selftests/bpf: Fix spurious test failures in core_retro selftest

core_retro selftest uses BPF program that's triggered on sys_enter
system-wide, but has no protection from some unrelated process doing syscall
while selftest is running. This leads to occasional test failures with
unexpected PIDs being returned. Fix that by filtering out all processes that
are not test_progs process.

Fixes: fcda189a5133 ("selftests/bpf: Add test relying only on CO-RE and no recent kernel features")
Signed-off-by: Andrii Nakryiko <[email protected]>
Signed-off-by: Alexei Starovoitov <[email protected]>
Acked-by: John Fastabend <[email protected]>
Link: https://lore.kernel.org/bpf/[email protected]

1 files changed, 13 insertions, 0 deletions

diff --git a/tools/testing/selftests/bpf/progs/test_core_retro.c b/tools/testing/selftests/bpf/progs/test_core_retro.c
index 75c60c3c29cf..20861ec2f674 100644
--- a/tools/testing/selftests/bpf/progs/test_core_retro.c
+++ b/tools/testing/selftests/bpf/progs/test_core_retro.c
@@ -13,6 +13,13 @@ struct {
 	__uint(max_entries, 1);
 	__type(key, int);
 	__type(value, int);
+} exp_tgid_map SEC(".maps");
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__uint(max_entries, 1);
+	__type(key, int);
+	__type(value, int);
 } results SEC(".maps");
 
 SEC("tp/raw_syscalls/sys_enter")
@@ -21,6 +28,12 @@ int handle_sys_enter(void *ctx)
 	struct task_struct *task = (void *)bpf_get_current_task();
 	int tgid = BPF_CORE_READ(task, tgid);
 	int zero = 0;
+	int real_tgid = bpf_get_current_pid_tgid() >> 32;
+	int *exp_tgid = bpf_map_lookup_elem(&exp_tgid_map, &zero);
+
+	/* only pass through sys_enters from test process */
+	if (!exp_tgid || *exp_tgid != real_tgid)
+		return 0;
 
 	bpf_map_update_elem(&results, &zero, &tgid, 0);