selftests/bpf: Add tests for raw_tp null handling

Ensure that trusted PTR_TO_BTF_ID accesses perform PROBE_MEM handling in
raw_tp program. Without the previous fix, this selftest crashes the
kernel due to a NULL-pointer dereference. Also ensure that dead code
elimination does not kick in for checks on the pointer.

Signed-off-by: Kumar Kartikeya Dwivedi <[email protected]>

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod-events.h

@@ -40,6 +40,14 @@ DECLARE_TRACE(bpf_testmod_test_nullable_bare,
 	TP_ARGS(ctx__nullable)
 );
 
+struct sk_buff;
+
+DECLARE_TRACE(bpf_testmod_test_raw_tp_null,
+	TP_PROTO(struct sk_buff *skb),
+	TP_ARGS(skb)
+);
+
+
 #undef BPF_TESTMOD_DECLARE_TRACE
 #ifdef DECLARE_TRACE_WRITABLE
 #define BPF_TESTMOD_DECLARE_TRACE(call, proto, args, size) \

tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c

@@ -380,6 +380,8 @@ bpf_testmod_test_read(struct file *file, struct kobject *kobj,
 
 	(void)bpf_testmod_test_arg_ptr_to_struct(&struct_arg1_2);
 
+	(void)trace_bpf_testmod_test_raw_tp_null(NULL);
+
 	struct_arg3 = kmalloc((sizeof(struct bpf_testmod_struct_arg_3) +
 				sizeof(int)), GFP_KERNEL);
 	if (struct_arg3 != NULL) {

tools/testing/selftests/bpf/prog_tests/raw_tp_null.c

@@ -0,0 +1,25 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Meta Platforms, Inc. and affiliates. */
+
+#include <test_progs.h>
+#include "raw_tp_null.skel.h"
+
+void test_raw_tp_null(void)
+{
+	struct raw_tp_null *skel;
+
+	skel = raw_tp_null__open_and_load();
+	if (!ASSERT_OK_PTR(skel, "raw_tp_null__open_and_load"))
+		return;
+
+	skel->bss->tid = gettid();
+
+	if (!ASSERT_OK(raw_tp_null__attach(skel), "raw_tp_null__attach"))
+		goto end;
+
+	ASSERT_OK(trigger_module_test_read(2), "trigger testmod read");
+	ASSERT_EQ(skel->bss->i, 3, "invocations");
+
+end:
+	raw_tp_null__destroy(skel);
+}

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

@@ -0,0 +1,27 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2024 Meta Platforms, Inc. and affiliates. */
+
+#include <vmlinux.h>
+#include <bpf/bpf_tracing.h>
+
+char _license[] SEC("license") = "GPL";
+
+int tid;
+int i;
+
+SEC("tp_btf/bpf_testmod_test_raw_tp_null")
+int BPF_PROG(test_raw_tp_null, struct sk_buff *skb)
+{
+	if (bpf_get_current_task_btf()->pid == tid) {
+		i = i + skb->mark + 1;
+
+		/* If dead code elimination kicks in, the increment below will
+		 * be removed. For raw_tp programs, we mark input arguments as
+		 * PTR_MAYBE_NULL, so branch prediction should never kick in.
+		 */
+		if (!skb)
+			i += 2;
+	}
+
+	return 0;
+}