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Skip __ksym externs for standard BPF helpers
Signed-off-by: Cong Wang <cwang@multikernel.io>
1 parent dd5f6b7 commit 93314d7

4 files changed

Lines changed: 321 additions & 21 deletions

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‎src/bpf_helpers.ml‎

Lines changed: 254 additions & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -0,0 +1,254 @@
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(*
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* Copyright 2026 Multikernel Technologies, Inc.
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*
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* Licensed under the Apache License, Version 2.0 (the "License");
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* you may not use this file except in compliance with the License.
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* You may obtain a copy of the License at
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*
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* http://www.apache.org/licenses/LICENSE-2.0
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*
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* Unless required by applicable law or agreed to in writing, software
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* distributed under the License is distributed on an "AS IS" BASIS,
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* See the License for the specific language governing permissions and
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* limitations under the License.
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*)
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(** Standard BPF helper functions.
18+
19+
These are the legacy BPF helpers invoked through a fixed helper ID. libbpf's
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[<bpf/bpf_helpers.h>] (via the auto-generated [bpf_helper_defs.h]) already
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declares every one of them as a function pointer, so the eBPF C backend must
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NOT emit its own [extern ... __ksym;] declaration for them - doing so clashes
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with the libbpf declaration ("redefinition as different kind of symbol").
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KernelScript uses the [extern] keyword for both kfuncs and helpers, so the
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backend consults this set to tell them apart: a name here is a helper (skip
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the __ksym declaration); anything else is a real kfunc (needs __ksym).
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The list mirrors libbpf's [bpf_helper_defs.h], which is generated from the
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kernel's [__BPF_FUNC_MAPPER] and only ever grows. *)
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let helper_names = [
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"bpf_bind";
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"bpf_bprm_opts_set";
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"bpf_btf_find_by_name_kind";
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"bpf_cgrp_storage_delete";
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"bpf_cgrp_storage_get";
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"bpf_check_mtu";
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"bpf_clone_redirect";
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"bpf_copy_from_user";
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"bpf_copy_from_user_task";
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"bpf_csum_diff";
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"bpf_csum_level";
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"bpf_csum_update";
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"bpf_current_task_under_cgroup";
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"bpf_d_path";
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"bpf_dynptr_data";
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"bpf_dynptr_from_mem";
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"bpf_dynptr_read";
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"bpf_dynptr_write";
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"bpf_fib_lookup";
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"bpf_find_vma";
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"bpf_for_each_map_elem";
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"bpf_get_attach_cookie";
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"bpf_get_branch_snapshot";
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"bpf_get_cgroup_classid";
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"bpf_get_current_ancestor_cgroup_id";
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"bpf_get_current_cgroup_id";
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"bpf_get_current_comm";
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"bpf_get_current_pid_tgid";
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"bpf_get_current_task";
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"bpf_get_current_task_btf";
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"bpf_get_current_uid_gid";
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"bpf_get_func_arg";
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"bpf_get_func_arg_cnt";
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"bpf_get_func_ip";
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"bpf_get_func_ret";
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"bpf_get_hash_recalc";
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"bpf_get_listener_sock";
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"bpf_get_local_storage";
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"bpf_get_netns_cookie";
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"bpf_get_ns_current_pid_tgid";
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"bpf_get_numa_node_id";
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"bpf_get_prandom_u32";
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"bpf_get_retval";
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"bpf_get_route_realm";
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"bpf_get_smp_processor_id";
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"bpf_get_socket_cookie";
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"bpf_get_socket_uid";
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"bpf_getsockopt";
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"bpf_get_stack";
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"bpf_get_stackid";
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"bpf_get_task_stack";
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"bpf_ima_file_hash";
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"bpf_ima_inode_hash";
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"bpf_inode_storage_delete";
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"bpf_inode_storage_get";
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"bpf_jiffies64";
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"bpf_kallsyms_lookup_name";
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"bpf_kptr_xchg";
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"bpf_ktime_get_boot_ns";
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"bpf_ktime_get_coarse_ns";
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"bpf_ktime_get_ns";
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"bpf_ktime_get_tai_ns";
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"bpf_l3_csum_replace";
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"bpf_l4_csum_replace";
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"bpf_load_hdr_opt";
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"bpf_loop";
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"bpf_lwt_push_encap";
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"bpf_lwt_seg6_action";
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"bpf_lwt_seg6_adjust_srh";
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"bpf_lwt_seg6_store_bytes";
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"bpf_map_delete_elem";
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"bpf_map_lookup_elem";
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"bpf_map_lookup_percpu_elem";
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"bpf_map_peek_elem";
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"bpf_map_pop_elem";
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"bpf_map_push_elem";
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"bpf_map_update_elem";
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"bpf_msg_apply_bytes";
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"bpf_msg_cork_bytes";
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"bpf_msg_pop_data";
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"bpf_msg_pull_data";
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"bpf_msg_push_data";
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"bpf_msg_redirect_hash";
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"bpf_msg_redirect_map";
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"bpf_override_return";
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"bpf_per_cpu_ptr";
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"bpf_perf_event_output";
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"bpf_perf_event_read";
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"bpf_perf_event_read_value";
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"bpf_perf_prog_read_value";
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"bpf_probe_read";
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"bpf_probe_read_kernel";
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"bpf_probe_read_kernel_str";
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"bpf_probe_read_str";
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"bpf_probe_read_user";
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"bpf_probe_read_user_str";
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"bpf_probe_write_user";
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"bpf_rc_keydown";
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"bpf_rc_pointer_rel";
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"bpf_rc_repeat";
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"bpf_read_branch_records";
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"bpf_redirect";
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"bpf_redirect_map";
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"bpf_redirect_neigh";
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"bpf_redirect_peer";
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"bpf_reserve_hdr_opt";
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"bpf_ringbuf_discard";
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"bpf_ringbuf_discard_dynptr";
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"bpf_ringbuf_output";
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"bpf_ringbuf_query";
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"bpf_ringbuf_reserve";
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"bpf_ringbuf_reserve_dynptr";
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"bpf_ringbuf_submit";
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"bpf_ringbuf_submit_dynptr";
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"bpf_send_signal";
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"bpf_send_signal_thread";
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"bpf_seq_printf";
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"bpf_seq_printf_btf";
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"bpf_seq_write";
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"bpf_set_hash";
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"bpf_set_hash_invalid";
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"bpf_set_retval";
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"bpf_setsockopt";
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"bpf_sk_ancestor_cgroup_id";
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"bpf_sk_assign";
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"bpf_skb_adjust_room";
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"bpf_skb_ancestor_cgroup_id";
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"bpf_skb_cgroup_classid";
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"bpf_skb_cgroup_id";
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"bpf_skb_change_head";
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"bpf_skb_change_proto";
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"bpf_skb_change_tail";
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"bpf_skb_change_type";
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"bpf_skb_ecn_set_ce";
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"bpf_skb_get_tunnel_key";
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"bpf_skb_get_tunnel_opt";
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"bpf_skb_get_xfrm_state";
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"bpf_skb_load_bytes";
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"bpf_skb_load_bytes_relative";
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"bpf_skb_output";
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"bpf_skb_pull_data";
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"bpf_skb_set_tstamp";
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"bpf_skb_set_tunnel_key";
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"bpf_skb_set_tunnel_opt";
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"bpf_skb_store_bytes";
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"bpf_skb_under_cgroup";
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"bpf_skb_vlan_pop";
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"bpf_skb_vlan_push";
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"bpf_sk_cgroup_id";
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"bpf_skc_lookup_tcp";
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"bpf_skc_to_mptcp_sock";
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"bpf_skc_to_tcp6_sock";
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"bpf_skc_to_tcp_request_sock";
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"bpf_skc_to_tcp_sock";
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"bpf_skc_to_tcp_timewait_sock";
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"bpf_skc_to_udp6_sock";
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"bpf_skc_to_unix_sock";
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"bpf_sk_fullsock";
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"bpf_sk_lookup_tcp";
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"bpf_sk_lookup_udp";
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"bpf_sk_redirect_hash";
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"bpf_sk_redirect_map";
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"bpf_sk_release";
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"bpf_sk_select_reuseport";
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"bpf_sk_storage_delete";
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"bpf_sk_storage_get";
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"bpf_snprintf";
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"bpf_snprintf_btf";
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"bpf_sock_from_file";
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"bpf_sock_hash_update";
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"bpf_sock_map_update";
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"bpf_sock_ops_cb_flags_set";
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"bpf_spin_lock";
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"bpf_spin_unlock";
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"bpf_store_hdr_opt";
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"bpf_strncmp";
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"bpf_strtol";
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"bpf_strtoul";
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"bpf_sys_bpf";
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"bpf_sys_close";
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"bpf_sysctl_get_current_value";
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"bpf_sysctl_get_name";
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"bpf_sysctl_get_new_value";
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"bpf_sysctl_set_new_value";
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"bpf_tail_call";
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"bpf_task_pt_regs";
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"bpf_task_storage_delete";
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"bpf_task_storage_get";
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"bpf_tcp_check_syncookie";
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"bpf_tcp_gen_syncookie";
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"bpf_tcp_raw_check_syncookie_ipv4";
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"bpf_tcp_raw_check_syncookie_ipv6";
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"bpf_tcp_raw_gen_syncookie_ipv4";
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"bpf_tcp_raw_gen_syncookie_ipv6";
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"bpf_tcp_send_ack";
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"bpf_tcp_sock";
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"bpf_this_cpu_ptr";
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"bpf_timer_cancel";
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"bpf_timer_init";
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"bpf_timer_set_callback";
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"bpf_timer_start";
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"bpf_trace_printk";
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"bpf_trace_vprintk";
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"bpf_user_ringbuf_drain";
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"bpf_xdp_adjust_head";
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"bpf_xdp_adjust_meta";
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"bpf_xdp_adjust_tail";
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"bpf_xdp_get_buff_len";
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"bpf_xdp_load_bytes";
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"bpf_xdp_output";
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"bpf_xdp_store_bytes";
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]
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let helper_set =
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let tbl = Hashtbl.create 256 in
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List.iter (fun name -> Hashtbl.replace tbl name ()) helper_names;
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tbl
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(** [is_bpf_helper name] is true if [name] is a standard BPF helper already
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declared by libbpf's bpf_helpers.h, and therefore must not be re-declared
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as a [__ksym] extern by the eBPF C backend. *)
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let is_bpf_helper name = Hashtbl.mem helper_set name

‎src/dune‎

Lines changed: 1 addition & 0 deletions
Original file line numberDiff line numberDiff line change
@@ -2,6 +2,7 @@
22
(name kernelscript)
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(modules ast parser lexer parse type_checker symbol_table maps map_assignment
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map_operations ir ir_generator ir_analysis loop_analysis ir_function_system codegen_common
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bpf_helpers
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multi_program_analyzer multi_program_ir_optimizer ebpf_c_codegen
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userspace_codegen evaluator safety_checker stdlib test_codegen
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tail_call_analyzer kernel_module_codegen dynptr_bridge

‎src/ebpf_c_codegen.ml‎

Lines changed: 22 additions & 16 deletions
Original file line numberDiff line numberDiff line change
@@ -3050,23 +3050,29 @@ let generate_declarations_in_source_order_unified ctx ir_multi_prog ~_btf_path _
30503050
| Ir.IRDeclKfuncDecl kfunc_decl ->
30513051
(* Emit a function-signature declaration so eBPF programs can call the kfunc.
30523052
Kernel-provided kfuncs need `extern ... __ksym;`; locally-defined @kfunc
3053-
functions just need a forward prototype. *)
3054-
let params_str = match kfunc_decl.Ir.ikfunc_params with
3055-
| [] -> "void"
3056-
| params -> String.concat ", " (List.map (fun (name, ir_type) ->
3057-
sprintf "%s %s" (kfunc_signature_type_to_c ir_type) name
3058-
) params)
3059-
in
3060-
let return_type_str = kfunc_signature_type_to_c kfunc_decl.Ir.ikfunc_return_type in
3061-
if kfunc_decl.Ir.ikfunc_is_extern then
3062-
emit_line ctx (sprintf "extern %s %s(%s) __ksym;"
3063-
return_type_str kfunc_decl.Ir.ikfunc_name params_str)
3053+
functions just need a forward prototype. Standard BPF helpers are skipped
3054+
entirely - libbpf's bpf_helpers.h already declares them, and a __ksym
3055+
extern would clash with that declaration. *)
3056+
let name = kfunc_decl.Ir.ikfunc_name in
3057+
if kfunc_decl.Ir.ikfunc_is_extern && Bpf_helpers.is_bpf_helper name then
3058+
()
30643059
else (
3065-
emit_line ctx "/* kfunc declaration */";
3066-
emit_line ctx (sprintf "%s %s(%s);"
3067-
return_type_str kfunc_decl.Ir.ikfunc_name params_str)
3068-
);
3069-
emit_blank_line ctx
3060+
let params_str = match kfunc_decl.Ir.ikfunc_params with
3061+
| [] -> "void"
3062+
| params -> String.concat ", " (List.map (fun (pname, ir_type) ->
3063+
sprintf "%s %s" (kfunc_signature_type_to_c ir_type) pname
3064+
) params)
3065+
in
3066+
let return_type_str = kfunc_signature_type_to_c kfunc_decl.Ir.ikfunc_return_type in
3067+
if kfunc_decl.Ir.ikfunc_is_extern then
3068+
emit_line ctx (sprintf "extern %s %s(%s) __ksym;"
3069+
return_type_str name params_str)
3070+
else (
3071+
emit_line ctx "/* kfunc declaration */";
3072+
emit_line ctx (sprintf "%s %s(%s);" return_type_str name params_str)
3073+
);
3074+
emit_blank_line ctx
3075+
)
30703076
) ir_multi_prog.Ir.source_declarations;
30713077

30723078
(* Emit callbacks at the end if no functions were found (fallback) *)

‎tests/test_extern.ml‎

Lines changed: 44 additions & 5 deletions
Original file line numberDiff line numberDiff line change
@@ -158,13 +158,13 @@ let test_extern_kfunc_string_representation () =
158158
(** Test extern kfunc declarations are emitted into generated eBPF C with __ksym *)
159159
let test_extern_kfunc_ebpf_codegen () =
160160
let program = {|
161-
extern bpf_ktime_get_ns() -> u64
161+
extern scx_bpf_select_cpu_dfl(p: *u8, prev_cpu: i32) -> u64
162162
extern scx_bpf_dsq_insert(p: *u8, dsq_id: u64, slice: u64, enq_flags: u64) -> void
163163

164164
@xdp
165165
fn test_program(ctx: *xdp_md) -> xdp_action {
166-
var ts = bpf_ktime_get_ns()
167-
scx_bpf_dsq_insert(null, 0, ts, 0)
166+
var cpu = scx_bpf_select_cpu_dfl(null, 0)
167+
scx_bpf_dsq_insert(null, 0, cpu, 0)
168168
return 2
169169
}
170170

@@ -186,11 +186,49 @@ let test_extern_kfunc_ebpf_codegen () =
186186
try ignore (Str.search_forward (Str.regexp_string substr) generated_code 0); true
187187
with Not_found -> false
188188
in
189-
check bool "Contains bpf_ktime_get_ns __ksym extern" true
190-
(contains "extern __u64 bpf_ktime_get_ns(void) __ksym;");
189+
check bool "Contains scx_bpf_select_cpu_dfl __ksym extern" true
190+
(contains "extern __u64 scx_bpf_select_cpu_dfl(__u8* p, __s32 prev_cpu) __ksym;");
191191
check bool "Contains scx_bpf_dsq_insert __ksym extern" true
192192
(contains "extern void scx_bpf_dsq_insert(__u8* p, __u64 dsq_id, __u64 slice, __u64 enq_flags) __ksym;")
193193

194+
(** Test that extern declarations naming standard BPF helpers are NOT re-declared
195+
as __ksym externs (libbpf's bpf_helpers.h already declares them; a __ksym extern
196+
would clash with the helper pointer definition). Real kfuncs still get __ksym. *)
197+
let test_extern_bpf_helper_not_redeclared () =
198+
let program = {|
199+
extern bpf_ktime_get_ns() -> u64
200+
extern my_real_kfunc(x: u64) -> i32
201+
202+
@xdp
203+
fn test_program(ctx: *xdp_md) -> xdp_action {
204+
var ts = bpf_ktime_get_ns()
205+
var r = my_real_kfunc(ts)
206+
if (r > 0) {
207+
return 1
208+
}
209+
return 2
210+
}
211+
212+
fn main() -> i32 {
213+
return 0
214+
}
215+
|} in
216+
217+
let ast = Parse.parse_string program in
218+
let symbol_table = Symbol_table.build_symbol_table ast in
219+
let (typed_ast, _) = Type_checker.type_check_and_annotate_ast ast in
220+
let ir = Ir_generator.generate_ir typed_ast symbol_table "test" in
221+
let (generated_code, _) = Ebpf_c_codegen.compile_multi_to_c_with_analysis ir in
222+
223+
let contains substr =
224+
try ignore (Str.search_forward (Str.regexp_string substr) generated_code 0); true
225+
with Not_found -> false
226+
in
227+
check bool "BPF helper bpf_ktime_get_ns is not re-declared as __ksym extern" false
228+
(contains "bpf_ktime_get_ns(void) __ksym;");
229+
check bool "Real kfunc my_real_kfunc is declared as __ksym extern" true
230+
(contains "extern __s32 my_real_kfunc(__u64 x) __ksym;")
231+
194232
(** Test extern keyword cannot be used in function definitions *)
195233
let test_extern_in_function_definition_fails () =
196234
let program = {|
@@ -270,6 +308,7 @@ let tests = [
270308
"extern kfunc userspace restriction", `Quick, test_extern_kfunc_userspace_restriction;
271309
"extern kfunc string representation", `Quick, test_extern_kfunc_string_representation;
272310
"extern kfunc ebpf codegen", `Quick, test_extern_kfunc_ebpf_codegen;
311+
"extern bpf helper not redeclared", `Quick, test_extern_bpf_helper_not_redeclared;
273312
"extern in function definition fails", `Quick, test_extern_in_function_definition_fails;
274313
"extern with body fails", `Quick, test_extern_with_body_fails;
275314
"extern mixed keywords fails", `Quick, test_extern_mixed_keywords_fails;

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