selftests/bpf: Fix map_kptr grace period wait

Commit c27cea4416 ("rcu: Re-implement RCU Tasks Trace in terms of SRCU-fast")
broke map_kptr selftest since it removed the function we were kprobing.
Use a new kfunc that invokes call_rcu_tasks_trace and sets a program
provided pointer to an integer to 1. Technically this can be unsafe if
the memory being written to from the callback disappears, but this is
just for usage in a test where we ensure we spin until we see the value
to be set to 1, so it's ok.

Reported-by: Shung-Hsi Yu <shung-hsi.yu@suse.com>
Fixes: c27cea4416 ("rcu: Re-implement RCU Tasks Trace in terms of SRCU-fast")
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Link: https://lore.kernel.org/r/20260211185747.3630539-1-memxor@gmail.com
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

tools/testing/selftests/bpf/prog_tests/map_kptr.c

@@ -118,15 +118,16 @@ static void test_map_kptr_success(bool test_run)
 
 static int kern_sync_rcu_tasks_trace(struct rcu_tasks_trace_gp *rcu)
 {
-	long gp_seq = READ_ONCE(rcu->bss->gp_seq);
 	LIBBPF_OPTS(bpf_test_run_opts, opts);
+	int ret;
 
-	if (!ASSERT_OK(bpf_prog_test_run_opts(bpf_program__fd(rcu->progs.do_call_rcu_tasks_trace),
-					      &opts), "do_call_rcu_tasks_trace"))
+	WRITE_ONCE(rcu->bss->done, 0);
+	ret = bpf_prog_test_run_opts(bpf_program__fd(rcu->progs.call_rcu_tasks_trace), &opts);
+	if (!ASSERT_OK(ret, "call_rcu_tasks_trace"))
 		return -EFAULT;
-	if (!ASSERT_OK(opts.retval, "opts.retval == 0"))
+	if (!ASSERT_OK(opts.retval, "call_rcu_tasks_trace retval"))
 		return -EFAULT;
-	while (gp_seq == READ_ONCE(rcu->bss->gp_seq))
+	while (!READ_ONCE(rcu->bss->done))
 		sched_yield();
 	return 0;
 }
@@ -159,8 +160,6 @@ void serial_test_map_kptr(void)
 	skel = rcu_tasks_trace_gp__open_and_load();
 	if (!ASSERT_OK_PTR(skel, "rcu_tasks_trace_gp__open_and_load"))
 		return;
-	if (!ASSERT_OK(rcu_tasks_trace_gp__attach(skel), "rcu_tasks_trace_gp__attach"))
-		goto end;
 
 	if (test__start_subtest("success-map")) {
 		test_map_kptr_success(true);
@@ -180,7 +179,5 @@ void serial_test_map_kptr(void)
 		test_map_kptr_success(true);
 	}
 
-end:
 	rcu_tasks_trace_gp__destroy(skel);
-	return;
 }

tools/testing/selftests/bpf/progs/rcu_tasks_trace_gp.c

@@ -1,36 +1,14 @@
 // SPDX-License-Identifier: GPL-2.0
 #include <vmlinux.h>
-#include <bpf/bpf_tracing.h>
 #include <bpf/bpf_helpers.h>
+#include "../test_kmods/bpf_testmod_kfunc.h"
 
-struct task_ls_map {
-	__uint(type, BPF_MAP_TYPE_TASK_STORAGE);
-	__uint(map_flags, BPF_F_NO_PREALLOC);
-	__type(key, int);
-	__type(value, int);
-} task_ls_map SEC(".maps");
-
-long gp_seq;
+int done;
 
 SEC("syscall")
-int do_call_rcu_tasks_trace(void *ctx)
+int call_rcu_tasks_trace(void *ctx)
 {
-    struct task_struct *current;
-    int *v;
-
-    current = bpf_get_current_task_btf();
-    v = bpf_task_storage_get(&task_ls_map, current, NULL, BPF_LOCAL_STORAGE_GET_F_CREATE);
-    if (!v)
-        return 1;
-    /* Invoke call_rcu_tasks_trace */
-    return bpf_task_storage_delete(&task_ls_map, current);
-}
-
-SEC("kprobe/rcu_tasks_trace_postgp")
-int rcu_tasks_trace_postgp(void *ctx)
-{
-    __sync_add_and_fetch(&gp_seq, 1);
-    return 0;
+	return bpf_kfunc_call_test_call_rcu_tasks_trace(&done);
 }
 
 char _license[] SEC("license") = "GPL";

tools/testing/selftests/bpf/test_kmods/bpf_testmod.c

@@ -18,6 +18,7 @@
 #include <linux/in6.h>
 #include <linux/un.h>
 #include <linux/filter.h>
+#include <linux/rcupdate_trace.h>
 #include <net/sock.h>
 #include <linux/namei.h>
 #include "bpf_testmod.h"
@@ -885,6 +886,32 @@ __bpf_kfunc void bpf_kfunc_call_test_sleepable(void)
 {
 }
 
+struct bpf_kfunc_rcu_tasks_trace_data {
+	struct rcu_head rcu;
+	int *done;
+};
+
+static void bpf_kfunc_rcu_tasks_trace_cb(struct rcu_head *rhp)
+{
+	struct bpf_kfunc_rcu_tasks_trace_data *data;
+
+	data = container_of(rhp, struct bpf_kfunc_rcu_tasks_trace_data, rcu);
+	WRITE_ONCE(*data->done, 1);
+	kfree(data);
+}
+
+__bpf_kfunc int bpf_kfunc_call_test_call_rcu_tasks_trace(int *done)
+{
+	struct bpf_kfunc_rcu_tasks_trace_data *data;
+
+	data = kmalloc(sizeof(*data), GFP_ATOMIC);
+	if (!data)
+		return -ENOMEM;
+	data->done = done;
+	call_rcu_tasks_trace(&data->rcu, bpf_kfunc_rcu_tasks_trace_cb);
+	return 0;
+}
+
 __bpf_kfunc int bpf_kfunc_init_sock(struct init_sock_args *args)
 {
 	int proto;
@@ -1222,6 +1249,7 @@ BTF_ID_FLAGS(func, bpf_kfunc_call_test_destructive, KF_DESTRUCTIVE)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_static_unused_arg)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_offset)
 BTF_ID_FLAGS(func, bpf_kfunc_call_test_sleepable, KF_SLEEPABLE)
+BTF_ID_FLAGS(func, bpf_kfunc_call_test_call_rcu_tasks_trace)
 BTF_ID_FLAGS(func, bpf_kfunc_init_sock, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_kfunc_close_sock, KF_SLEEPABLE)
 BTF_ID_FLAGS(func, bpf_kfunc_call_kernel_connect, KF_SLEEPABLE)

tools/testing/selftests/bpf/test_kmods/bpf_testmod_kfunc.h

@@ -118,6 +118,7 @@ void bpf_kfunc_call_test_mem_len_fail2(__u64 *mem, int len) __ksym;
 
 void bpf_kfunc_call_test_destructive(void) __ksym;
 void bpf_kfunc_call_test_sleepable(void) __ksym;
+int bpf_kfunc_call_test_call_rcu_tasks_trace(int *done) __ksym;
 
 void bpf_kfunc_call_test_offset(struct prog_test_ref_kfunc *p);
 struct prog_test_member *bpf_kfunc_call_memb_acquire(void);