selftests/bpf: Add a test cases for sync_linked_regs regarding zext propagation

selftests/bpf: Add a test cases for sync_linked_regs regarding zext propagation

Add multiple test cases for linked register tracking with alu32 ops:

  - Add a test that checks sync_linked_regs() regarding reg->id (the linked
    target register) for BPF_ADD_CONST32 rather than known_reg->id (the
    branch register).

  - Add a test case for linked register tracking that exposes the cross-type
    sync_linked_regs() bug. One register uses alu32 (w7 += 1, BPF_ADD_CONST32)
    and another uses alu64 (r8 += 2, BPF_ADD_CONST64), both linked to the
    same base register.

  - Add a test case that exercises regsafe() path pruning when two execution
    paths reach the same program point with linked registers carrying
    different ADD_CONST flags (BPF_ADD_CONST32 from alu32 vs BPF_ADD_CONST64
    from alu64). This particular test passes with and without the fix since
    the pruning will fail due to different ranges, but it would still be
    useful to carry this one as a regression test for the unreachable div
    by zero.

With the fix applied all the tests pass:

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t verifier_linked_scalars
  [...]
  ./test_progs -t verifier_linked_scalars
  #602/1   verifier_linked_scalars/scalars: find linked scalars:OK
  #602/2   verifier_linked_scalars/sync_linked_regs_preserves_id:OK
  #602/3   verifier_linked_scalars/scalars_neg:OK
  #602/4   verifier_linked_scalars/scalars_neg_sub:OK
  #602/5   verifier_linked_scalars/scalars_neg_alu32_add:OK
  #602/6   verifier_linked_scalars/scalars_neg_alu32_sub:OK
  #602/7   verifier_linked_scalars/scalars_pos:OK
  #602/8   verifier_linked_scalars/scalars_sub_neg_imm:OK
  #602/9   verifier_linked_scalars/scalars_double_add:OK
  #602/10  verifier_linked_scalars/scalars_sync_delta_overflow:OK
  #602/11  verifier_linked_scalars/scalars_sync_delta_overflow_large_range:OK
  #602/12  verifier_linked_scalars/scalars_alu32_big_offset:OK
  #602/13  verifier_linked_scalars/scalars_alu32_basic:OK
  #602/14  verifier_linked_scalars/scalars_alu32_wrap:OK
  #602/15  verifier_linked_scalars/scalars_alu32_zext_linked_reg:OK
  #602/16  verifier_linked_scalars/scalars_alu32_alu64_cross_type:OK
  #602/17  verifier_linked_scalars/scalars_alu32_alu64_regsafe_pruning:OK
  #602/18  verifier_linked_scalars/alu32_negative_offset:OK
  #602/19  verifier_linked_scalars/spurious_precision_marks:OK
  #602     verifier_linked_scalars:OK
  Summary: 1/19 PASSED, 0 SKIPPED, 0 FAILED

Co-developed-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20260319211507.213816-2-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>