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#+title: BPF
#+date: 2026-05-11 Mon
#+author: W. Kosior
#+email: wkosior@agh.edu.pl
* BPF
- 1991 (BSD)
- Berkeley Packet Filter
- VM in a kernel
- tcpdump
- https://www.tcpdump.org/papers/bpf-usenix93.pdf
* eBPF
- 2014 (kernel 3.18)
- 2021 (Windows implementation)
- extended BPF
- C → BPF program
- loading
- =CAP_SYS_ADMIN=
- =CAP_BPF= since kernel 5.8
- verification
- efficient in-kernel operation
- unambigious naming: eBPF & cBPF (classical BPF)
* eBPF VM
- registers R0-R10
- R10 — read-only frame pointer
- 64-bit
- helper functions (side-effects)
- safely read kernel & userspace
- =bpf_probe_read_kernel()=, =bpf_core_read()= & friends
- direct pointer dereference = verification failure
- traverse kernel data structures
- kill a process
- write to userspace
- …
- dedicated LLVM & GCC target
- use of object file sections
- https://eunomia.dev/tutorials/1-helloworld/
* eBPF Verification
- no arbitrary memory writes
- static bounds checking
- no direct kernel memory access
- loops
- none
- bounded only (2019; kernel 5.3)
- execution paths checked during loading
- helper functions
* eBPF Compilation & Loading
- libbpf
- =program.bpf.c= → =program.skel.h=
- compiled bpf → C array blob
- =.bpf.c= — one of conventions
- =program_bpf__open()= & =program_bpf__load()= & =bootstrap_bpf__attach()=
- https://gothub.r4fo.com/libbpf/libbpf-bootstrap/blob/master/examples/c/fentry.c
- other toolsets
* eBPF Communication with Userspace
- =bpf_printk()=
- =sudo cat /sys/kernel/debug/tracing/trace_pipe=
- data structures (maps)
* eBPF Data Structures
- array (=BPF_MAP_TYPE_ARRAY=)
- optionally per-cpu
- https://gothub.r4fo.com/libbpf/libbpf-bootstrap/blob/master/examples/c/minimal_legacy.bpf.c
- https://gothub.r4fo.com/libbpf/libbpf-bootstrap/blob/master/examples/c/minimal_legacy.c
- hash (=BPF_MAP_TYPE_HASH=)
- optionally per-cpu
- perf buffers (=BPF_MAP_TYPE_PERF_EVENT_ARRAY=)
- per-cpu
- efficient polling possible
- https://gothub.r4fo.com/libbpf/libbpf-bootstrap/blob/master/examples/c/bootstrap_legacy.bpf.c
- https://gothub.r4fo.com/libbpf/libbpf-bootstrap/blob/master/examples/c/bootstrap_legacy.c
- ring buffers (=BPF_MAP_TYPE_RINGBUF=)
- reserve & submit (spinlock)
- global alterantive to perf buffers
* eBPF Attachment Points
- kprobes
- & kretprobes
- uprobes
- & uretprobes
- tracepoints
- ingress / egress
- XDP
- LSM hooks
- …
* Kprobe eBPF
- (almost) arbitrary kernel code
- even inside functions!
- =int3= (or equivalent on non-x86) injection
- function exit (kretprobe)
- *predates eBPF*
- usable from kernel modules
- analyze registers
- function arguments
- traverse kernel data structures
- https://gothub.r4fo.com/libbpf/libbpf-bootstrap/blob/master/examples/c/kprobe.bpf.c
* Uprobe eBPF
- attach to arbitrary program code
- e.g., TLS routines :)
- function exit (uretprobe)
- *also predates eBPF*
- can read userspace
- access controls? *beware of TOCTOU*
- *can modify userspace*
- no ptrace needed
- higher efficiency, less intrusive
- https://gothub.r4fo.com/libbpf/libbpf-bootstrap/blob/master/examples/c/uprobe.bpf.c
- User Statically Defined Tracepoints (USDT)
- non-pre-compiled languages
* Tracepoint eBPF
- attach to non-arbitrary points in kernel
- explicitly exposed by kernel programmers
- higher stability than kprobes
- =/sys/kernel/debug/tracing/events/=
- =bootstrap_legacy.bpf.c= & =minimal_legacy.bpf.c= from before
* Tracepoint Format Listing
#+begin_example
# cat /sys/kernel/debug/tracing/events/sched/sched_process_exec/format
name: sched_process_exec
ID: 296
format:
field:unsigned short common_type; offset:0; size:2; signed:0;
field:unsigned char common_flags; offset:2; size:1; signed:0;
field:unsigned char common_preempt_count; offset:3; size:1; signed:0;
field:int common_pid; offset:4; size:4; signed:1;
field:__data_loc char[] filename; offset:8; size:4; signed:0;
field:pid_t pid; offset:12; size:4; signed:1;
field:pid_t old_pid; offset:16; size:4; signed:1;
print fmt: "filename=%s pid=%d old_pid=%d", __get_str(filename), REC->pid, REC->old_pid
#+end_example
* ingress / egress eBPF
- Traffic Control (*also predates eBPF*)
- packet dropping, redirection & modification
- =__sk_buff= (writable)
- return codes
- =TC_ACT_UNSPEC=
- =TC_ACT_OK=
- =TC_ACT_RECLASSIFY=
- =TC_ACT_SHOT=
- =TC_ACT_PIPE=
- …
- https://upload.wikimedia.org/wikipedia/commons/3/37/Netfilter-packet-flow.svg
- https://gothub.r4fo.com/libbpf/libbpf-bootstrap/blob/master/examples/c/tc.bpf.c
- https://gothub.r4fo.com/libbpf/libbpf-bootstrap/blob/master/examples/c/tc.c
- if name → if index
* eXpress Data Path
- "zero-copy"
- even quicker redirects, etc.
- return codes
- =XDP_ABORTED=
- =XDP_DROP=
- =XDP_PASS=
- =XDP_TX= — back out on the same NIC!
- =XDP_REDIRECT=
- NIC offloading
- few cards
- https://gothub.r4fo.com/haolipeng/ebpf-tutorial/blob/master/src/18-xdp-filter/xdp_filter.bpf.c
* Kernel Struct Field Accesses
- direct pointer dereference only on BPF-owned memory
- =bpf_probe_read_kernel()= — read kernel emmory
- size & offset
- =bpf_probe_read_kernel()= → =bpf_core_read()=
- type safety
- =bpf_core_read()= → =BPF_CORE_READ()=
- replaces multiple =bpf_core_read()=
- https://gothub.r4fo.com/libbpf/libbpf-bootstrap/blob/master/examples/c/bootstrap.bpf.c
- =BPF_CORE_READ(task, real_parent, tgid)= → =task->real_parent->tgid=
- BPF Type Format (BTF)
- "Compile Once Run Everywhere" ("CO-RE")
* BPF Use-Cases
- load-balancing (XDP)
- DDoS mitigation (XDP)
- QoS / traffic shaping
- monitoring
- access restrictions (seccomp-bpf, LSM)
- debugging
- snooping on / hijacking userspace
* Tools (Development Libraries)
- libbpf (C)
- libbpfgo (Go) (libbpf wrapper)
- Cilium/eBPF (Go)
- aya (Rust)
- libbpf-rs (Rust)
* Tools (BPF Users)
- bpftrace
- https://bpftrace.org/docs/release_023/cli
- bpfilter (more efficient itables?)
- seccomp-bpf (cBPF)
- https://kernel-internals.org/security/seccomp/
* Omitted Tracing Facilities
- DTrace
- ftrace (can also be used by eBPF)
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