trampoline_count fills all trampoline attachment slots for a single
target function and verifies that one extra attach fails with -E2BIG.

It currently targets bpf_modify_return_test, which is also used by
other selftests such as modify_return, get_func_ip_test, and
get_func_args_test. When such tests run in parallel, they can contend
for the same per-function trampoline quota and cause unexpected attach
failures. This issue is currently masked by harness serialization.

Move trampoline_count to a dedicated bpf_testmod target and register it
for fmod_ret attachment. This keeps the test semantics unchanged while
isolating it from other selftests, so it no longer needs to run in
serial mode. Remove the TODO comment as well.

Tested:
  ./test_progs -t trampoline_count -vv
  ./test_progs -t modify_return -vv
  ./test_progs -t get_func_ip_test -vv
  ./test_progs -t get_func_args_test -vv
  ./test_progs -j$(nproc) -t trampoline_count -vv
  ./test_progs -j$(nproc) -t
    trampoline_count,modify_return,get_func_ip_test,get_func_args_test,\
kprobe_multi_test -vv
  20 runs of:
    ./test_progs -j$(nproc) -t
    trampoline_count,modify_return,get_func_ip_test,get_func_args_test,\
kprobe_multi_test

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
---
 .../selftests/bpf/prog_tests/trampoline_count.c |  3 +--
 .../selftests/bpf/progs/test_trampoline_count.c |  6 +++---
 .../selftests/bpf/test_kmods/bpf_testmod.c      | 17 +++++++++++++++++
 3 files changed, 21 insertions(+), 5 deletions(-)

diff --git a/tools/testing/selftests/bpf/prog_tests/trampoline_count.c b/tools/testing/selftests/bpf/prog_tests/trampoline_count.c
index 6cd7349d4a2b..b19358aed126 100644
--- a/tools/testing/selftests/bpf/prog_tests/trampoline_count.c
+++ b/tools/testing/selftests/bpf/prog_tests/trampoline_count.c
@@ -30,8 +30,7 @@ static struct bpf_program *load_prog(char *file, char *name, struct inst *inst)
 	return prog;
 }
 
-/* TODO: use different target function to run in concurrent mode */
-void serial_test_trampoline_count(void)
+void test_trampoline_count(void)
 {
 	char *file = "test_trampoline_count.bpf.o";
 	char *const progs[] = { "fentry_test", "fmod_ret_test", "fexit_test" };
diff --git a/tools/testing/selftests/bpf/progs/test_trampoline_count.c b/tools/testing/selftests/bpf/progs/test_trampoline_count.c
index 7765720da7d5..14ad2f53cf33 100644
--- a/tools/testing/selftests/bpf/progs/test_trampoline_count.c
+++ b/tools/testing/selftests/bpf/progs/test_trampoline_count.c
@@ -3,19 +3,19 @@
 #include <bpf/bpf_helpers.h>
 #include <bpf/bpf_tracing.h>
 
-SEC("fentry/bpf_modify_return_test")
+SEC("fentry/bpf_testmod_trampoline_count_test")
 int BPF_PROG(fentry_test, int a, int *b)
 {
 	return 0;
 }
 
-SEC("fmod_ret/bpf_modify_return_test")
+SEC("fmod_ret/bpf_testmod_trampoline_count_test")
 int BPF_PROG(fmod_ret_test, int a, int *b, int ret)
 {
 	return 0;
 }
 
-SEC("fexit/bpf_modify_return_test")
+SEC("fexit/bpf_testmod_trampoline_count_test")
 int BPF_PROG(fexit_test, int a, int *b, int ret)
 {
 	return 0;
diff --git a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
index e62c6b78657f..6eaceb4dfbec 100644
--- a/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
+++ b/tools/testing/selftests/bpf/test_kmods/bpf_testmod.c
@@ -581,6 +581,12 @@ noinline int bpf_fentry_shadow_test(int a)
 }
 EXPORT_SYMBOL_GPL(bpf_fentry_shadow_test);
 
+noinline int bpf_testmod_trampoline_count_test(int a, int *b)
+{
+	*b += 1;
+	return a + *b;
+}
+
 __bpf_hook_end();
 
 static struct bin_attribute bin_attr_bpf_testmod_file __ro_after_init = {
@@ -1843,6 +1849,16 @@ struct bpf_struct_ops testmod_multi_st_ops = {
 
 extern int bpf_fentry_test1(int a);
 
+BTF_KFUNCS_START(bpf_testmod_trampoline_count_ids)
+BTF_ID_FLAGS(func, bpf_testmod_trampoline_count_test)
+BTF_KFUNCS_END(bpf_testmod_trampoline_count_ids)
+
+static const struct
+btf_kfunc_id_set bpf_testmod_trampoline_count_fmodret_set = {
+	.owner = THIS_MODULE,
+	.set = &bpf_testmod_trampoline_count_ids,
+};
+
 static int bpf_testmod_init(void)
 {
 	const struct btf_id_dtor_kfunc bpf_testmod_dtors[] = {
@@ -1859,6 +1875,7 @@ static int bpf_testmod_init(void)
 	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &bpf_testmod_kfunc_set);
 	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_SYSCALL, &bpf_testmod_kfunc_set);
 	ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &bpf_testmod_kfunc_set);
+	ret = ret ?: register_btf_fmodret_id_set(&bpf_testmod_trampoline_count_fmodret_set);
 	ret = ret ?: register_bpf_struct_ops(&bpf_bpf_testmod_ops, bpf_testmod_ops);
 	ret = ret ?: register_bpf_struct_ops(&bpf_testmod_ops2, bpf_testmod_ops2);
 	ret = ret ?: register_bpf_struct_ops(&bpf_testmod_ops3, bpf_testmod_ops3);

base-commit: 8a30aeb0d1b4e4aaf7f7bae72f20f2ae75385ccb
-- 
2.43.0