On Wed, Aug 01, 2018 at 06:52:09PM +0200, Dmitry Vyukov wrote: > On Wed, Aug 1, 2018 at 6:35 PM, Will Deacon <will.deacon@xxxxxxx> wrote: > > Thanks for tracking these cases down and going through each of them. The > > obvious follow-up question is: how do we ensure that we keep on top of > > this in mainline? Are you going to repeat your experiment at every kernel > > release or every -rc or something else? I really can't see how we can > > maintain this in the long run, especially given that the coverage we have > > is only dynamic -- do you have an idea of how much coverage you're actually > > getting for, say, a defconfig+modules build? > > > > I'd really like to enable pointer tagging in the kernel, I'm just still > > failing to see how we can do it in a controlled manner where we can reason > > about the semantic changes using something other than a best-effort, > > case-by-case basis which is likely to be fragile and error-prone. > > Unfortunately, if that's all we have, then this gets relegated to a > > debug feature, which sort of defeats the point in my opinion. > > Well, in some cases there is no other way as resorting to dynamic testing. > How do we ensure that kernel does not dereference NULL pointers, does > not access objects after free or out of bounds? Nohow. And, yes, it's > constant maintenance burden resolved via dynamic testing. ... and the advantage of NULL pointer issues is that you're likely to see them as a synchronous exception at runtime, regardless of architecture and regardless of Kconfig options. With pointer tagging, that's certainly not the case, and so I don't think we can just treat issues there like we do for NULL pointers. Will
