bpf: cel: require __V511_BPF_PROG or later

Loading the programs in 5.4 and 4.19 fails with:

   ; tail_call(ctx, tailcalls, TAIL_CALL_ARGS);
        3573: (bf) r1 = r6
        3574: (18) r2 = 0xffff888138f0e400
        3576: (b7) r3 = 3
        3577: (85) call bpf_tail_call#12
        tail_calls are not allowed in programs with bpf-to-bpf calls

Because tail-calls cannot mix with bpf-to-bpf calls.
Add an appropriate guard for this.

Signed-off-by: Kornilios Kourtis <[email protected]>

Author: kkourt

bpf/process/cel_expr.h

@@ -8,7 +8,12 @@
  * The code for the cel_expr_N functions will be generated and linked whtn the policy is loaded
  */
 
-#if defined(GENERIC_KPROBE) && !defined(__MULTI_KPROBE)
+/*
+ * NB: The cel_expr code uses bpf-to-bpf calls, so it needs to be guarded by __V511_BPF_PROG because
+ * previous kernel vesions do not allow mixing bpf-to-bpf calls and tail-calls. Ref:
+ * https://lore.kernel.org/bpf/[email protected]/T/
+ */
+#if defined(__V511_BPF_PROG) && (defined(GENERIC_KPROBE) && !defined(__MULTI_KPROBE))
 int cel_expr_0(long *argsoff, char *args);
 int cel_expr_1(long *argsoff, char *args);
 int cel_expr_2(long *argsoff, char *args);