Why are rbp and rsp called general purpose registers?

Question

According to Intel in x64 the following registers are called general purpose registers (RAX, RBX, RCX, RDX, RBP, RSI, RDI, RSP and R8-R15) https://software.intel.com/en-us/a

Answer 1

Dereferencing rbp might result in a #SS(stack segment) fault.

Recently, I hit a linux kernel crash with a 'stack segment fault'.

crash> dmesg
[...]
stack segment: 0000 [#1] SMP
[...]
RIP: 0010:[]  lock_get_status+0x9b/0x3b0
RSP: 0018:ffff89954a317d90  EFLAGS: 00010282
[...]
RBP: 800000fa8c251867 R08: 0000000000001000 R09: 000000000000ffff
[...]
crash> dis lock_get_status+0x9b
0xffffffff8125fa8b :      mov    0x28(%rbp),%rax

The memory address in rbp is non-canonical address. That's the reason for this crash. What I learned from this crash is that accessing rbp implicitly accesses ss segment register even through rbp is not used as a stack frame base pointer.

According to Intel SDMv1 3.4.1 General-Purpose Registers:

EBP — Pointer to data on the stack (in the SS segment)