selftests/bpf: validate precision logic in partial_stack_load_preserves_zeros

Enhance partial_stack_load_preserves_zeros subtest with detailed
precision propagation log checks. We know expect fp-16 to be spilled,
initially imprecise, zero const register, which is later marked as
precise even when partial stack slot load is performed, even if it's not
a register fill (!).
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20231205184248.1502704-10-andrii@kernel.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/progs/verifier_spill_fill.c

@@ -495,6 +495,22 @@ char single_byte_buf[1] SEC(".data.single_byte_buf");
 SEC("raw_tp")
 __log_level(2)
 __success
+/* make sure fp-8 is all STACK_ZERO */
+__msg("2: (7a) *(u64 *)(r10 -8) = 0          ; R10=fp0 fp-8_w=00000000")
+/* but fp-16 is spilled IMPRECISE zero const reg */
+__msg("4: (7b) *(u64 *)(r10 -16) = r0        ; R0_w=0 R10=fp0 fp-16_w=0")
+/* and now check that precision propagation works even for such tricky case */
+__msg("10: (71) r2 = *(u8 *)(r10 -9)         ; R2_w=P0 R10=fp0 fp-16_w=0")
+__msg("11: (0f) r1 += r2")
+__msg("mark_precise: frame0: last_idx 11 first_idx 0 subseq_idx -1")
+__msg("mark_precise: frame0: regs=r2 stack= before 10: (71) r2 = *(u8 *)(r10 -9)")
+__msg("mark_precise: frame0: regs= stack=-16 before 9: (bf) r1 = r6")
+__msg("mark_precise: frame0: regs= stack=-16 before 8: (73) *(u8 *)(r1 +0) = r2")
+__msg("mark_precise: frame0: regs= stack=-16 before 7: (0f) r1 += r2")
+__msg("mark_precise: frame0: regs= stack=-16 before 6: (71) r2 = *(u8 *)(r10 -1)")
+__msg("mark_precise: frame0: regs= stack=-16 before 5: (bf) r1 = r6")
+__msg("mark_precise: frame0: regs= stack=-16 before 4: (7b) *(u64 *)(r10 -16) = r0")
+__msg("mark_precise: frame0: regs=r0 stack= before 3: (b7) r0 = 0")
 __naked void partial_stack_load_preserves_zeros(void)
 {
 	asm volatile (