selftests/bpf: Isolate fmod_ret hooks by pid

modify_return's fmod_ret programs can override bpf_modify_return_test()'s
return value, which conflicts with get_func_ip_test when selftests run in
parallel.

Store current tgid in BSS and make modify_return hooks act only for that
tgid. For other tasks, fentry/fexit become no-ops and fmod_ret returns the
original ret.

Drop the serial-only restriction and remove the TODO comment.

Tested:
  sudo ./test_progs -t modify_return
  sudo ./test_progs -t get_func_ip_test
  sudo ./test_progs -j$(nproc) -t modify_return
  sudo ./test_progs -j$(nproc) -t get_func_ip_test

Signed-off-by: Sun Jian <sun.jian.kdev@gmail.com>
Link: https://lore.kernel.org/r/20260313034540.255805-1-sun.jian.kdev@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

2 files changed, 18 insertions, 3 deletions

diff --git a/tools/testing/selftests/bpf/prog_tests/modify_return.c b/tools/testing/selftests/bpf/prog_tests/modify_return.c
index a70c99c2f8c8..4661d77ebdfc 100644
--- a/tools/testing/selftests/bpf/prog_tests/modify_return.c
+++ b/tools/testing/selftests/bpf/prog_tests/modify_return.c
@@ -5,6 +5,7 @@
  */
 
 #include <test_progs.h>
+#include <unistd.h>
 #include "modify_return.skel.h"
 
 #define LOWER(x) ((x) & 0xffff)
@@ -23,11 +24,13 @@ static void run_test(__u32 input_retval, __u16 want_side_effect, __s16 want_ret)
 	if (!ASSERT_OK_PTR(skel, "skel_load"))
 		goto cleanup;
 
+	skel->bss->input_retval = input_retval;
+	skel->bss->test_pid = getpid();
+
 	err = modify_return__attach(skel);
 	if (!ASSERT_OK(err, "modify_return__attach failed"))
 		goto cleanup;
 
-	skel->bss->input_retval = input_retval;
 	prog_fd = bpf_program__fd(skel->progs.fmod_ret_test);
 	err = bpf_prog_test_run_opts(prog_fd, &topts);
 	ASSERT_OK(err, "test_run");
@@ -49,8 +52,7 @@ cleanup:
 	modify_return__destroy(skel);
 }
 
-/* TODO: conflict with get_func_ip_test */
-void serial_test_modify_return(void)
+void test_modify_return(void)
 {
 	run_test(0 /* input_retval */,
 		 2 /* want_side_effect */,
diff --git a/tools/testing/selftests/bpf/progs/modify_return.c b/tools/testing/selftests/bpf/progs/modify_return.c
index 3376d4849f58..68fabd2efe8d 100644
--- a/tools/testing/selftests/bpf/progs/modify_return.c
+++ b/tools/testing/selftests/bpf/progs/modify_return.c
@@ -12,11 +12,14 @@ char _license[] SEC("license") = "GPL";
 
 static int sequence = 0;
 __s32 input_retval = 0;
+__u32 test_pid = 0;
 
 __u64 fentry_result = 0;
 SEC("fentry/bpf_modify_return_test")
 int BPF_PROG(fentry_test, int a, __u64 b)
 {
+	if (bpf_get_current_pid_tgid() >> 32 != test_pid)
+		return 0;
 	sequence++;
 	fentry_result = (sequence == 1);
 	return 0;
@@ -26,6 +29,8 @@ __u64 fmod_ret_result = 0;
 SEC("fmod_ret/bpf_modify_return_test")
 int BPF_PROG(fmod_ret_test, int a, int *b, int ret)
 {
+	if (bpf_get_current_pid_tgid() >> 32 != test_pid)
+		return ret;
 	sequence++;
 	/* This is the first fmod_ret program, the ret passed should be 0 */
 	fmod_ret_result = (sequence == 2 && ret == 0);
@@ -36,6 +41,8 @@ __u64 fexit_result = 0;
 SEC("fexit/bpf_modify_return_test")
 int BPF_PROG(fexit_test, int a, __u64 b, int ret)
 {
+	if (bpf_get_current_pid_tgid() >> 32 != test_pid)
+		return 0;
 	sequence++;
 	/* If the input_reval is non-zero a successful modification should have
 	 * occurred.
@@ -55,6 +62,8 @@ SEC("fentry/bpf_modify_return_test2")
 int BPF_PROG(fentry_test2, int a, int *b, short c, int d, void *e, char f,
 	     int g)
 {
+	if (bpf_get_current_pid_tgid() >> 32 != test_pid)
+		return 0;
 	sequence2++;
 	fentry_result2 = (sequence2 == 1);
 	return 0;
@@ -65,6 +74,8 @@ SEC("fmod_ret/bpf_modify_return_test2")
 int BPF_PROG(fmod_ret_test2, int a, int *b, short c, int d, void *e, char f,
 	     int g, int ret)
 {
+	if (bpf_get_current_pid_tgid() >> 32 != test_pid)
+		return ret;
 	sequence2++;
 	/* This is the first fmod_ret program, the ret passed should be 0 */
 	fmod_ret_result2 = (sequence2 == 2 && ret == 0);
@@ -76,6 +87,8 @@ SEC("fexit/bpf_modify_return_test2")
 int BPF_PROG(fexit_test2, int a, int *b, short c, int d, void *e, char f,
 	     int g, int ret)
 {
+	if (bpf_get_current_pid_tgid() >> 32 != test_pid)
+		return 0;
 	sequence2++;
 	/* If the input_reval is non-zero a successful modification should have
 	 * occurred.