[PATCH bpf-next v6 2/2] selftests/bpf: Add tests for kprobe attachment with duplicate symbols

Andrey Grodzovsky
2026-04-07T16:52:09Z

bpf_fentry_shadow_test exists in both vmlinux (net/bpf/test_run.c) and
bpf_testmod (bpf_testmod.c), creating a duplicate symbol condition when
bpf_testmod is loaded. Add subtests that verify kprobe behavior with
this duplicate symbol:

In attach_probe:
- dup-sym-{default,legacy,perf,link}: unqualified attach succeeds
across all four modes, preferring vmlinux over module shadow.
- dup-sym-module-{default,legacy,perf,link}: MOD:SYM qualification
(bpf_testmod:bpf_fentry_shadow_test) attaches to the module version.

In kprobe_multi_test:
- dup_sym: kprobe_multi attach with kprobe and kretprobe succeeds.

Signed-off-by: Andrey Grodzovsky <andrey.grodzovsky@crowdstrike.com>
---
.../selftests/bpf/prog_tests/attach_probe.c | 68 +++++++++++++++++++
.../bpf/prog_tests/kprobe_multi_test.c | 39 +++++++++++
2 files changed, 107 insertions(+)

diff --git a/tools/testing/selftests/bpf/prog_tests/attach_probe.c b/tools/testing/selftests/bpf/prog_tests/attach_probe.c
index 12a841afda68..2d52547c395e 100644
--- a/tools/testing/selftests/bpf/prog_tests/attach_probe.c
+++ b/tools/testing/selftests/bpf/prog_tests/attach_probe.c
@@ -197,6 +197,65 @@ static void test_attach_kprobe_legacy_by_addr_reject(void)
test_attach_probe_manual__destroy(skel);
}

+/* bpf_fentry_shadow_test exists in both vmlinux (net/bpf/test_run.c) and
+ * bpf_testmod (bpf_testmod.c). When bpf_testmod is loaded the symbol is
+ * duplicated. Test that kprobe attachment handles this correctly:
+ * - Unqualified name ("bpf_fentry_shadow_test") attaches to vmlinux.
+ * - MOD:SYM name ("bpf_testmod:bpf_fentry_shadow_test") attaches to module.
+ *
+ * Note: bpf_fentry_shadow_test is not invoked via test_run, so we only
+ * verify that attach and detach succeed without triggering the probe.
+ */
+static void test_attach_probe_dup_sym(enum probe_attach_mode attach_mode)
+{
+ DECLARE_LIBBPF_OPTS(bpf_kprobe_opts, kprobe_opts);
+ struct bpf_link *kprobe_link, *kretprobe_link;
+ struct test_attach_probe_manual *skel;
+
+ skel = test_attach_probe_manual__open_and_load();
+ if (!ASSERT_OK_PTR(skel, "skel_dup_sym_open_and_load"))
+ return;
+
+ kprobe_opts.attach_mode = attach_mode;
+
+ /* Unqualified: should attach to vmlinux symbol */
+ kprobe_opts.retprobe = false;
+ kprobe_link = bpf_program__attach_kprobe_opts(skel->progs.handle_kprobe,
+ "bpf_fentry_shadow_test",
+ &kprobe_opts);
+ if (!ASSERT_OK_PTR(kprobe_link, "attach_kprobe_vmlinux"))
+ goto cleanup;
+ bpf_link__destroy(kprobe_link);
+
+ kprobe_opts.retprobe = true;
+ kretprobe_link = bpf_program__attach_kprobe_opts(skel->progs.handle_kretprobe,
+ "bpf_fentry_shadow_test",
+ &kprobe_opts);
+ if (!ASSERT_OK_PTR(kretprobe_link, "attach_kretprobe_vmlinux"))
+ goto cleanup;
+ bpf_link__destroy(kretprobe_link);
+
+ /* MOD:SYM qualified: should attach to module symbol */
+ kprobe_opts.retprobe = false;
+ kprobe_link = bpf_program__attach_kprobe_opts(skel->progs.handle_kprobe,
+ "bpf_testmod:bpf_fentry_shadow_test",
+ &kprobe_opts);
+ if (!ASSERT_OK_PTR(kprobe_link, "attach_kprobe_module"))
+ goto cleanup;
+ bpf_link__destroy(kprobe_link);
+
+ kprobe_opts.retprobe = true;
+ kretprobe_link = bpf_program__attach_kprobe_opts(skel->progs.handle_kretprobe,
+ "bpf_testmod:bpf_fentry_shadow_test",
+ &kprobe_opts);
+ if (!ASSERT_OK_PTR(kretprobe_link, "attach_kretprobe_module"))
+ goto cleanup;
+ bpf_link__destroy(kretprobe_link);
+
+cleanup:
+ test_attach_probe_manual__destroy(skel);
+}
+
/* attach uprobe/uretprobe long event name testings */
static void test_attach_uprobe_long_event_name(void)
{
@@ -559,6 +618,15 @@ void test_attach_probe(void)
if (test__start_subtest("kprobe-legacy-by-addr-reject"))
test_attach_kprobe_legacy_by_addr_reject();

+ if (test__start_subtest("dup-sym-default"))
+ test_attach_probe_dup_sym(PROBE_ATTACH_MODE_DEFAULT);
+ if (test__start_subtest("dup-sym-legacy"))
+ test_attach_probe_dup_sym(PROBE_ATTACH_MODE_LEGACY);
+ if (test__start_subtest("dup-sym-perf"))
+ test_attach_probe_dup_sym(PROBE_ATTACH_MODE_PERF);
+ if (test__start_subtest("dup-sym-link"))
+ test_attach_probe_dup_sym(PROBE_ATTACH_MODE_LINK);
+
if (test__start_subtest("auto"))
test_attach_probe_auto(skel);
if (test__start_subtest("kprobe-sleepable"))
diff --git a/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c b/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
index 78c974d4ea33..d89650e31fd3 100644
--- a/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
+++ b/tools/testing/selftests/bpf/prog_tests/kprobe_multi_test.c
@@ -633,6 +633,43 @@ static void test_attach_write_ctx(void)
}
#endif

+/* Test kprobe_multi handles shadow symbols (vmlinux + module duplicate).
+ * bpf_fentry_shadow_test exists in both vmlinux and bpf_testmod.
+ * kprobe_multi resolves via ftrace_lookup_symbols() which finds the
+ * vmlinux symbol first and stops, so this should always succeed.
+ */
+static void test_attach_probe_dup_sym(void)
+{
+ LIBBPF_OPTS(bpf_kprobe_multi_opts, opts);
+ const char *syms[1] = { "bpf_fentry_shadow_test" };
+ struct kprobe_multi *skel = NULL;
+ struct bpf_link *link1 = NULL, *link2 = NULL;
+
+ skel = kprobe_multi__open_and_load();
+ if (!ASSERT_OK_PTR(skel, "kprobe_multi__open_and_load"))
+ goto cleanup;
+
+ skel->bss->pid = getpid();
+ opts.syms = syms;
+ opts.cnt = ARRAY_SIZE(syms);
+
+ link1 = bpf_program__attach_kprobe_multi_opts(skel->progs.test_kprobe_manual,
+ NULL, &opts);
+ if (!ASSERT_OK_PTR(link1, "attach_kprobe_multi_dup_sym"))
+ goto cleanup;
+
+ opts.retprobe = true;
+ link2 = bpf_program__attach_kprobe_multi_opts(skel->progs.test_kretprobe_manual,
+ NULL, &opts);
+ if (!ASSERT_OK_PTR(link2, "attach_kretprobe_multi_dup_sym"))
+ goto cleanup;
+
+cleanup:
+ bpf_link__destroy(link2);
+ bpf_link__destroy(link1);
+ kprobe_multi__destroy(skel);
+}
+
void serial_test_kprobe_multi_bench_attach(void)
{
if (test__start_subtest("kernel"))
@@ -676,5 +713,7 @@ void test_kprobe_multi_test(void)
test_unique_match();
if (test__start_subtest("attach_write_ctx"))
test_attach_write_ctx();
+ if (test__start_subtest("dup_sym"))
+ test_attach_probe_dup_sym();
RUN_TESTS(kprobe_multi_verifier);
}
--
2.34.1