[PATCH bpf-next] selftests/bpf: validate eliminated global subprog is not freplaceable

[Andrii Nakryiko <andrii@kernel.org>]

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").

Suggested-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 .../bpf/prog_tests/global_func_dead_code.c    | 60 +++++++++++++++++++
 .../bpf/progs/freplace_dead_global_func.c     | 11 ++++
 .../bpf/progs/verifier_global_subprogs.c      | 32 ++++++----
 3 files changed, 92 insertions(+), 11 deletions(-)
 create mode 100644 tools/testing/selftests/bpf/prog_tests/global_func_dead_code.c
 create mode 100644 tools/testing/selftests/bpf/progs/freplace_dead_global_func.c

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..2716f6ccfe22
--- /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;
+
+	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);
+}
diff --git a/tools/testing/selftests/bpf/progs/freplace_dead_global_func.c b/tools/testing/selftests/bpf/progs/freplace_dead_global_func.c
new file mode 100644
index 000000000000..808738eac578
--- /dev/null
+++ b/tools/testing/selftests/bpf/progs/freplace_dead_global_func.c
@@ -0,0 +1,11 @@
+// SPDX-License-Identifier: GPL-2.0
+#include <linux/bpf.h>
+#include <bpf/bpf_helpers.h>
+
+SEC("freplace")
+int freplace_prog(int x)
+{
+	return 0;
+}
+
+char _license[] SEC("license") = "GPL";
diff --git a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
index a0a5efd1caa1..7f9b21a1c5a7 100644
--- a/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
+++ b/tools/testing/selftests/bpf/progs/verifier_global_subprogs.c
@@ -10,25 +10,31 @@
 
 int arr[1];
 int unkn_idx;
+const volatile bool call_dead_subprog = false;
 
-__noinline long global_bad(void)
+__noinline long global_bad(int x)
 {
-	return arr[unkn_idx]; /* BOOM */
+	return arr[unkn_idx] + x; /* BOOM */
 }
 
-__noinline long global_good(void)
+__noinline long global_good(int x)
 {
-	return arr[0];
+	return arr[0] + x;
 }
 
-__noinline long global_calls_bad(void)
+__noinline long global_calls_bad(int x)
 {
-	return global_good() + global_bad() /* does BOOM indirectly */;
+	return global_good(x) + global_bad(x) /* does BOOM indirectly */;
 }
 
-__noinline long global_calls_good_only(void)
+__noinline long global_calls_good_only(int x)
 {
-	return global_good();
+	return global_good(x);
+}
+
+__noinline long global_dead(int x)
+{
+	return x * 2;
 }
 
 SEC("?raw_tp")
@@ -41,19 +47,23 @@ __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(42);
+	return global_calls_good_only(42) + sum;
 }
 
 SEC("?raw_tp")
 __failure __log_level(2)
 /* main prog validated successfully first */
-__msg("1: (95) exit")
+__msg("2: (95) exit")
 /* eventually we validate global_bad() and fail */
 __msg("Validating global_bad() func")
 __msg("math between map_value pointer and register") /* BOOM */
 int chained_global_func_calls_bad(void)
 {
-	return global_calls_bad();
+	return global_calls_bad(13);
 }
 
 /* do out of bounds access forcing verifier to fail verification if this
-- 
2.34.1