bpf: tail calls do not modify packet data

kernel/bpf/verifier.c

@@ -17827,7 +17827,8 @@ static int visit_insn(int t, struct bpf_verifier_env *env)
 			 */
 			if (ret == 0 && fp->might_sleep)
 				mark_subprog_might_sleep(env, t);
-			if (bpf_helper_changes_pkt_data(insn->imm))
+			if (bpf_helper_changes_pkt_data(insn->imm)
+					|| insn->imm == BPF_FUNC_tail_call)
 				mark_subprog_changes_pkt_data(env, t);
 		} else if (insn->src_reg == BPF_PSEUDO_KFUNC_CALL) {
 			struct bpf_kfunc_call_arg_meta meta;

net/core/filter.c

@@ -8032,8 +8032,6 @@ bool bpf_helper_changes_pkt_data(enum bpf_func_id func_id)
 	case BPF_FUNC_xdp_adjust_head:
 	case BPF_FUNC_xdp_adjust_meta:
 	case BPF_FUNC_xdp_adjust_tail:
-	/* tail-called program could call any of the above */
-	case BPF_FUNC_tail_call:
 		return true;
 	default:
 		return false;

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

@@ -1117,10 +1117,10 @@ int tail_call(struct __sk_buff *sk)
 	return 0;
 }
 
-/* Tail calls invalidate packet pointers. */
+/* Tail calls in sub-programs invalidate packet pointers. */
 SEC("tc")
 __failure __msg("invalid mem access")
-int invalidate_pkt_pointers_by_tail_call(struct __sk_buff *sk)
+int invalidate_pkt_pointers_by_indirect_tail_call(struct __sk_buff *sk)
 {
 	int *p = (void *)(long)sk->data;
 
@@ -1131,4 +1131,18 @@ int invalidate_pkt_pointers_by_tail_call(struct __sk_buff *sk)
 	return TCX_PASS;
 }
 
+/* Direct tail calls do not invalidate packet pointers. */
+SEC("tc")
+__success
+int invalidate_pkt_pointers_by_tail_call(struct __sk_buff *sk)
+{
+	int *p = (void *)(long)sk->data;
+
+	if ((void *)(p + 1) > (void *)(long)sk->data_end)
+		return TCX_DROP;
+	bpf_tail_call_static(sk, &jmp_table, 0);
+	*p = 42; /* this is NOT unsafe: tail calls don't return */
+	return TCX_PASS;
+}
+
 char _license[] SEC("license") = "GPL";