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.
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 BPF_kfunc_nodef.
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");
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 BPF_kfunc_nodef.
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.
This annotation is used to indicate a memory and size pair in the argument list. An example is given below:
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.
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:
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.
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.
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.
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.
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.
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.
The KF_KPTR_GET flag is used to indicate that the kfunc takes the first argument as a pointer to kptr, safely increments the refcount of the object it points to, and returns a reference to the user. The rest of the arguments may be normal arguments of a kfunc. The KF_KPTR_GET flag should be used in conjunction with KF_ACQUIRE and KF_RET_NULL flags.
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).
There are two types of pointers to kernel objects which are considered "valid":
- Pointers which are passed as tracepoint or struct_ops callback arguments.
- Pointers which were returned from a KF_ACQUIRE or KF_KPTR_GET 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.
The KF_SLEEPABLE flag is used for kfuncs that may sleep. Such kfuncs can only be called by sleepable BPF programs (BPF_F_SLEEPABLE).
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.
The KF_RCU flag is used for kfuncs which have a rcu ptr as its argument. When used together with KF_ACQUIRE, it indicates the kfunc should have a single argument which must be a trusted argument or a MEM_RCU pointer. The argument may have reference count of 0 and the kfunc must take this into consideration.
2.4.9 KF_CHANGES_PKT flag -----------------
The KF_CHANGES_PKT is used for kfuncs that may change packet data. After calls to such kfuncs, existing packet pointers will be invalidated and must be revalidated before the prog can access packet data.
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);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.
There are a number of kfuncs that allow struct task_struct * objects to be used as kptrs:
kernel/bpf/helpers.c
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);
/*
* 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;
}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/bpf/helpers.c
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;
}struct cgroup * objects also have acquire and release functions:
kernel/bpf/helpers.c
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.
You may also acquire a reference to a struct cgroup kptr that's already stored in a map using bpf_cgroup_kptr_get():
kernel/bpf/helpers.c
Here's an example of how it can be used:
/* struct containing the struct task_struct kptr which is actually stored in the map. */
struct __cgroups_kfunc_map_value {
struct cgroup __kptr_ref * cgroup;
};
/* The map containing struct __cgroups_kfunc_map_value entries. */
struct {
__uint(type, BPF_MAP_TYPE_HASH);
__type(key, int);
__type(value, struct __cgroups_kfunc_map_value);
__uint(max_entries, 1);
} __cgroups_kfunc_map SEC(".maps");
/* ... */
/**
* A simple example tracepoint program showing how a
* struct cgroup kptr that is stored in a map can
* be acquired using the bpf_cgroup_kptr_get() kfunc.
*/
SEC("tp_btf/cgroup_mkdir")
int BPF_PROG(cgroup_kptr_get_example, struct cgroup *cgrp, const char *path)
{
struct cgroup *kptr;
struct __cgroups_kfunc_map_value *v;
s32 id = cgrp->self.id;
/* Assume a cgroup kptr was previously stored in the map. */
v = bpf_map_lookup_elem(&__cgroups_kfunc_map, &id);
if (!v)
return -ENOENT;
/* Acquire a reference to the cgroup kptr that's already stored in the map. */
kptr = bpf_cgroup_kptr_get(&v->cgroup);
if (!kptr)
/* If no cgroup was present in the map, it's because
* we're racing with another CPU that removed it with
* bpf_kptr_xchg() between the bpf_map_lookup_elem()
* above, and our call to bpf_cgroup_kptr_get().
* bpf_cgroup_kptr_get() internally safely handles this
* race, and will return NULL if the task is no longer
* present in the map by the time we invoke the kfunc.
*/
return -EBUSY;
/* Free the reference we just took above. Note that the
* original struct cgroup kptr is still in the map. It will
* be freed either at a later time if another context deletes
* it from the map, or automatically by the BPF subsystem if
* it's still present when the map is destroyed.
*/
bpf_cgroup_release(kptr);
return 0;
}Another kfunc available for interacting with struct cgroup * objects is bpf_cgroup_ancestor(). This allows callers to access the ancestor of a cgroup, and return it as a cgroup kptr.
kernel/bpf/helpers.c
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;
}