Libbpf eBPF macro __kptr

v0.8.0

The __kptr macros is used to tag a pointer to tell the verifier it holds pointers to kernel memory.

Definition

#define __kptr __attribute__((btf_type_tag("kptr")))

Usage

This macro can used on type definitions for both global variables and fields in map values. It informs the verifier that the pointer is a kernel pointer.

Docs could be improved

This part of the docs is incomplete, contributions are very welcome

Example

struct cpumask_map_value {
        struct bpf_cpumask __kptr * cpumask;
};

struct array_map {
        __uint(type, BPF_MAP_TYPE_ARRAY);
        __type(key, int);
        __type(value, struct cpumask_map_value);
        __uint(max_entries, 65536);
} cpumask_map SEC(".maps");

static int cpumask_map_insert(struct bpf_cpumask *mask, u32 pid)
{
        struct cpumask_map_value local, *v;
        long status;
        struct bpf_cpumask *old;
        u32 key = pid;

        local.cpumask = NULL;
        status = bpf_map_update_elem(&cpumask_map, &key, &local, 0);
        if (status) {
                bpf_cpumask_release(mask);
                return status;
        }

        v = bpf_map_lookup_elem(&cpumask_map, &key);
        if (!v) {
                bpf_cpumask_release(mask);
                return -ENOENT;
        }

        old = bpf_kptr_xchg(&v->cpumask, mask);
        if (old)
                bpf_cpumask_release(old);

        return 0;
}

/**
    * A sample tracepoint showing how a task's cpumask can be queried and
    * recorded as a kptr.
    */
SEC("tp_btf/task_newtask")
int BPF_PROG(record_task_cpumask, struct task_struct *task, u64 clone_flags)
{
        struct bpf_cpumask *cpumask;
        int ret;

        cpumask = bpf_cpumask_create();
        if (!cpumask)
                return -ENOMEM;

        if (!bpf_cpumask_full(task->cpus_ptr))
                bpf_printk("task %s has CPU affinity", task->comm);

        bpf_cpumask_copy(cpumask, task->cpus_ptr);
        return cpumask_map_insert(cpumask, task->pid);
}