful() should be much more lightweight - and we already know that the memory range at least _used to_ be a valid kernel address. (Theoretical race: with memory hotplug that kernel pointer address could have gotten unmapped and we could get device memory there - with side-effects if accessed. Wont happen in practice.) Ingo -- To unsubscribe from this list: send the line "unsubscribe linux-fsdevel" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
