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 3. 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 2.3 Using an existing kernel function.

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 */
__bpf_kfunc_start_defs();

__bpf_kfunc struct task_struct *bpf_find_get_task_by_vpid(pid_t nr)
{
        return find_get_task_by_vpid(nr);
}

__bpf_kfunc_end_defs();

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 2.3 Using an existing kernel function.

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.

2.2.5 __str Annotation

This annotation is used to indicate that the argument is a constant string.

An example is given below:

__bpf_kfunc bpf_get_file_xattr(..., const char *name__str, ...)
{
...
}

In this case, bpf_get_file_xattr() can be called as:

bpf_get_file_xattr(..., "xattr_name", ...);

Or:

const char name[] = "xattr_name";  /* This need to be global */
int BPF_PROG(...)
{
        ...
        bpf_get_file_xattr(..., name, ...);
        ...
}

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_KFUNCS_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_KFUNCS_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,

BTF_TYPE_SAFE_TRUSTED(struct socket) {
        struct sock *sk;
};

or

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:

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.

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 3. 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_KFUNCS_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_KFUNCS_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:

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:

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.

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:

1. 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.

2. 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.

3. 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 2.4.8 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 above, 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:

__bpf_kfunc struct task_struct *bpf_task_acquire(struct task_struct *p)

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().

Parameters

struct task_struct *p

The task on which a reference is being acquired.

__bpf_kfunc void bpf_task_release(struct task_struct *p)

Release the reference acquired on a task.

Parameters

struct task_struct *p

The task on which a reference is being released.

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:

/**
 * 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:

#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().

__bpf_kfunc struct task_struct *bpf_task_from_pid(s32 pid)

Find a struct task_struct from its pid by looking it up in the root pid namespace idr. If a task is returned, it must either be stored in a map, or released with bpf_task_release().

Parameters

s32 pid

The pid of the task being looked up.

Here is an example of it being used:

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:

__bpf_kfunc struct cgroup *bpf_cgroup_acquire(struct cgroup *cgrp)

Acquire a reference to a cgroup. A cgroup acquired by this kfunc which is not stored in a map as a kptr, must be released by calling bpf_cgroup_release().

Parameters

struct cgroup *cgrp

The cgroup on which a reference is being acquired.

__bpf_kfunc void bpf_cgroup_release(struct cgroup *cgrp)

Release the reference acquired on a cgroup. If this kfunc is invoked in an RCU read region, the cgroup is guaranteed to not be freed until the current grace period has ended, even if its refcount drops to 0.

Parameters

struct cgroup *cgrp

The cgroup on which a reference is being released.

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.

__bpf_kfunc struct cgroup *bpf_cgroup_ancestor(struct cgroup *cgrp, int level)

Perform a lookup on an entry in a cgroup’s ancestor array. A cgroup returned by this kfunc which is not subsequently stored in a map, must be released by calling bpf_cgroup_release().

Parameters

struct cgroup *cgrp

The cgroup for which we’re performing a lookup.

int level

The level of ancestor to look up.

__bpf_kfunc struct cgroup *bpf_cgroup_from_id(u64 cgid)

Find a cgroup from its ID. A cgroup returned by this kfunc which is not subsequently stored in a map, must be released by calling bpf_cgroup_release().

Parameters

u64 cgid

cgroup 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:

/**
 * 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 BPF cpumask kfuncs for more details.