Maps and Helpers

Relevant source files

Map types

Four map types can be created at runtime through BPF_MAP_CREATE (BpfManager::create_map_for). Any other type value returns InvalidInstruction.

Type	Value	Constraints at creation	Storage and behaviour
Hash (`BPF_MAP_TYPE_HASH`)	1	non-zero key and value size	One flat buffer of `state, key, value` buckets with linear probing; fixed key and value sizes
Array (`BPF_MAP_TYPE_ARRAY`)	2	key size must be 4, non-zero value size	Contiguous value bytes indexed by `u32`
Ring buffer (`BPF_MAP_TYPE_RINGBUF`)	27	key size 0, value size 0, `max_entries` a power of two (it is the byte size)	Reserve, submit and poll with an 8-byte event header; polling copies one committed event and advances the tail; nonblocking
Time series (`BPF_MAP_TYPE_TIMESERIES`)	100	key size 8 (a timestamp), non-zero value size	Circular storage of timestamped fixed-size values

The kernel_bpf::maps::MapType enum names more types (prog array, per-CPU maps, stack trace, cgroup array, and cloud-only LRU hash, LRU per-CPU hash and LPM trie) so that ELF map definitions can be parsed, but the manager does not create them. The ABI catalog lists only the four above.

All constructors validate dimensions, use checked arithmetic for allocation sizes and reserve memory fallibly, returning MapError on invalid or exhausted requests. The resize operation exists only in cloud builds. Embedded constructors additionally reject any single map whose full allocation exceeds MEMORY_BUDGET (64 KiB). There is no static map pool.

Reserved envelope map

Handle 0 (ENVELOPE_MAP_ID) is a kernel-owned, read-only envelope map created at manager construction from the profile’s actuation constants. RESERVED_MAP_COUNT is 1, so user maps start after it. Mutating helpers on this handle are rejected by the verifier (read-only) and at runtime.

Kernel quotas (`BpfLimits`)

Limit	Cloud	Embedded
Live programs	128	32
Program slots	1024	128
Per-owner programs	32	8
Total program bytes	16 MiB	2 MiB
Single program bytes	1 MiB	800 KiB
Per-owner program bytes	4 MiB	512 KiB
Max ELF bytes	1 MiB	1 MiB
Live maps	64	16
Map slots	512	64
Per-owner maps	16	4
Total map bytes	64 MiB	64 KiB
Single map bytes	16 MiB	64 KiB
Per-owner map bytes	16 MiB	32 KiB
Pinned maps	128	32
Key size	512	256
Value size	64 KiB	4 KiB
Max entries	1,048,576	4096

The verifier receives the caller’s authoritative view of map sizes, generations and permissions (MapPerm: Unavailable, ReadOnly, WriteOnly, ReadWrite), and ProgramRuntime retains only the maps a program references and is authorized to use. At run time helpers re-resolve the handle through the program’s captured map set and re-check access. During execution each map is leased (MapLease, up to 128 leases per execution); userspace mutation or destruction of a leased map fails rather than blocks. Pinning and cross-process grants are described in Trust Model.

Helper functions

Helper IDs are defined once in kernel_abi and mirrored by the verifier’s HelperId enum, which is also what the loader relocation table, the WCET cost table and the interpreter dispatch use. The loader source states the IDs are the kernel’s own runtime ABI, not the upstream Linux uapi numbering; rkBPF extensions start at 1000.

The table lists every ID in HelperId. “Runtime” says whether the interpreter implements it; the others are known to the verifier and signature table but return InvalidHelper if executed. Tier is the minimum caller tier (P = privileged). “Act.” marks helpers that require the actuation capability.

ID	Name	Purpose	Runtime	Tier	Notes
1	`bpf_ktime_get_ns`	Kernel time in nanoseconds	yes	any
2	`bpf_trace_printk`	Log a message to the kernel log	yes	any	May log; not available on embedded; rejected on timer, GPIO and sched_switch hooks
3	`bpf_get_prandom_u32`	Pseudo-random 32-bit value	no	any
4	`bpf_get_smp_processor_id`	Current CPU ID	no	any
5	`bpf_map_lookup_elem`	Look up a map element; returns a maybe-null value pointer	yes	any
6	`bpf_map_update_elem`	Insert or update an element	yes	any	Requires writable map
7	`bpf_map_delete_elem`	Delete an element	yes	any	Requires writable map
8	`bpf_ringbuf_output`	Copy an event into a ring buffer map	yes	any	Requires writable map
9	`bpf_timeseries_push`	Push a timestamped value into a time-series map	yes	any	Requires writable map
10	`bpf_get_current_pid_tgid`	Current PID and TGID	no	any
11	`bpf_get_current_uid_gid`	Current UID and GID	no	any
12	`bpf_get_current_comm`	Current process command name into a stack buffer	no	any
13	`bpf_get_interrupt_latency_ns`	Interrupt latency stored in the context	yes	any
14	`bpf_probe_read`	Read from an arbitrary address with safety checks	no	P
15	`bpf_get_boot_time_ms`	Boot time in milliseconds from the context	yes	P
16	`bpf_get_kernel_heap_kb`	Kernel heap usage in KiB from the context	yes	P
17	`bpf_get_kernel_image_mb`	Kernel image size in MiB from the context	yes	P
40	`bpf_ringbuf_reserve`	Reserve ring buffer space; returns a maybe-null pointer	no	any	Not available on embedded
41	`bpf_ringbuf_submit`	Submit a reserved entry	no	any
42	`bpf_ringbuf_discard`	Discard a reserved entry	no	any
1002	`bpf_sensor_last_timestamp`	Last timestamp from a sensor	no	any	Relocatable by name
1003	`bpf_gpio_set`	Set a GPIO output level	yes	any	Act.; passes through the actuation monitor
1004	`bpf_gpio_get`	Read a GPIO input level	yes	any	Returns -1 on invalid pin or non-Pi 5 builds
1005	`bpf_pwm_write`	Write a PWM duty (pwm id 0 or 1, channel 1 or 2)	yes	any	Act.; guarded by the actuation monitor; link-owned motor channels are routed over the Shrike link
1006	`bpf_iio_read`	Read an IIO sensor value	no	any
1007	`bpf_can_send`	Send a CAN message	no	any	Act.

Which helpers the ABI catalog advertises as supported: bpf_ktime_get_ns, bpf_trace_printk (cloud profile only), map lookup/update/delete, bpf_ringbuf_output, bpf_timeseries_push, bpf_get_interrupt_latency_ns, bpf_get_boot_time_ms, bpf_get_kernel_heap_kb, bpf_get_kernel_image_mb, and, on Raspberry Pi 5, bpf_gpio_set, bpf_gpio_get and bpf_pwm_write.

Argument checking

Each helper has a HelperSignature of up to five argument types (Scalar, PtrToMapKey, PtrToMapValue, PtrToStack, PtrToMem, MemSize, Const, ring buffer types, and others) and a return type (Integer, PtrToMapValueOrNull, PtrToAllocMemOrNull, Void). Pointer returns become maybe-null pointers whose accessible size comes from the map’s value size. Memory arguments paired with a size are bounds-checked against the pointed region.

Actuation

Actuation helpers (gpio_set, pwm_write, can_send) require both the verifier’s allow_actuation flag (from the process ACTUATE capability at load time) and, at runtime, the ARM-A monitor in kernel_bpf::actuation. The profile supplies the envelope: on embedded a PWM duty ceiling of 90 percent and a slew limit of 20 per 1 ms window; on cloud these are no-op bounds. A program’s authorization snapshot is stored with it, so later privilege changes do not widen an existing program. Kernel helpers bpf_gpio_toggle and bpf_gpio_set_output exist in kernel/src/bpf/helpers.rs but are explicitly not exposed to the BPF ABI.

The loader’s relocation table recognises only 17 helper names (see Loader); bpf_gpio_*, bpf_pwm_write, bpf_iio_read and bpf_can_send cannot be linked by name from an ELF and are called by numeric ID in the bundled demo. The kernel’s bpf_trace_printk implementation reads a NUL-terminated string from the pointer it is given.