selftests/bpf: validate eliminated global subprog is not freplaceable

Add selftest that establishes dead code-eliminated valid global subprog
(global_dead) and makes sure that it's not possible to freplace it, as
it's effectively not there. This test will fail with unexpected success
before 2afae08c9dcb ("bpf: Validate global subprogs lazily").

v2->v3:
  - add missing err assignment (Alan);
  - undo unnecessary signature changes in verifier_global_subprogs.c (Eduard);
v1->v2:
  - don't rely on assembly output in verifier log, which changes between
    compiler versions (CI).

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: Alan Maguire <alan.maguire@oracle.com>
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: John Fastabend <john.fastabend@gmail.com>
Link: https://lore.kernel.org/r/20231211174131.2324306-1-andrii@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

1 files changed, 60 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/global_func_dead_code.c b/tools/testing/selftests/bpf/prog_tests/global_func_dead_code.c
new file mode 100644
index 000000000000..65309894b27a
--- /dev/null
+++ b/tools/testing/selftests/bpf/prog_tests/global_func_dead_code.c
@@ -0,0 +1,60 @@
+// SPDX-License-Identifier: GPL-2.0
+/* Copyright (c) 2023 Meta Platforms, Inc. and affiliates. */
+
+#include <test_progs.h>
+#include "verifier_global_subprogs.skel.h"
+#include "freplace_dead_global_func.skel.h"
+
+void test_global_func_dead_code(void)
+{
+	struct verifier_global_subprogs *tgt_skel = NULL;
+	struct freplace_dead_global_func *skel = NULL;
+	char log_buf[4096];
+	int err, tgt_fd;
+
+	/* first, try to load target with good global subprog */
+	tgt_skel = verifier_global_subprogs__open();
+	if (!ASSERT_OK_PTR(tgt_skel, "tgt_skel_good_open"))
+		return;
+
+	bpf_program__set_autoload(tgt_skel->progs.chained_global_func_calls_success, true);
+
+	err = verifier_global_subprogs__load(tgt_skel);
+	if (!ASSERT_OK(err, "tgt_skel_good_load"))
+		goto out;
+
+	tgt_fd = bpf_program__fd(tgt_skel->progs.chained_global_func_calls_success);
+
+	/* Attach to good non-eliminated subprog */
+	skel = freplace_dead_global_func__open();
+	if (!ASSERT_OK_PTR(skel, "skel_good_open"))
+		goto out;
+
+	err = bpf_program__set_attach_target(skel->progs.freplace_prog, tgt_fd, "global_good");
+	ASSERT_OK(err, "attach_target_good");
+
+	err = freplace_dead_global_func__load(skel);
+	if (!ASSERT_OK(err, "skel_good_load"))
+		goto out;
+
+	freplace_dead_global_func__destroy(skel);
+
+	/* Try attaching to dead code-eliminated subprog */
+	skel = freplace_dead_global_func__open();
+	if (!ASSERT_OK_PTR(skel, "skel_dead_open"))
+		goto out;
+
+	bpf_program__set_log_buf(skel->progs.freplace_prog, log_buf, sizeof(log_buf));
+	err = bpf_program__set_attach_target(skel->progs.freplace_prog, tgt_fd, "global_dead");
+	ASSERT_OK(err, "attach_target_dead");
+
+	err = freplace_dead_global_func__load(skel);
+	if (!ASSERT_ERR(err, "skel_dead_load"))
+		goto out;
+
+	ASSERT_HAS_SUBSTR(log_buf, "Subprog global_dead doesn't exist", "dead_subprog_missing_msg");
+
+out:
+	verifier_global_subprogs__destroy(tgt_skel);
+	freplace_dead_global_func__destroy(skel);
+}