On Thu, May 23, 2019 at 11:42:57AM +0100, Dave P Martin wrote: > On Wed, May 22, 2019 at 09:20:52PM -0300, Jason Gunthorpe wrote: > > On Wed, May 22, 2019 at 02:49:28PM +0100, Dave Martin wrote: > > > If multiple people will care about this, perhaps we should try to > > > annotate types more explicitly in SYSCALL_DEFINEx() and ABI data > > > structures. > > > > > > For example, we could have a couple of mutually exclusive modifiers > > > > > > T __object * > > > T __vaddr * (or U __vaddr) > > > > > > In the first case the pointer points to an object (in the C sense) > > > that the call may dereference but not use for any other purpose. > > > > How would you use these two differently? > > > > So far the kernel has worked that __user should tag any pointer that > > is from userspace and then you can't do anything with it until you > > transform it into a kernel something > > Ultimately it would be good to disallow casting __object pointers execpt > to compatible __object pointer types, and to make get_user etc. demand > __object. > > __vaddr pointers / addresses would be freely castable, but not to > __object and so would not be dereferenceable even indirectly. I think it gets too complicated and there are ambiguous cases that we may not be able to distinguish. For example copy_from_user() may be used to copy a user data structure into the kernel, hence __object would work, while the same function may be used to copy opaque data to a file, so __vaddr may be a better option (unless I misunderstood your proposal). We currently have T __user * and I think it's a good starting point. The prior attempt [1] was shut down because it was just hiding the cast using __force. We'd need to work through those cases again and rather start changing the function prototypes to avoid unnecessary casting in the callers (e.g. get_user_pages(void __user *) or come up with a new type) while changing the explicit casting to a macro where it needs to be obvious that we are converting a user pointer, potentially typed (tagged), to an untyped address range. We may need a user_ptr_to_ulong() macro or similar (it seems that we have a u64_to_user_ptr, wasn't aware of it). It may actually not be far from what you suggested but I'd keep the current T __user * to denote possible dereference. [1] https://lore.kernel.org/lkml/5d54526e5ff2e5ad63d0dfdd9ab17cf359afa4f2.1535629099.git.andreyknvl@xxxxxxxxxx/ -- Catalin
