selftests/bpf: Add test for writes to .rodata

selftests/bpf: Add test for writes to .rodata

Add a small test to write a (verification-time) fixed vs unknown but
bounded-sized buffer into .rodata BPF map and assert that both get
rejected.

  # ./vmtest.sh -- ./test_progs -t verifier_const
  [...]
  ./test_progs -t verifier_const
  [    1.418717] tsc: Refined TSC clocksource calibration: 3407.994 MHz
  [    1.419113] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x311fcde90a1, max_idle_ns: 440795222066 ns
  [    1.419972] clocksource: Switched to clocksource tsc
  [    1.449596] bpf_testmod: loading out-of-tree module taints kernel.
  [    1.449958] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
  #475/1   verifier_const/rodata/strtol: write rejected:OK
  #475/2   verifier_const/bss/strtol: write accepted:OK
  #475/3   verifier_const/data/strtol: write accepted:OK
  #475/4   verifier_const/rodata/mtu: write rejected:OK
  #475/5   verifier_const/bss/mtu: write accepted:OK
  #475/6   verifier_const/data/mtu: write accepted:OK
  #475/7   verifier_const/rodata/mark: write with unknown reg rejected:OK
  #475/8   verifier_const/rodata/mark: write with unknown reg rejected:OK
  #475     verifier_const:OK
  #476/1   verifier_const_or/constant register |= constant should keep constant type:OK
  #476/2   verifier_const_or/constant register |= constant should not bypass stack boundary checks:OK
  #476/3   verifier_const_or/constant register |= constant register should keep constant type:OK
  #476/4   verifier_const_or/constant register |= constant register should not bypass stack boundary checks:OK
  #476     verifier_const_or:OK
  Summary: 2/12 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20241021152809.33343-4-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>