Which correctly describes the capabilities used in the instruction-set extension called CHERI?

Answer: C. A pointer carries the range it may access and its permissions, and a 1-bit tag that software cannot write distinguishes in memory whether it was created legitimately

CHERI replaces address-only pointers with capabilities that bundle the address, the upper and lower bounds of the range it may point to, and permissions such as read, write and execute (for 64-bit addresses they are compressed into 128 bits, with a separate 1-bit tag). The CPU checks the bounds and permissions on every access, so memory-safety errors such as reading or writing outside an array can be stopped in hardware. The tag is set only when a capability is created with the proper instructions and is cleared when it is overwritten as ordinary data, so pointers cannot be forged. The University of Cambridge and others have led the research, and in the early 2020s Arm made Morello, a prototype processor and evaluation board based on it, available to researchers.

• A. It does not hide pointers; it gives them a usable range and permissions and stops out-of-bounds accesses.
• B. Checking the signature of an executable is done by secure boot and similar mechanisms and is separate from capabilities.
• C. Correct. Bounds, permissions and the tag prevent out-of-bounds accesses and pointer forgery.