selftests/bpf: Add more precision tracking tests for atomics

selftests/bpf: Add more precision tracking tests for atomics

Add verifier precision tracking tests for BPF atomic fetch operations.
Validate that backtrack_insn correctly propagates precision from the
fetch dst_reg to the stack slot for {fetch_add,xchg,cmpxchg} atomics.
For the first two src_reg gets the old memory value, and for the last
one r0. The fetched register is used for pointer arithmetic to trigger
backtracking. Also add coverage for fetch_{or,and,xor} flavors which
exercises the bitwise atomic fetch variants going through the same
insn->imm & BPF_FETCH check but with different imm values.

Add dual-precision regression tests for fetch_add and cmpxchg where
both the fetched value and a reread of the same stack slot are tracked
for precision. After the atomic operation, the stack slot is STACK_MISC,
so the ldx does not set INSN_F_STACK_ACCESS. These tests verify that
stack precision propagates solely through the atomic fetch's load side.

Add map-based tests for fetch_add and cmpxchg which validate that non-
stack atomic fetch completes precision tracking without falling back
to mark_all_scalars_precise. Lastly, add 32-bit variants for {fetch_add,
cmpxchg} on map values to cover the second valid atomic operand size.

  # LDLIBS=-static PKG_CONFIG='pkg-config --static' ./vmtest.sh -- ./test_progs -t verifier_precision
  [...]
  + /etc/rcS.d/S50-startup
  ./test_progs -t verifier_precision
  [    1.697105] bpf_testmod: loading out-of-tree module taints kernel.
  [    1.700220] bpf_testmod: module verification failed: signature and/or required key missing - tainting kernel
  [    1.777043] tsc: Refined TSC clocksource calibration: 3407.986 MHz
  [    1.777619] clocksource: tsc: mask: 0xffffffffffffffff max_cycles: 0x311fc6d7268, max_idle_ns: 440795260133 ns
  [    1.778658] clocksource: Switched to clocksource tsc
  #633/1   verifier_precision/bpf_neg:OK
  #633/2   verifier_precision/bpf_end_to_le:OK
  #633/3   verifier_precision/bpf_end_to_be:OK
  #633/4   verifier_precision/bpf_end_bswap:OK
  #633/5   verifier_precision/bpf_load_acquire:OK
  #633/6   verifier_precision/bpf_store_release:OK
  #633/7   verifier_precision/state_loop_first_last_equal:OK
  #633/8   verifier_precision/bpf_cond_op_r10:OK
  #633/9   verifier_precision/bpf_cond_op_not_r10:OK
  #633/10  verifier_precision/bpf_atomic_fetch_add_precision:OK
  #633/11  verifier_precision/bpf_atomic_xchg_precision:OK
  #633/12  verifier_precision/bpf_atomic_fetch_or_precision:OK
  #633/13  verifier_precision/bpf_atomic_fetch_and_precision:OK
  #633/14  verifier_precision/bpf_atomic_fetch_xor_precision:OK
  #633/15  verifier_precision/bpf_atomic_cmpxchg_precision:OK
  #633/16  verifier_precision/bpf_atomic_fetch_add_dual_precision:OK
  #633/17  verifier_precision/bpf_atomic_cmpxchg_dual_precision:OK
  #633/18  verifier_precision/bpf_atomic_fetch_add_map_precision:OK
  #633/19  verifier_precision/bpf_atomic_cmpxchg_map_precision:OK
  #633/20  verifier_precision/bpf_atomic_fetch_add_32bit_precision:OK
  #633/21  verifier_precision/bpf_atomic_cmpxchg_32bit_precision:OK
  #633/22  verifier_precision/bpf_neg_2:OK
  #633/23  verifier_precision/bpf_neg_3:OK
  #633/24  verifier_precision/bpf_neg_4:OK
  #633/25  verifier_precision/bpf_neg_5:OK
  #633     verifier_precision:OK
  Summary: 1/25 PASSED, 0 SKIPPED, 0 FAILED

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/r/20260331222020.401848-2-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>