In order to generate the prologue and epilogue, the BPF JIT needs to
know which registers that are clobbered. Therefore, the during
pre-final passes, the prologue is generated after the body of the
program body-prologue-epilogue. Then, in the final pass, a proper
prologue-body-epilogue JITted image is generated.

This scheme has worked most of the time. However, for some large
programs with many jumps, e.g. the test_kmod.sh BPF selftest with
hardening enabled (blinding constants), this has shown to be
incorrect. For the final pass, when the proper prologue-body-epilogue
is generated, the image has not converged. This will lead to that the
final image will have incorrect jump offsets. The following is an
excerpt from an incorrect image:

  | ...
  |     3b8:       00c50663                beq     a0,a2,3c4 <.text+0x3c4>
  |     3bc:       0020e317                auipc   t1,0x20e
  |     3c0:       49630067                jalr    zero,1174(t1) # 20e852 <.text+0x20e852>
  | ...
  |  20e84c:       8796                    c.mv    a5,t0
  |  20e84e:       6422                    c.ldsp  s0,8(sp)    # Epilogue start
  |  20e850:       6141                    c.addi16sp      sp,16
  |  20e852:       853e                    c.mv    a0,a5       # Incorrect jump target
  |  20e854:       8082                    c.jr    ra

The image has shrunk, and the epilogue offset is incorrect in the
final pass.

Correct the problem by always generating proper prologue-body-epilogue
outputs, which means that the first pass will only generate the body
to track what registers that are touched.

Fixes: 2353ecc6 ("bpf, riscv: add BPF JIT for RV64G")
Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230710074131.19596-1-bjorn@kernel.org