On Tue, Jun 22, 2021 at 09:25:57AM +0100, Richard Sandiford wrote: > Kees Cook <keesc...@chromium.org> writes: > > On Mon, Jun 21, 2021 at 03:39:45PM +0000, Qing Zhao wrote: > >> So, if “pattern value” is “0xFFFFFFFFFFFFFFFF”, then it’s a valid > >> canonical virtual memory address. However, for most OS, > >> “0xFFFFFFFFFFFFFFFF” should be not in user space. > >> > >> My question is, is “0xFFFFFFFFFFFFFFFFF” good for pointer? Or > >> “0xAAAAAAAAAAAAAAAA” better? > > > > I think 0xFF repeating is fine for this version. Everything else is a > > "nice to have" for the pattern-init, IMO. :) > > Sorry to be awkward, but 0xFF seems worse than 0xAA to me. > > For integer types, all values are valid representations, and we're > relying on the pattern being “obviously” wrong in context. 0xAAAA… > is unlikely to be a correct integer but 0xFFFF… would instead be a > “nice” -1. It would be difficult to tell in a debugger that a -1 > came from pattern init rather than a deliberate choice.
I can live with 0xAA. On x86_64, this puts it nicely in the middle of the middle of the non-canonical space: 0x800000000000 - 0xffff7fffffffffff The only trouble is with 32-bit, where the value 0xAAAAAAAA is a legitimate allocatable userspace address. If we want some kind-of middle ground, how about 0xFE? That'll be non-canonical on x86_64, and at the high end of the i386 kernel address space. > I agree that, all other things being equal, it would be nice to use NaNs > for floats. But relying on wrong numerical values for floats doesn't > seem worse than doing that for integers. > > 0xAA… for float is (if I've got this right) -3.0316488252093987e-13, > which admittedly doesn't stand out as wrong. But I'm not sure we > should sacrifice integer debugging for float debugging here. In some future version type-specific patterns would be a nice improvement, but I don't want that to block getting the zero-init portion landed. :) -- Kees Cook
