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Subject[PATCH v4 2/3] Now that BPF supports adding new kernel functions with kfuncs, and storing kernel objects in maps with kptrs, we can add a set of kfuncs which allow struct task_struct objects to be stored in maps as referenced kptrs. The possible use case
Date
In order to enable this, this patch adds three new kfuncs:

struct task_struct *bpf_task_acquire(struct task_struct *p);
struct task_struct *bpf_task_kptr_get(struct task_struct **pp);
void bpf_task_release(struct task_struct *p);

A follow-on patch will add selftests validating these kfuncs.

Signed-off-by: David Vernet <void@manifault.com>
---
kernel/bpf/helpers.c | 86 +++++++++++++++++++++++++++++++++++++++++---
1 file changed, 81 insertions(+), 5 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index a6b04faed282..9d0307969977 100644
--- a/kernel/bpf/helpers.c
+++ b/kernel/bpf/helpers.c
@@ -1700,20 +1700,96 @@ bpf_base_func_proto(enum bpf_func_id func_id)
}
}

-BTF_SET8_START(tracing_btf_ids)
+__diag_push();
+__diag_ignore_all("-Wmissing-prototypes",
+ "Global functions as their definitions will be in vmlinux BTF");
+
+/**
+ * bpf_task_acquire - Acquire a reference to a task. A task acquired by this
+ * kfunc which is not stored in a map as a kptr, must be released by calling
+ * bpf_task_release().
+ * @p: The task on which a reference is being acquired.
+ */
+__used noinline
+struct task_struct *bpf_task_acquire(struct task_struct *p)
+{
+ if (!p)
+ return NULL;
+
+ refcount_inc(&p->rcu_users);
+ return p;
+}
+
+/**
+ * bpf_task_kptr_get - Acquire a reference on a struct task_struct kptr. A task
+ * kptr acquired by this kfunc which is not subsequently stored in a map, must
+ * be released by calling bpf_task_release().
+ * @pp: A pointer to a task kptr on which a reference is being acquired.
+ */
+__used noinline
+struct task_struct *bpf_task_kptr_get(struct task_struct **pp)
+{
+ struct task_struct *p;
+
+ rcu_read_lock();
+ p = READ_ONCE(*pp);
+ if (p && !refcount_inc_not_zero(&p->rcu_users))
+ p = NULL;
+ rcu_read_unlock();
+
+ return p;
+}
+
+/**
+ * bpf_task_release - Release the reference acquired on a struct task_struct *.
+ * If this kfunc is invoked in an RCU read region, the task_struct is
+ * guaranteed to not be freed until the current grace period has ended, even if
+ * its refcount drops to 0.
+ * @p: The task on which a reference is being released.
+ */
+__used noinline void bpf_task_release(struct task_struct *p)
+{
+ if (!p)
+ return;
+
+ put_task_struct_rcu_user(p);
+}
+
+__diag_pop();
+
+BTF_SET8_START(generic_kfunc_btf_ids)
#ifdef CONFIG_KEXEC_CORE
BTF_ID_FLAGS(func, crash_kexec, KF_DESTRUCTIVE)
#endif
-BTF_SET8_END(tracing_btf_ids)
+BTF_ID_FLAGS(func, bpf_task_acquire, KF_ACQUIRE | KF_RET_NULL | KF_TRUSTED_ARGS)
+BTF_ID_FLAGS(func, bpf_task_kptr_get, KF_ACQUIRE | KF_KPTR_GET | KF_RET_NULL)
+BTF_ID_FLAGS(func, bpf_task_release, KF_RELEASE | KF_TRUSTED_ARGS)
+BTF_SET8_END(generic_kfunc_btf_ids)

-static const struct btf_kfunc_id_set tracing_kfunc_set = {
+static const struct btf_kfunc_id_set generic_kfunc_set = {
.owner = THIS_MODULE,
- .set = &tracing_btf_ids,
+ .set = &generic_kfunc_btf_ids,
};

+BTF_ID_LIST(generic_kfunc_dtor_ids)
+BTF_ID(struct, task_struct)
+BTF_ID(func, bpf_task_release)
+
static int __init kfunc_init(void)
{
- return register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &tracing_kfunc_set);
+ int ret;
+ const struct btf_id_dtor_kfunc generic_kfunc_dtors[] = {
+ {
+ .btf_id = generic_kfunc_dtor_ids[0],
+ .kfunc_btf_id = generic_kfunc_dtor_ids[1]
+ },
+ };
+
+ ret = register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &generic_kfunc_set);
+ ret = ret ?: register_btf_kfunc_id_set(BPF_PROG_TYPE_STRUCT_OPS, &generic_kfunc_set);
+ return ret ?: register_btf_id_dtor_kfuncs(generic_kfunc_dtors,
+ ARRAY_SIZE(generic_kfunc_dtors),
+ THIS_MODULE);
}

late_initcall(kfunc_init);
--
2.38.0
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