selftests/bpf: Add a test case to write mtu result into .rodata

Add a test which attempts to call bpf_check_mtu() and writes the MTU
into .rodata section of the BPF program, and for comparison this adds
test cases also for .bss and .data section again. The bpf_check_mtu()
is a bit more special in that the passed mtu argument is read and
written by the helper (instead of just written to). Assert that writes
into .rodata remain rejected by the verifier.

  # ./vmtest.sh -- ./test_progs -t verifier_const
  [...]
  ./test_progs -t verifier_const
  [    1.657367] bpf_testmod: loading out-of-tree module taints kernel.
  [    1.657773] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
  #473/1   verifier_const/rodata/strtol: write rejected:OK
  #473/2   verifier_const/bss/strtol: write accepted:OK
  #473/3   verifier_const/data/strtol: write accepted:OK
  #473/4   verifier_const/rodata/mtu: write rejected:OK
  #473/5   verifier_const/bss/mtu: write accepted:OK
  #473/6   verifier_const/data/mtu: write accepted:OK
  #473     verifier_const:OK
  [...]
  Summary: 2/10 PASSED, 0 SKIPPED, 0 FAILED

For comparison, without the MEM_UNINIT on bpf_check_mtu's proto:

  # ./vmtest.sh -- ./test_progs -t verifier_const
  [...]
  #473/3   verifier_const/data/strtol: write accepted:OK
  run_subtest:PASS:obj_open_mem 0 nsec
  run_subtest:FAIL:unexpected_load_success unexpected success: 0
  #473/4   verifier_const/rodata/mtu: write rejected:FAIL
  #473/5   verifier_const/bss/mtu: write accepted:OK
  #473/6   verifier_const/data/mtu: write accepted:OK
  #473     verifier_const:FAIL
  [...]

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20240913191754.13290-9-daniel@iogearbox.net


Signed-off-by: Alexei Starovoitov <ast@kernel.org>