rbx is a reserved register?

[weiwenhao]

  >0x60e1a0 <libucontext_trampoline>       mov    (%rbx),%rdi                                                                                                                                                   │
│   0x60e1a3 <libucontext_trampoline+3>     test   %rdi,%rdi                                                                                                                                                     │
│   0x60e1a6 <libucontext_trampoline+6>     je     0x60e1b0 <libucontext_trampoline+16>                                                                                                                          │
│   0x60e1a8 <libucontext_trampoline+8>     jmpq   0x60e1ba <setcontext>                                                                                                                                         │
│   0x60e1ad <libucontext_trampoline+13>    nopl   (%rax)                                                                                                                                                        │
│   0x60e1b0 <libucontext_trampoline+16>    sub    $0x8,%rsp                                                                                                                                                     │
│   0x60e1b4 <libucontext_trampoline+20>    callq  0x60e69c <exit>

When I finish executing ctx fn and modify the rbx register within fn, returning to the old ctx, mov (%rbx) is unable to read the correct stack base address.