selftests/bpf: Test RCU pointer invalidation after spin unlock

The verifier previously accepted a task kptr after the final spin unlock
ended its RCU protection in a sleepable program. The pointer could then be
used after the task was freed.

Add a negative test for that case. Add positive controls showing that the
pointer remains valid in a non-sleepable program and while an explicit RCU
read-side section is still active.

Assisted-by: Codex:gpt-5.6-sol
Assisted-by: ChatGPT:GPT-5.6-Pro
Signed-off-by: Ning Ding <dingning04@gmail.com>
Link: https://lore.kernel.org/bpf/20260803112615.3362122-3-dingning04@gmail.com
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>

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

@@ -176,6 +176,8 @@ static const char * const success_tests[] = {
 	"test_task_from_pid_current",
 	"test_task_from_pid_invalid",
 	"task_kfunc_acquire_trusted_walked",
+	"task_kfunc_acquire_after_spin_unlock_non_sleepable",
+	"task_kfunc_acquire_after_spin_unlock_explicit_rcu",
 	"test_task_kfunc_flavor_relo",
 	"test_task_kfunc_flavor_relo_not_found",
 };

tools/testing/selftests/bpf/progs/task_kfunc_common.h

@@ -20,6 +20,18 @@ struct {
 	__uint(max_entries, 1);
 } __tasks_kfunc_map SEC(".maps");
 
+struct task_kptr_lock_value {
+	struct bpf_spin_lock lock;
+	struct task_struct __kptr * task;
+};
+
+struct {
+	__uint(type, BPF_MAP_TYPE_ARRAY);
+	__type(key, int);
+	__type(value, struct task_kptr_lock_value);
+	__uint(max_entries, 1);
+} task_kptr_lock_map SEC(".maps");
+
 struct task_struct *bpf_task_acquire(struct task_struct *p) __ksym;
 void bpf_task_release(struct task_struct *p) __ksym;
 struct task_struct *bpf_task_from_pid(s32 pid) __ksym;

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

@@ -378,3 +378,27 @@ int BPF_PROG(task_kfunc_release_in_map, struct task_struct *task, u64 clone_flag
 
 	return 0;
 }
+
+SEC("?fentry.s/" SYS_PREFIX "sys_getpgid")
+__failure __msg("R1 must be a rcu pointer")
+int BPF_PROG(task_kfunc_acquire_after_final_spin_unlock)
+{
+	struct task_kptr_lock_value *v;
+	struct task_struct *task, *acquired;
+	int key = 0;
+
+	v = bpf_map_lookup_elem(&task_kptr_lock_map, &key);
+	if (!v)
+		return 0;
+
+	bpf_spin_lock(&v->lock);
+	task = v->task;
+	bpf_spin_unlock(&v->lock);
+	if (!task)
+		return 0;
+
+	acquired = bpf_task_acquire(task);
+	if (acquired)
+		bpf_task_release(acquired);
+	return 0;
+}

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

@@ -6,6 +6,7 @@
 #include <bpf/bpf_helpers.h>
 
 #include "../bpf_experimental.h"
+#include "bpf_misc.h"
 #include "task_kfunc_common.h"
 
 char _license[] SEC("license") = "GPL";
@@ -366,6 +367,53 @@ int BPF_PROG(task_kfunc_acquire_trusted_walked, struct task_struct *task, u64 cl
 	return 0;
 }
 
+SEC("fentry/" SYS_PREFIX "sys_getpgid")
+int BPF_PROG(task_kfunc_acquire_after_spin_unlock_non_sleepable)
+{
+	struct task_kptr_lock_value *v;
+	struct task_struct *task, *acquired;
+	int key = 0;
+
+	v = bpf_map_lookup_elem(&task_kptr_lock_map, &key);
+	if (!v)
+		return 0;
+
+	bpf_spin_lock(&v->lock);
+	task = v->task;
+	bpf_spin_unlock(&v->lock);
+	if (!task)
+		return 0;
+
+	acquired = bpf_task_acquire(task);
+	if (acquired)
+		bpf_task_release(acquired);
+	return 0;
+}
+
+SEC("fentry.s/" SYS_PREFIX "sys_getpgid")
+int BPF_PROG(task_kfunc_acquire_after_spin_unlock_explicit_rcu)
+{
+	struct task_kptr_lock_value *v;
+	struct task_struct *task, *acquired;
+	int key = 0;
+
+	v = bpf_map_lookup_elem(&task_kptr_lock_map, &key);
+	if (!v)
+		return 0;
+
+	bpf_rcu_read_lock();
+	bpf_spin_lock(&v->lock);
+	task = v->task;
+	bpf_spin_unlock(&v->lock);
+	if (task) {
+		acquired = bpf_task_acquire(task);
+		if (acquired)
+			bpf_task_release(acquired);
+	}
+	bpf_rcu_read_unlock();
+	return 0;
+}
+
 SEC("syscall")
 int test_task_from_vpid_current(const void *ctx)
 {