I was looking at some usercopy stuff and noticed that arch_within_stack_frames() (the helper used by the usercopy bounds-checking logic to detect copies that cross stack frames) seems to be a no-op on almost all modern kernel configurations. It is only defined for x86 - no implementation for e.g. arm64 exists at all. The x86 version requires CONFIG_FRAME_POINTER, which is only selected by CONFIG_UNWINDER_FRAME_POINTER (whereas the more modern choice, and default, for x86-64 is CONFIG_UNWINDER_ORC). Personally, I don't feel very attached to that check; but if people are interested in keeping it, it should probably be reworked to use the proper x86 unwinder API: unwind_start(), unwind_next_frame(), unwind_get_return_address_ptr() and unwind_done() together would probably help with this. Otherwise, it should probably be removed, since in that case it's pretty much going to just be bitrot?
