gobpf - utilizing eBPF from Go @ FOSDEM 2017

Transcript

1. FOSDEM • 2017-02-05 • michael@kinvolk.io gobpf utilizing eBPF from Go
2. FOSDEM • 2017-02-05 • michael@kinvolk.io bpf_trace_printk("hello"); Michael, Software Engineer at Kinvolk We mostly work on Linux core system software (kernel, container, etc.) … and like it! https://kinvolk.io/about/ We are hiring, too!
3. FOSDEM • 2017-02-05 • michael@kinvolk.io What is gobpf? • library to create, load and use eBPF programs from Go • bring together eBPF capabilities and Go software
4. FOSDEM • 2017-02-05 • michael@kinvolk.io Agenda Introduction to eBPF • Berkeley Packet Filter / cBPF • Programs • Maps gobpf • BPF Compiler Collection (bcc) • elf • CI
5. FOSDEM • 2017-02-05 • michael@kinvolk.io What is eBPF? • “bytecode virtual machine” in the Linux kernel • used for tracing kernel functions, networking, performance analysis, ...
6. FOSDEM • 2017-02-05 • michael@kinvolk.io Berkeley Packet Filter / LSF / cBPF • original use case packet filtering • based on paper “The BSD Packet Filter: A New Architecture for User-level Packet Capture” from December 19, 1992 sudo tcpdump -p -ni eth0 -d "ip and udp" (000) ldh [12] (001) jeq #0x800 jt 2 jf 5 (002) ldb [23] (003) jeq #0x11 jt 4 jf 5 (004) ret #262144 (005) ret #0 https://blog.cloudflare.com/bpf-the-forgotten-bytecode/
7. FOSDEM • 2017-02-05 • michael@kinvolk.io Why eBPF? • enables you to ◦ attach to different kernel events ◦ do networking (SDN), e.g. packet parsing & modification ◦ use bpf as a security mechanism, e.g. filtering ◦ ... • safety guarantee through the bpf verifier • fast, running in the kernel ◦ JIT compiled (for x86-64, arm64, and s390 if enabled, /proc/sys/net/core/bpf_jit_enable )
8. FOSDEM • 2017-02-05 • michael@kinvolk.io How does it work? kernel user-space eBPF maps eBPF program program - uses bpf() system call or library to interact with BPF subsystem eBPF program ...
9. FOSDEM • 2017-02-05 • michael@kinvolk.io eBPF programs Programs can be attached to ... • sockets ◦ execute eBPF program for each packet ◦ https://www.kernel.org/doc/Documentation/networking/filter.txt ◦ non-privileged operation contrary to other bpf(2) ops • kernel tracepoints ◦ linux:include/trace/events/ ◦ https://www.kernel.org/doc/Documentation/trace/tracepoints.txt https://github.com/torvalds/linux/blob/v4.9/include/uapi/linux/bpf.h#L90-L99
10. FOSDEM • 2017-02-05 • michael@kinvolk.io eBPF programs Programs can be attached to ... • kprobes ◦ “a set of handlers placed on a certain instruction address” - https://lwn.net/Articles/132196/ ◦ pre-handler and a post-handler: kretprobe ◦ https://www.kernel.org/doc/Documentation/kprobes.txt • uprobes ◦ user-space counterpart of kprobes
11. FOSDEM • 2017-02-05 • michael@kinvolk.io eBPF maps Maps are a generic data structure to share “data between eBPF kernel programs, and also between kernel and user-space applications.” • a key/value for a given map can have an arbitrary structure, as specified by the user at map-creation time • one special map BPF_MAP_TYPE_PROG_ARRAY holding file descriptors referring to other eBPF programs • man (2) bpf notoriously not up-to-date
12. FOSDEM • 2017-02-05 • michael@kinvolk.io eBPF maps • Available may types: ◦ HASH ◦ ARRAY ◦ PROG_ARRAY ◦ PERF_EVENT_ARRAY ◦ PERCPU_HASH, PERCPU_ARRAY ◦ STACK_TRACE ◦ CGROUP_ARRAY • Linux v4.10 adds LRU_HASH https://github.com/torvalds/linux/blob/v4.9/include/uapi/linux/bpf.h#L78-L88
13. FOSDEM • 2017-02-05 • michael@kinvolk.io • an array of file descriptors (one per cpu, created with perf_event_open(2)) to in-kernel ring buffers containing perf events • allows sending a lot of events very fast • user-space program can read asynchronously from mmap’ed memory BPF_MAP_TYPE_PERF_EVENT_ARRAY
14. FOSDEM • 2017-02-05 • michael@kinvolk.io Warning: somewhat scary code on next slide “... to start by coding a struct bpf_insn is starting with difficulty setting Ultra-Violence” - Brendan Gregg
15. FOSDEM • 2017-02-05 • michael@kinvolk.io Example: fchownat(2) count kprobe /* Put 0 (the map key) on the stack */ BPF_ST_MEM(BPF_W, BPF_REG_10, -4, 0), /* Put frame pointer into R2 */ BPF_MOV64_REG(BPF_REG_2, BPF_REG_10), /* Decrement pointer by four */ BPF_ALU64_IMM(BPF_ADD, BPF_REG_2, -4), /* Put map_fd into R1 */ BPF_LD_MAP_FD(BPF_REG_1, map_fd), /* Load current count from map into R0 */ BPF_RAW_INSN(BPF_JMP | BPF_CALL, 0, 0, 0, BPF_FUNC_map_lookup_elem), /* If returned value NULL, skip two instructions and exit */ BPF_JMP_IMM(BPF_JEQ, BPF_REG_0, 0, 2), /* Put 1 into R1 */ BPF_MOV64_IMM(BPF_REG_1, 1), /* Increment value by 1 */ BPF_RAW_INSN(BPF_STX | BPF_XADD | BPF_W, BPF_REG_0, BPF_REG_1, 0, 0), BPF_EXIT_INSN(), https://kinvolk.io/blog/2016/11/introducing-gobpf---using-ebpf-from-go/
16. FOSDEM • 2017-02-05 • michael@kinvolk.io The IO Visor Project “[...] open source project and a community of developers to accelerate the innovation, development, and sharing of virtualized in-kernel IO services for tracing, analytics, monitoring, security and networking functions.” https://www.iovisor.org/about https://github.com/iovisor
17. FOSDEM • 2017-02-05 • michael@kinvolk.io gobpf library to create, load and use eBPF programs from Go • use Cgo + the BPF Compiler Collection (bcc) or • load and use pre-build elf object files • https://github.com/iovisor/gobpf
18. FOSDEM • 2017-02-05 • michael@kinvolk.io Why gobpf? • There was no library which does what we need ◦ … but the Hover Framework: https://github.com/iovisor/iomodules • We like Go @ Kinvolk + use it a lot • We work on Weave Scope, which is written in Go
19. FOSDEM • 2017-02-05 • michael@kinvolk.io Why gobpf? • Scope probes (read: agents) need to gather a lot of system data ◦ https://github.com/weaveworks/scope/tree/master/probe/endpoint • Doing that with eBPF is often faster and/or more reliable than e.g. /proc or conntrack (Linux connection tracking) parsing
20. FOSDEM • 2017-02-05 • michael@kinvolk.io gobpf/bcc • import bpf “github.com/iovisor/gobpf/bcc” • write eBPF program in C • load with gobpf
21. FOSDEM • 2017-02-05 • michael@kinvolk.io gobpf/bcc • “A modified C language for BPF backends” - https://github.com/iovisor/bcc/blob/master/README.md • When using bcc, we can rely on bcc helper functions which make it easier to e.g. work with maps: map.update(&key, &value);
22. FOSDEM • 2017-02-05 • michael@kinvolk.io gobpf/bcc libbcc.so is the core of bcc and not only a library but also a compiler to translate eBPF programs written in aforementioned modified C language into byte code for bpf(2) • uses clang + llvm-bpf backend • allows you to verify program before loading it • spares you “a kludgy workflow, sometimes involving compiling directly in a linux kernel source tree”
23. FOSDEM • 2017-02-05 • michael@kinvolk.io gobpf/bcc const source string = ` #include <uapi/linux/ptrace.h> #include <bcc/proto.h> typedef struct { u32 pid; uid_t uid; gid_t gid; int ret; char filename[256]; } chown_event_t; BPF_PERF_OUTPUT(chown_events); BPF_HASH(chowncall, u64, chown_event_t); int kprobe__sys_fchownat( struct pt_regs *ctx, int dfd, const char *filename, uid_t uid, gid_t gid, int flag) { u64 pid = bpf_get_current_pid_tgid(); chown_event_t event = { .pid = pid >> 32, .uid = uid, .gid = gid, }; bpf_probe_read(&event.filename, sizeof(event.filename), (void *)filename); chowncall.update(&pid, &event); return 0; } ...
24. FOSDEM • 2017-02-05 • michael@kinvolk.io gobpf/bcc m := bpf.NewModule(source, []string{}) … chownKprobe, err := m.LoadKprobe("kprobe__sys_fchownat") err = m.AttachKprobe("sys_fchownat", chownKprobe) … table := bpf.NewTable(0, m) perfMap, err := bpf.InitPerfMap(table, channel) … go func() { var event chownEvent for { data := <-channel err := binary.Read(bytes.NewBuffer(data), binary.LittleEndian, &event) … filename := (*C.char)(unsafe.Pointer(&event.Filename)) fmt.Printf("uid %d gid %d pid %d called fchownat(2) on %s (return value: %d)\n", event.Uid, event.Gid, event.Pid, C.GoString(filename), event.ReturnValue) } }()
25. FOSDEM • 2017-02-05 • michael@kinvolk.io gobpf/bcc // Gateway function as required with CGO Go >= 1.6 // ... //export callback_to_go func callback_to_go(cbCookie unsafe.Pointer, raw unsafe.Pointer, rawSize C.int) { callbackData := lookupCallback(uint64(uintptr(cbCookie))) receiverChan := callbackData.receiverChan go func() { receiverChan <- C.GoBytes(raw, rawSize) }() } … C.bpf_open_perf_buffer( (C.perf_reader_raw_cb)(unsafe.Pointer(C.callback_to_go)), unsafe.Pointer(uintptr(callbackDataIndex)), -1, C.int(cpu))
26. FOSDEM • 2017-02-05 • michael@kinvolk.io Demo: fchownat(2) snoop
27. FOSDEM • 2017-02-05 • michael@kinvolk.io gobpf/elf • import bpf “github.com/iovisor/gobpf/elf” • load + use pre-built elf object
28. FOSDEM • 2017-02-05 • michael@kinvolk.io gobpf/elf • relies on elf sections: #define SEC(NAME) __attribute__((section(NAME), used)) … SEC("kretprobe/tcp_v4_connect") int kretprobe__tcp_v4_connect(struct pt_regs *ctx) { … • pkg debug/elf used to find kprobes + maps, resolve relocatable sections and load data • https://github.com/weaveworks/tcptracer-bpf
29. FOSDEM • 2017-02-05 • michael@kinvolk.io gobpf/elf clang -O2 \ -emit-llvm \ # produce LLVM bitcode object file -c ebpf.c -o - | \ # write result to stdout llc \ # LLVM static compiler -march=bpf \ # generate eBPF code -filetype=obj -o ebpf.o # write object file
30. FOSDEM • 2017-02-05 • michael@kinvolk.io gobpf/elf • allows you to build a single object for multiple kernel versions by setting the kernel version to a constant: 0xFFFFFFFE ◦ gobpf/elf then gets your kernel version from uname(2) • use feature at your own risk; requires careful consideration as ◦ in k{ret,}probes, used kernel functions and accessed structs could change ◦ those are internal functions / structures not covered by the ABI guarantee
31. FOSDEM • 2017-02-05 • michael@kinvolk.io Continuous integration for eBPF programs • problem: requires modern Linux kernel + root access • currently, we use semaphoreci.com + custom rkt stage1-kvm images ◦ experimental build script for custom stage1 images: https://github.com/kinvolk/stage1-builder
32. FOSDEM • 2017-02-05 • michael@kinvolk.io Questions? https://fosdem.org/2017/schedule/event/go_bpf ← Submit feedback, if you want https://www.speakerdeck.com/schu schu schux00 michael@kinvolk.io