Libbpf eBPF macro __arg_nullable

v1.4.0

The __arg_nullable macros is used to tag a function argument to tell the verifier that its value may be null.

Definition

#define __arg_nullable __attribute((btf_decl_tag("arg:nullable")))

Usage

This macro can be used to tag a function argument of a global function to tell the verifier that it can assume the argument can be NULL. It was introduced alongside the __arg_trusted macro which tells the verifier that an argument is a trusted pointer to kernel memory. The verifier will by default assume that any trusted pointer argument is never NULL (the opposite of normal pointers see __arg_nonnull). Adding the __arg_nullable attribute to a trusted pointer argument will tell the verifier that the argument can be NULL, which requires the function to add a NULL check, but allows the caller to pass a NULL pointer. Thus making the argument optional.

Example

__weak int subprog_nullable_task_flavor(
    struct task_struct___local *task __arg_trusted __arg_nullable)
{
    char buf[16];

    if (!task)
        return 0;

    return bpf_copy_from_user_task(&buf, sizeof(buf), NULL, (void *)task, 0);
}

SEC("?uprobe.s")
int flavor_ptr_nullable(void *ctx)
{
    struct task_struct___local *t = (void *)bpf_get_current_task_btf();

    return subprog_nullable_task_flavor(t);
}

__weak int subprog_nonnull_task_flavor(struct task_struct___local *task __arg_trusted)
{
    char buf[16];

    return bpf_copy_from_user_task(&buf, sizeof(buf), NULL, (void *)task, 0);
}

SEC("?uprobe.s")
int flavor_ptr_nonnull(void *ctx)
{
    struct task_struct *t = bpf_get_current_task_btf();

    return subprog_nonnull_task_flavor((void *)t);
}