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+.. SPDX-License-Identifier: GPL-2.0
+
+.. _kfuncs-header-label:
+
+=============================
+BPF Kernel Functions (kfuncs)
+=============================
+
+1. Introduction
+===============
+
+BPF Kernel Functions or more commonly known as kfuncs are functions in the Linux
+kernel which are exposed for use by BPF programs. Unlike normal BPF helpers,
+kfuncs do not have a stable interface and can change from one kernel release to
+another. Hence, BPF programs need to be updated in response to changes in the
+kernel. See :ref:`BPF_kfunc_lifecycle_expectations` for more information.
+
+2. Defining a kfunc
+===================
+
+There are two ways to expose a kernel function to BPF programs, either make an
+existing function in the kernel visible, or add a new wrapper for BPF. In both
+cases, care must be taken that BPF program can only call such function in a
+valid context. To enforce this, visibility of a kfunc can be per program type.
+
+If you are not creating a BPF wrapper for existing kernel function, skip ahead
+to :ref:`BPF_kfunc_nodef`.
+
+2.1 Creating a wrapper kfunc
+----------------------------
+
+When defining a wrapper kfunc, the wrapper function should have extern linkage.
+This prevents the compiler from optimizing away dead code, as this wrapper kfunc
+is not invoked anywhere in the kernel itself. It is not necessary to provide a
+prototype in a header for the wrapper kfunc.
+
+An example is given below::
+
+ /* Disables missing prototype warnings */
+ __diag_push();
+ __diag_ignore_all("-Wmissing-prototypes",
+ "Global kfuncs as their definitions will be in BTF");
+
+ __bpf_kfunc struct task_struct *bpf_find_get_task_by_vpid(pid_t nr)
+ {
+ return find_get_task_by_vpid(nr);
+ }
+
+ __diag_pop();
+
+A wrapper kfunc is often needed when we need to annotate parameters of the
+kfunc. Otherwise one may directly make the kfunc visible to the BPF program by
+registering it with the BPF subsystem. See :ref:`BPF_kfunc_nodef`.
+
+2.2 Annotating kfunc parameters
+-------------------------------
+
+Similar to BPF helpers, there is sometime need for additional context required
+by the verifier to make the usage of kernel functions safer and more useful.
+Hence, we can annotate a parameter by suffixing the name of the argument of the
+kfunc with a __tag, where tag may be one of the supported annotations.
+
+2.2.1 __sz Annotation
+---------------------
+
+This annotation is used to indicate a memory and size pair in the argument list.
+An example is given below::
+
+ __bpf_kfunc void bpf_memzero(void *mem, int mem__sz)
+ {
+ ...
+ }
+
+Here, the verifier will treat first argument as a PTR_TO_MEM, and second
+argument as its size. By default, without __sz annotation, the size of the type
+of the pointer is used. Without __sz annotation, a kfunc cannot accept a void
+pointer.
+
+2.2.2 __k Annotation
+--------------------
+
+This annotation is only understood for scalar arguments, where it indicates that
+the verifier must check the scalar argument to be a known constant, which does
+not indicate a size parameter, and the value of the constant is relevant to the
+safety of the program.
+
+An example is given below::
+
+ __bpf_kfunc void *bpf_obj_new(u32 local_type_id__k, ...)
+ {
+ ...
+ }
+
+Here, bpf_obj_new uses local_type_id argument to find out the size of that type
+ID in program's BTF and return a sized pointer to it. Each type ID will have a
+distinct size, hence it is crucial to treat each such call as distinct when
+values don't match during verifier state pruning checks.
+
+Hence, whenever a constant scalar argument is accepted by a kfunc which is not a
+size parameter, and the value of the constant matters for program safety, __k
+suffix should be used.
+
+2.2.3 __uninit Annotation
+-------------------------
+
+This annotation is used to indicate that the argument will be treated as
+uninitialized.
+
+An example is given below::
+
+ __bpf_kfunc int bpf_dynptr_from_skb(..., struct bpf_dynptr_kern *ptr__uninit)
+ {
+ ...
+ }
+
+Here, the dynptr will be treated as an uninitialized dynptr. Without this
+annotation, the verifier will reject the program if the dynptr passed in is
+not initialized.
+
+2.2.4 __opt Annotation
+-------------------------
+
+This annotation is used to indicate that the buffer associated with an __sz or __szk
+argument may be null. If the function is passed a nullptr in place of the buffer,
+the verifier will not check that length is appropriate for the buffer. The kfunc is
+responsible for checking if this buffer is null before using it.
+
+An example is given below::
+
+ __bpf_kfunc void *bpf_dynptr_slice(..., void *buffer__opt, u32 buffer__szk)
+ {
+ ...
+ }
+
+Here, the buffer may be null. If buffer is not null, it at least of size buffer_szk.
+Either way, the returned buffer is either NULL, or of size buffer_szk. Without this
+annotation, the verifier will reject the program if a null pointer is passed in with
+a nonzero size.
+
+
+.. _BPF_kfunc_nodef:
+
+2.3 Using an existing kernel function
+-------------------------------------
+
+When an existing function in the kernel is fit for consumption by BPF programs,
+it can be directly registered with the BPF subsystem. However, care must still
+be taken to review the context in which it will be invoked by the BPF program
+and whether it is safe to do so.
+
+2.4 Annotating kfuncs
+---------------------
+
+In addition to kfuncs' arguments, verifier may need more information about the
+type of kfunc(s) being registered with the BPF subsystem. To do so, we define
+flags on a set of kfuncs as follows::
+
+ BTF_SET8_START(bpf_task_set)
+ BTF_ID_FLAGS(func, bpf_get_task_pid, KF_ACQUIRE | KF_RET_NULL)
+ BTF_ID_FLAGS(func, bpf_put_pid, KF_RELEASE)
+ BTF_SET8_END(bpf_task_set)
+
+This set encodes the BTF ID of each kfunc listed above, and encodes the flags
+along with it. Ofcourse, it is also allowed to specify no flags.
+
+kfunc definitions should also always be annotated with the ``__bpf_kfunc``
+macro. This prevents issues such as the compiler inlining the kfunc if it's a
+static kernel function, or the function being elided in an LTO build as it's
+not used in the rest of the kernel. Developers should not manually add
+annotations to their kfunc to prevent these issues. If an annotation is
+required to prevent such an issue with your kfunc, it is a bug and should be
+added to the definition of the macro so that other kfuncs are similarly
+protected. An example is given below::
+
+ __bpf_kfunc struct task_struct *bpf_get_task_pid(s32 pid)
+ {
+ ...
+ }
+
+2.4.1 KF_ACQUIRE flag
+---------------------
+
+The KF_ACQUIRE flag is used to indicate that the kfunc returns a pointer to a
+refcounted object. The verifier will then ensure that the pointer to the object
+is eventually released using a release kfunc, or transferred to a map using a
+referenced kptr (by invoking bpf_kptr_xchg). If not, the verifier fails the
+loading of the BPF program until no lingering references remain in all possible
+explored states of the program.
+
+2.4.2 KF_RET_NULL flag
+----------------------
+
+The KF_RET_NULL flag is used to indicate that the pointer returned by the kfunc
+may be NULL. Hence, it forces the user to do a NULL check on the pointer
+returned from the kfunc before making use of it (dereferencing or passing to
+another helper). This flag is often used in pairing with KF_ACQUIRE flag, but
+both are orthogonal to each other.
+
+2.4.3 KF_RELEASE flag
+---------------------
+
+The KF_RELEASE flag is used to indicate that the kfunc releases the pointer
+passed in to it. There can be only one referenced pointer that can be passed
+in. All copies of the pointer being released are invalidated as a result of
+invoking kfunc with this flag. KF_RELEASE kfuncs automatically receive the
+protection afforded by the KF_TRUSTED_ARGS flag described below.
+
+2.4.4 KF_TRUSTED_ARGS flag
+--------------------------
+
+The KF_TRUSTED_ARGS flag is used for kfuncs taking pointer arguments. It
+indicates that the all pointer arguments are valid, and that all pointers to
+BTF objects have been passed in their unmodified form (that is, at a zero
+offset, and without having been obtained from walking another pointer, with one
+exception described below).
+
+There are two types of pointers to kernel objects which are considered "valid":
+
+1. Pointers which are passed as tracepoint or struct_ops callback arguments.
+2. Pointers which were returned from a KF_ACQUIRE kfunc.
+
+Pointers to non-BTF objects (e.g. scalar pointers) may also be passed to
+KF_TRUSTED_ARGS kfuncs, and may have a non-zero offset.
+
+The definition of "valid" pointers is subject to change at any time, and has
+absolutely no ABI stability guarantees.
+
+As mentioned above, a nested pointer obtained from walking a trusted pointer is
+no longer trusted, with one exception. If a struct type has a field that is
+guaranteed to be valid (trusted or rcu, as in KF_RCU description below) as long
+as its parent pointer is valid, the following macros can be used to express
+that to the verifier:
+
+* ``BTF_TYPE_SAFE_TRUSTED``
+* ``BTF_TYPE_SAFE_RCU``
+* ``BTF_TYPE_SAFE_RCU_OR_NULL``
+
+For example,
+
+.. code-block:: c
+
+ BTF_TYPE_SAFE_TRUSTED(struct socket) {
+ struct sock *sk;
+ };
+
+or
+
+.. code-block:: c
+
+ BTF_TYPE_SAFE_RCU(struct task_struct) {
+ const cpumask_t *cpus_ptr;
+ struct css_set __rcu *cgroups;
+ struct task_struct __rcu *real_parent;
+ struct task_struct *group_leader;
+ };
+
+In other words, you must:
+
+1. Wrap the valid pointer type in a ``BTF_TYPE_SAFE_*`` macro.
+
+2. Specify the type and name of the valid nested field. This field must match
+ the field in the original type definition exactly.
+
+A new type declared by a ``BTF_TYPE_SAFE_*`` macro also needs to be emitted so
+that it appears in BTF. For example, ``BTF_TYPE_SAFE_TRUSTED(struct socket)``
+is emitted in the ``type_is_trusted()`` function as follows:
+
+.. code-block:: c
+
+ BTF_TYPE_EMIT(BTF_TYPE_SAFE_TRUSTED(struct socket));
+
+
+2.4.5 KF_SLEEPABLE flag
+-----------------------
+
+The KF_SLEEPABLE flag is used for kfuncs that may sleep. Such kfuncs can only
+be called by sleepable BPF programs (BPF_F_SLEEPABLE).
+
+2.4.6 KF_DESTRUCTIVE flag
+--------------------------
+
+The KF_DESTRUCTIVE flag is used to indicate functions calling which is
+destructive to the system. For example such a call can result in system
+rebooting or panicking. Due to this additional restrictions apply to these
+calls. At the moment they only require CAP_SYS_BOOT capability, but more can be
+added later.
+
+2.4.7 KF_RCU flag
+-----------------
+
+The KF_RCU flag is a weaker version of KF_TRUSTED_ARGS. The kfuncs marked with
+KF_RCU expect either PTR_TRUSTED or MEM_RCU arguments. The verifier guarantees
+that the objects are valid and there is no use-after-free. The pointers are not
+NULL, but the object's refcount could have reached zero. The kfuncs need to
+consider doing refcnt != 0 check, especially when returning a KF_ACQUIRE
+pointer. Note as well that a KF_ACQUIRE kfunc that is KF_RCU should very likely
+also be KF_RET_NULL.
+
+.. _KF_deprecated_flag:
+
+2.4.8 KF_DEPRECATED flag
+------------------------
+
+The KF_DEPRECATED flag is used for kfuncs which are scheduled to be
+changed or removed in a subsequent kernel release. A kfunc that is
+marked with KF_DEPRECATED should also have any relevant information
+captured in its kernel doc. Such information typically includes the
+kfunc's expected remaining lifespan, a recommendation for new
+functionality that can replace it if any is available, and possibly a
+rationale for why it is being removed.
+
+Note that while on some occasions, a KF_DEPRECATED kfunc may continue to be
+supported and have its KF_DEPRECATED flag removed, it is likely to be far more
+difficult to remove a KF_DEPRECATED flag after it's been added than it is to
+prevent it from being added in the first place. As described in
+:ref:`BPF_kfunc_lifecycle_expectations`, users that rely on specific kfuncs are
+encouraged to make their use-cases known as early as possible, and participate
+in upstream discussions regarding whether to keep, change, deprecate, or remove
+those kfuncs if and when such discussions occur.
+
+2.5 Registering the kfuncs
+--------------------------
+
+Once the kfunc is prepared for use, the final step to making it visible is
+registering it with the BPF subsystem. Registration is done per BPF program
+type. An example is shown below::
+
+ BTF_SET8_START(bpf_task_set)
+ BTF_ID_FLAGS(func, bpf_get_task_pid, KF_ACQUIRE | KF_RET_NULL)
+ BTF_ID_FLAGS(func, bpf_put_pid, KF_RELEASE)
+ BTF_SET8_END(bpf_task_set)
+
+ static const struct btf_kfunc_id_set bpf_task_kfunc_set = {
+ .owner = THIS_MODULE,
+ .set = &bpf_task_set,
+ };
+
+ static int init_subsystem(void)
+ {
+ return register_btf_kfunc_id_set(BPF_PROG_TYPE_TRACING, &bpf_task_kfunc_set);
+ }
+ late_initcall(init_subsystem);
+
+2.6 Specifying no-cast aliases with ___init
+--------------------------------------------
+
+The verifier will always enforce that the BTF type of a pointer passed to a
+kfunc by a BPF program, matches the type of pointer specified in the kfunc
+definition. The verifier, does, however, allow types that are equivalent
+according to the C standard to be passed to the same kfunc arg, even if their
+BTF_IDs differ.
+
+For example, for the following type definition:
+
+.. code-block:: c
+
+ struct bpf_cpumask {
+ cpumask_t cpumask;
+ refcount_t usage;
+ };
+
+The verifier would allow a ``struct bpf_cpumask *`` to be passed to a kfunc
+taking a ``cpumask_t *`` (which is a typedef of ``struct cpumask *``). For
+instance, both ``struct cpumask *`` and ``struct bpf_cpmuask *`` can be passed
+to bpf_cpumask_test_cpu().
+
+In some cases, this type-aliasing behavior is not desired. ``struct
+nf_conn___init`` is one such example:
+
+.. code-block:: c
+
+ struct nf_conn___init {
+ struct nf_conn ct;
+ };
+
+The C standard would consider these types to be equivalent, but it would not
+always be safe to pass either type to a trusted kfunc. ``struct
+nf_conn___init`` represents an allocated ``struct nf_conn`` object that has
+*not yet been initialized*, so it would therefore be unsafe to pass a ``struct
+nf_conn___init *`` to a kfunc that's expecting a fully initialized ``struct
+nf_conn *`` (e.g. ``bpf_ct_change_timeout()``).
+
+In order to accommodate such requirements, the verifier will enforce strict
+PTR_TO_BTF_ID type matching if two types have the exact same name, with one
+being suffixed with ``___init``.
+
+.. _BPF_kfunc_lifecycle_expectations:
+
+3. kfunc lifecycle expectations
+===============================
+
+kfuncs provide a kernel <-> kernel API, and thus are not bound by any of the
+strict stability restrictions associated with kernel <-> user UAPIs. This means
+they can be thought of as similar to EXPORT_SYMBOL_GPL, and can therefore be
+modified or removed by a maintainer of the subsystem they're defined in when
+it's deemed necessary.
+
+Like any other change to the kernel, maintainers will not change or remove a
+kfunc without having a reasonable justification. Whether or not they'll choose
+to change a kfunc will ultimately depend on a variety of factors, such as how
+widely used the kfunc is, how long the kfunc has been in the kernel, whether an
+alternative kfunc exists, what the norm is in terms of stability for the
+subsystem in question, and of course what the technical cost is of continuing
+to support the kfunc.
+
+There are several implications of this:
+
+a) kfuncs that are widely used or have been in the kernel for a long time will
+ be more difficult to justify being changed or removed by a maintainer. In
+ other words, kfuncs that are known to have a lot of users and provide
+ significant value provide stronger incentives for maintainers to invest the
+ time and complexity in supporting them. It is therefore important for
+ developers that are using kfuncs in their BPF programs to communicate and
+ explain how and why those kfuncs are being used, and to participate in
+ discussions regarding those kfuncs when they occur upstream.
+
+b) Unlike regular kernel symbols marked with EXPORT_SYMBOL_GPL, BPF programs
+ that call kfuncs are generally not part of the kernel tree. This means that
+ refactoring cannot typically change callers in-place when a kfunc changes,
+ as is done for e.g. an upstreamed driver being updated in place when a
+ kernel symbol is changed.
+
+ Unlike with regular kernel symbols, this is expected behavior for BPF
+ symbols, and out-of-tree BPF programs that use kfuncs should be considered
+ relevant to discussions and decisions around modifying and removing those
+ kfuncs. The BPF community will take an active role in participating in
+ upstream discussions when necessary to ensure that the perspectives of such
+ users are taken into account.
+
+c) A kfunc will never have any hard stability guarantees. BPF APIs cannot and
+ will not ever hard-block a change in the kernel purely for stability
+ reasons. That being said, kfuncs are features that are meant to solve
+ problems and provide value to users. The decision of whether to change or
+ remove a kfunc is a multivariate technical decision that is made on a
+ case-by-case basis, and which is informed by data points such as those
+ mentioned above. It is expected that a kfunc being removed or changed with
+ no warning will not be a common occurrence or take place without sound
+ justification, but it is a possibility that must be accepted if one is to
+ use kfuncs.
+
+3.1 kfunc deprecation
+---------------------
+
+As described above, while sometimes a maintainer may find that a kfunc must be
+changed or removed immediately to accommodate some changes in their subsystem,
+usually kfuncs will be able to accommodate a longer and more measured
+deprecation process. For example, if a new kfunc comes along which provides
+superior functionality to an existing kfunc, the existing kfunc may be
+deprecated for some period of time to allow users to migrate their BPF programs
+to use the new one. Or, if a kfunc has no known users, a decision may be made
+to remove the kfunc (without providing an alternative API) after some
+deprecation period so as to provide users with a window to notify the kfunc
+maintainer if it turns out that the kfunc is actually being used.
+
+It's expected that the common case will be that kfuncs will go through a
+deprecation period rather than being changed or removed without warning. As
+described in :ref:`KF_deprecated_flag`, the kfunc framework provides the
+KF_DEPRECATED flag to kfunc developers to signal to users that a kfunc has been
+deprecated. Once a kfunc has been marked with KF_DEPRECATED, the following
+procedure is followed for removal:
+
+1. Any relevant information for deprecated kfuncs is documented in the kfunc's
+ kernel docs. This documentation will typically include the kfunc's expected
+ remaining lifespan, a recommendation for new functionality that can replace
+ the usage of the deprecated function (or an explanation as to why no such
+ replacement exists), etc.
+
+2. The deprecated kfunc is kept in the kernel for some period of time after it
+ was first marked as deprecated. This time period will be chosen on a
+ case-by-case basis, and will typically depend on how widespread the use of
+ the kfunc is, how long it has been in the kernel, and how hard it is to move
+ to alternatives. This deprecation time period is "best effort", and as
+ described :ref:`above<BPF_kfunc_lifecycle_expectations>`, circumstances may
+ sometimes dictate that the kfunc be removed before the full intended
+ deprecation period has elapsed.
+
+3. After the deprecation period the kfunc will be removed. At this point, BPF
+ programs calling the kfunc will be rejected by the verifier.
+
+4. Core kfuncs
+==============
+
+The BPF subsystem provides a number of "core" kfuncs that are potentially
+applicable to a wide variety of different possible use cases and programs.
+Those kfuncs are documented here.
+
+4.1 struct task_struct * kfuncs
+-------------------------------
+
+There are a number of kfuncs that allow ``struct task_struct *`` objects to be
+used as kptrs:
+
+.. kernel-doc:: kernel/bpf/helpers.c
+ :identifiers: bpf_task_acquire bpf_task_release
+
+These kfuncs are useful when you want to acquire or release a reference to a
+``struct task_struct *`` that was passed as e.g. a tracepoint arg, or a
+struct_ops callback arg. For example:
+
+.. code-block:: c
+
+ /**
+ * A trivial example tracepoint program that shows how to
+ * acquire and release a struct task_struct * pointer.
+ */
+ SEC("tp_btf/task_newtask")
+ int BPF_PROG(task_acquire_release_example, struct task_struct *task, u64 clone_flags)
+ {
+ struct task_struct *acquired;
+
+ acquired = bpf_task_acquire(task);
+ if (acquired)
+ /*
+ * In a typical program you'd do something like store
+ * the task in a map, and the map will automatically
+ * release it later. Here, we release it manually.
+ */
+ bpf_task_release(acquired);
+ return 0;
+ }
+
+
+References acquired on ``struct task_struct *`` objects are RCU protected.
+Therefore, when in an RCU read region, you can obtain a pointer to a task
+embedded in a map value without having to acquire a reference:
+
+.. code-block:: c
+
+ #define private(name) SEC(".data." #name) __hidden __attribute__((aligned(8)))
+ private(TASK) static struct task_struct *global;
+
+ /**
+ * A trivial example showing how to access a task stored
+ * in a map using RCU.
+ */
+ SEC("tp_btf/task_newtask")
+ int BPF_PROG(task_rcu_read_example, struct task_struct *task, u64 clone_flags)
+ {
+ struct task_struct *local_copy;
+
+ bpf_rcu_read_lock();
+ local_copy = global;
+ if (local_copy)
+ /*
+ * We could also pass local_copy to kfuncs or helper functions here,
+ * as we're guaranteed that local_copy will be valid until we exit
+ * the RCU read region below.
+ */
+ bpf_printk("Global task %s is valid", local_copy->comm);
+ else
+ bpf_printk("No global task found");
+ bpf_rcu_read_unlock();
+
+ /* At this point we can no longer reference local_copy. */
+
+ return 0;
+ }
+
+----
+
+A BPF program can also look up a task from a pid. This can be useful if the
+caller doesn't have a trusted pointer to a ``struct task_struct *`` object that
+it can acquire a reference on with bpf_task_acquire().
+
+.. kernel-doc:: kernel/bpf/helpers.c
+ :identifiers: bpf_task_from_pid
+
+Here is an example of it being used:
+
+.. code-block:: c
+
+ SEC("tp_btf/task_newtask")
+ int BPF_PROG(task_get_pid_example, struct task_struct *task, u64 clone_flags)
+ {
+ struct task_struct *lookup;
+
+ lookup = bpf_task_from_pid(task->pid);
+ if (!lookup)
+ /* A task should always be found, as %task is a tracepoint arg. */
+ return -ENOENT;
+
+ if (lookup->pid != task->pid) {
+ /* bpf_task_from_pid() looks up the task via its
+ * globally-unique pid from the init_pid_ns. Thus,
+ * the pid of the lookup task should always be the
+ * same as the input task.
+ */
+ bpf_task_release(lookup);
+ return -EINVAL;
+ }
+
+ /* bpf_task_from_pid() returns an acquired reference,
+ * so it must be dropped before returning from the
+ * tracepoint handler.
+ */
+ bpf_task_release(lookup);
+ return 0;
+ }
+
+4.2 struct cgroup * kfuncs
+--------------------------
+
+``struct cgroup *`` objects also have acquire and release functions:
+
+.. kernel-doc:: kernel/bpf/helpers.c
+ :identifiers: bpf_cgroup_acquire bpf_cgroup_release
+
+These kfuncs are used in exactly the same manner as bpf_task_acquire() and
+bpf_task_release() respectively, so we won't provide examples for them.
+
+----
+
+Other kfuncs available for interacting with ``struct cgroup *`` objects are
+bpf_cgroup_ancestor() and bpf_cgroup_from_id(), allowing callers to access
+the ancestor of a cgroup and find a cgroup by its ID, respectively. Both
+return a cgroup kptr.
+
+.. kernel-doc:: kernel/bpf/helpers.c
+ :identifiers: bpf_cgroup_ancestor
+
+.. kernel-doc:: kernel/bpf/helpers.c
+ :identifiers: bpf_cgroup_from_id
+
+Eventually, BPF should be updated to allow this to happen with a normal memory
+load in the program itself. This is currently not possible without more work in
+the verifier. bpf_cgroup_ancestor() can be used as follows:
+
+.. code-block:: c
+
+ /**
+ * Simple tracepoint example that illustrates how a cgroup's
+ * ancestor can be accessed using bpf_cgroup_ancestor().
+ */
+ SEC("tp_btf/cgroup_mkdir")
+ int BPF_PROG(cgrp_ancestor_example, struct cgroup *cgrp, const char *path)
+ {
+ struct cgroup *parent;
+
+ /* The parent cgroup resides at the level before the current cgroup's level. */
+ parent = bpf_cgroup_ancestor(cgrp, cgrp->level - 1);
+ if (!parent)
+ return -ENOENT;
+
+ bpf_printk("Parent id is %d", parent->self.id);
+
+ /* Return the parent cgroup that was acquired above. */
+ bpf_cgroup_release(parent);
+ return 0;
+ }
+
+4.3 struct cpumask * kfuncs
+---------------------------
+
+BPF provides a set of kfuncs that can be used to query, allocate, mutate, and
+destroy struct cpumask * objects. Please refer to :ref:`cpumasks-header-label`
+for more details.