push %rbx
mov 8(%rsp),%rbx
xor %r12,%rbx
...
xor %r12,%rbx
cmp %rbx,8(%rsp)
jnz .error
pop %rbx
retn
.error:
ud2

; store the old value of rbx
push %rbx
; read the top value on the stack (the return pointer) and put it in rbx
mov 8(%rsp),%rbx
; xor rbx with a random value stored in r12
; rbx now contains the RAP canary
xor %r12,%rbx

; xor the RAP canary in rbx with the random value in r12
; this "decrypts" the stack pointer from the RAP canary
xor %r12,%rbx
; compare the top value on the stack (the return pointer)
; with the value decrypted from the RAP canary
cmp %rbx,8(%rsp)
; if the comparison fails, jump to error
jnz .error
; restore the original value of rbx
pop %rbx
; return
retn
.error:
; an error occured, quit the process
ud2