selftests/bpf: test bpf_for within spin lock section

Add a selftest to ensure BPF for loops within critical sections are
accepted by the verifier.

Signed-off-by: Emil Tsalapatis (Meta) <emil@etsalapatis.com>
Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Link: https://lore.kernel.org/r/20250104202528.882482-3-emil@etsalapatis.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

Author: etsal

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

@@ -530,4 +530,30 @@ l1_%=:	exit;						\
 	: __clobber_all);
 }
 
+SEC("tc")
+__description("spin_lock: loop within a locked region")
+__success __failure_unpriv __msg_unpriv("")
+__retval(0)
+int bpf_loop_inside_locked_region(void)
+{
+	const int zero = 0;
+	struct val *val;
+	int i, j = 0;
+
+	val = bpf_map_lookup_elem(&map_spin_lock, &zero);
+	if (!val)
+		return -1;
+
+	bpf_spin_lock(&val->l);
+	bpf_for(i, 0, 10) {
+		j++;
+		/* Silence "unused variable" warnings. */
+		if (j == 10)
+			break;
+	}
+	bpf_spin_unlock(&val->l);
+
+	return 0;
+}
+
 char _license[] SEC("license") = "GPL";