I no real complaints about CHERI here. What’s a pointer, anyway? Lots of old systems thought it was 8 or 16 bits that give a linear address. 8086 thought it was 16 + 16 bits split among two registers, with some interesting arithmetic [0]. You can’t add, say, 20000 to a pointer and get a pointer to a byte 20000 farther into memory. 80286 changed it so those high bits index into a table, and the actual segment registers are much wider than 16 bits and can’t be read or written directly [1]. Unprivileged code certainly cannot load arbitrary values into a segment register. 80386 added bits. Even x86_64 still technically has those extra segment registers, but they mostly don’t work any more.

So who am I to complain if CHERI pointers are even wider and have strange rules? At least you can write a pointer to memory and read it back again.

[0] I could be wrong. I’ve hacked on Linux’s v8086 support, but that’s virtual and I never really cared what its effect was in user mode so long as it worked.

[1] You can read and write them via SMM entry or using virtualization extensions.