On Tue, Jun 06, 2023 at 08:45:49AM -0700, Linus Torvalds wrote: > On Tue, Jun 6, 2023 at 8:31 AM Kees Cook <keescook@xxxxxxxxxxxx> wrote: > > > > nit: Linus's example was "(void *)8" (instead of 1) because we've had > > issues in the past with alignment warnings on archs that are sensitive > > to it. (e.g. see the __is_constexpr() macro which is doing NULL/!NULL > > comparisons.) > > Note that I don't think we ever saw such a warning, it was just a > theoretical observation that depending on type, the compiler might > warn about known mis-aligned pointer bits. > > So I'm not sure the 1-vs-8 actually matters. We do other things that > assume that low bits in a pointer are retained and valid, even if in > theory the C type system might have issues with it. > > But maybe I mis-remember - if you did get an actual warning, maybe we > should document that warning just to keep the memory alive. I've never seen a warning, but since this came up in the dissection of the __is_constexpr() behavior, it's been burned into my mind. ;) -- Kees Cook
