On Tue, May 21, 2019 at 03:48:56PM -0300, Jason Gunthorpe wrote: > On Fri, May 17, 2019 at 03:49:31PM +0100, Catalin Marinas wrote: > > > The tagged pointers (whether hwasan or MTE) should ideally be a > > transparent feature for the application writer but I don't think we can > > solve it entirely and make it seamless for the multitude of ioctls(). > > I'd say you only opt in to such feature if you know what you are doing > > and the user code takes care of specific cases like ioctl(), hence the > > prctl() proposal even for the hwasan. > > I'm not sure such a dire view is warrented.. > > The ioctl situation is not so bad, other than a few special cases, > most drivers just take a 'void __user *' and pass it as an argument to > some function that accepts a 'void __user *'. sparse et al verify > this. > > As long as the core functions do the right thing the drivers will be > OK. > > The only place things get dicy is if someone casts to unsigned long > (ie for vma work) but I think that reflects that our driver facing > APIs for VMAs are compatible with static analysis (ie I have no > earthly idea why get_user_pages() accepts an unsigned long), not that > this is too hard. 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. In the latter case the pointer (or other type) is a virtual address that the call does not dereference but my do other things with. Also U __really(T) to tell static analysers the real type of pointers smuggled through UAPI disguised as other types (*cough* KVM, etc.) We could gradually make sparse more strict about the presence of annotations and allowed conversions, add get/put_user() variants that demand explicit annotation, etc. find_vma() wouldn't work with a __object pointer, for example. A get_user_pages_for_dereference() might be needed for __object pointers (embodying a promise from the caller that only the object will be dereferenced within the mapped pages). Thoughts? This kind of thing would need widespread buy-in in order to be viable. Cheers ---Dave
