On Sun Jun 21, 2026 at 5:15 PM BST, Danilo Krummrich wrote: > On Sun Jun 21, 2026 at 5:41 PM CEST, Gary Guo wrote: >> On Sun Jun 21, 2026 at 10:17 AM BST, Alexandre Courbot wrote: >>> As you can see, this lets a 32-bit access be done on the upper half of a >>> 64-bit register, which sounds like it should not be allowed? Similarly one >>> could change register types, and so on. This might not be "unsafe" in the >>> sense that it is still aligned and in bounds, but it lets the structure set >>> by the type system be bypassed. It could also potentially be a violation of >>> the hardware contract if the access width is relevant for this particular >>> address. >> >> I see no reason to prevent any of the case, this can be done by `try_cast()` >> API as well. If we need to take access width restriction and other >> restrictions into consideration, then a lot of API cannot be exposed at all. >> E.g. it is not okay to add `copy_read`/`copy_write` like the patch 19, >> because >> it uses memcpy_from/toio which is possibility doing byte-width access. >> >> in my opinion think people should be able to type casting without reaching >> out >> to `unsafe` if it's not UB. Similar to the logic on why we have `zerocopy` >> that allows casting between to types, these are "bypassing the typesystem" as >> well! > > I think this is fine as-is. The natural, ergonomic path through the API > (io_read!/io_write!/io_project! macros, IoLoc-based accessors, etc.) leads > users > toward correct access widths. > > Whether through io_addr() or a custom IoLoc implementation, reinterpreting the > access type requires explicit, deliberate choices: picking a different type > and > computing a byte offset. This is not something anyone would do by accident. > > So, as long as the API doesn't provide a subtle way to do the wrong thing by > accident, I don't think we need to add restrictions here.
I'm still going to make this a standalone function to be used from `IoLoc` functions only, given these functions are not intended to be used by user directly anyway. But I'll leave `IoLoc` and `try_cast` as-is. Best, Gary
