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 2afae08c ("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.orgSigned-off-by: Alexei Starovoitov <ast@kernel.org>

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

+// 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);
+}

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

+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+SEC("freplace")
+int freplace_prog(void)
+{
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";

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

@@ -10,6 +10,7 @@
 
 int arr[1];
 int unkn_idx;
+const volatile bool call_dead_subprog = false;
 
 __noinline long global_bad(void)
 {
@@ -31,23 +32,31 @@ __noinline long global_calls_good_only(void)
 	return global_good();
 }
 
+__noinline long global_dead(void)
+{
+	return arr[0] * 2;
+}
+
 SEC("?raw_tp")
 __success __log_level(2)
 /* main prog is validated completely first */
 __msg("('global_calls_good_only') is global and assumed valid.")
-__msg("1: (95) exit")
 /* eventually global_good() is transitively validated as well */
 __msg("Validating global_good() func")
 __msg("('global_good') is safe for any args that match its prototype")
 int chained_global_func_calls_success(void)
 {
-	return global_calls_good_only();
+	int sum = 0;
+
+	if (call_dead_subprog)
+		sum += global_dead();
+	return global_calls_good_only() + sum;
 }
 
 SEC("?raw_tp")
 __failure __log_level(2)
 /* main prog validated successfully first */
-__msg("1: (95) exit")
+__msg("('global_calls_bad') is global and assumed valid.")
 /* eventually we validate global_bad() and fail */
 __msg("Validating global_bad() func")
 __msg("math between map_value pointer and register") /* BOOM */