1.
Understanding eBPF in a
Hurry!
LinkedIn Performance Engineering Meetup
June 2019
Ray Jenkins

2.
Hi, I’m Ray
@_rayjenkins
github.com/rjenkins
ray@segment.com

3.
Let’s say you have a
performance problem.

4.
Examples
● A developer claims boxes have “slow” I/O
● Network connections are randomly
terminated.
● Your service is crashing, you’re not sure why,
maybe it getting OOM killed?
● You think some process might be getting
starved.

5.
Someone suggests you
might be able to solve it
with eBPF.

6.
Now you got two problems.

7.
Goal: Can we understand
what eBPF is and how it
works?

8.
http://www.brendangregg.com/ebpf.html
This is our map

9.
What is eBPF? (Extended Berkeley Packet Filter)
● Fast and safe, in-kernel, register based,
bytecode VM.
● Designed to be JITed with direct mapping to
x86_64 and other modern architectures.
● eBPF programs are “attached” to code paths
within the kernel or user space programs and
are executed when the code path is traversed.
● Linux Kernel 3.18 (2014) - bpf(2) syscall
○ (4.1 for Kprobes)

10.
What is eBPF? … cont.
● Programs are written in restricted C. eBPF backend for
LLVM/Clang.
○ clang -O2 -emit-llvm -c bpf.c -o - | llc -march=bpf -filetype=obj -o bpf.o
● eBPF Verifier
○ Verified to finish (no loops), no unreachable instructions, reads to uninitialized registers, or
memory access to arbitrary pointers restricted kernel func calls and data structure access.
● eBPF Maps / Perf Events Ring Buffer
○ Memory-Mapped, bi-directional data structures for storage. Allow sharing of data between
eBPF kernel programs, and also between kernel and user-space applications.
● Helper Functions
○ Kernel functions exposed to eBPF programs.
○ Context sensitive to type of eBPF program.

11.
https://github.com/iovisor/bcc/blob/master/docs/kernel-versions.md

12.
Why do we need eBPF?

13.
Dynamically and
Programmatically Trace
Kernel or User Space
Functions and Events,
Safely and Efficiently.

14.
http://www.brendangregg.com/ebpf.html
This is our map YOU ARE HERE

15.
eBPF is appealing to different people for different reasons,
but its power resides in what you can attach it to.
For Performance Engineering
we’re primarily interested in
these hooks.
● Kprobes/Uprobes
● Tracepoints
● USDT
● PerfEvents
https://elixir.bootlin.com/linux/latest/source/include/uapi/linux/bpf.h#L145

16.
Tracepoints (2.6.32) - 2009
● Static places in the kernel where tracing is inserted.
● $ grep -ri TRACE_EVENT *
● https://github.com/brendangregg/perf-tools

17.
K/J(ret)probes (2.6.9) - 2004 / U(ret)probes 3.15 - (2014)
● Probe any instruction, dynamically
● grep <func> /proc/kallsyms
● Register kprobes copies instruction, inserts breakpoint.
(int3 on x86_64)
● Cpu hits breakpoints, trap occurs, registers saved and
control passed to Kprobe.
● Pre-handler function called, Kprobes single steps
instructions (Slow), Post-Handler called.
● CONFIG_OPTPROBES=Y (enabled on x86_64)

18.
https://vjordan.info/log/fpga/how-linux-kprobes-works.html

19.
https://vjordan.info/log/fpga/how-linux-kprobes-works.html

20.
Perf events (2.6.31) - 2009
● The “nearly un-googleable” - http://web.eece.maine.edu/~vweaver/projects/perf_events/
● Trace and count tracepoints and lower level events, PMU, HW events (L1
cache store/load/miss etc).
● Accesses data from user space efficiently by accessing the perf_events ring
buffer.

21.
USDT (BCC March 2016)
● Userland Statically Defined Tracepoints
● sudo ./tplist -l <library name>

22.
http://www.brendangregg.com/ebpf.html
This is our map YOU ARE HERE

23.
sudo apt-get install bpfcc-
tools

24.
Single Purpose Tools

25.
Multi-Purpose Tools

26.
So what does it look like?

27.
https://github.com/torvalds/linux/blob/master/samples/bpf/sock_example.c

28.
Ayyy, lol 😂 jk

29.
https://github.com/iovisor/bcc
https://github.com/iovisor/gobpf
BPF Compiler Collection (BCC)
Python, Lua, Golang

30.
Let’s Talk about the VM,
First Let’s Check our Map

31.
YOU ARE IN 1992

32.
https://www.tcpdump.org/papers/bpf-usenix93.pdf

33.
tcpdump -ni eth0 ip and udp

34.
tcpdump -ni eth0 ip and udp -d

35.
tcpdump
libpcap
bpf
Userspace
Kernel
tcp and udp
bytecode
packets
packets

36.
BPF - Berkeley Packet Filter
● Bytecode, register based VM, with a limited instruction set
● Runs in-kernel, designed for fast packet filtering
● 32-bit instructions (LOAD, STORE, ALU, BRANCH, RETURN)
● 2, 32-bit registers (A, X), hidden frame pointer

37.
Bpf bytecode for ‘tcpdump ip and udp’
(000) ldh [12] (load 2 bytes from packet, at offset 12)
(001) jeq #0x800 jt 2 jf 5
(002) ldb [23] (load byte at offset 23)
(003) jeq #0x11 jt 4jf 5 (0x11 == 17)
(004) ret #262144
(005) ret #0
https://blog.cloudflare.com/bpf-the-forgotten-bytecode/
http://www.networksorcery.com/enp/protocol/ip.htm

38.
http://www.brendangregg.com/ebpf.html
This is our map YOU ARE HERE

39.
eBPF - Extended Berkeley Packet Filter
● Bytecode, register based VM, with a extended instruction set
○ Designed to be JITed with direct mapping to x86_64
● 64-bit instructions, and 10 64-bit registers
○ R0 - return value from in-kernel function, and exit value for eBPF program
○ R1 - R5 - arguments from eBPF program to in-kernel function
○ R6 - R9 - callee saved registers that in-kernel function will preserve
○ R10 - read-only frame pointer to access stack
● BPF_CALL
○ hw register zero overhead calls to other kernel functions
● BPF_MAPS
○ Bi-directional data structures for storage. Allow sharing of data between eBPF kernel
programs, and also between kernel and user-space applications.
● Helper Functions
○ https://github.com/iovisor/bcc/blob/master/docs/reference_guide.md ← Very Important!

40.
eBPF - Extended Berkeley Packet Filter… cont
● Load programs via bpf(2) syscall (see: man bpf)
○ int bpf(int cmd, union bpf_attr *attr, unsigned int size);
● Cmd: BPF_PROG_LOAD
○ Verify and load an eBPF program, returning a new file descriptor associated with the
program. The close-on-exec file descriptor flag (see fcntl(2)) is automatically enabled for
the new file descriptor.

41.
Can we learn more about
eBPF VM like we did with
tcpdump?

42.
http://www.brendangregg.com/ebpf.html
This is our map YOU ARE HERE

43.
https://github.com/iovisor/bpf-docs/blob/master/eBPF.md

44.
0xb7 r1 imm: 72=114,
6c=108,64=100, (op) (dst)
0a=10
imm->ascii=”rldn”

45.
0x63 r1 r10 offset
(op) (src) (dst)

46.
0x18 r1 imm
(op) (dst) “hello wo”

47.
As you can imagine the next 4 instructions
copy the “hello wo” into a scratch space at
offset -16. Copy a “0” into r1 and then
copies “0” at offset -4. Finally we copy the
address of the variable from the frame
pointer at r10 into r1.

48.
To prepare for the call to
int bpf_trace_printk(const char *fmt, u32 fmt_size, ...)
We need to point r1 to the variable (which is -16 bytes
from the frame pointer) and in r2, we store the size of
“hello worldn0” = 13 bytes.

49.
0x85 Is a function call, with an imm of 6. We need to
look that up in bpf.h in order to figure out what that is.

50.
0
1
2
3
4
5
6

51.
Lastly we set our return value in r0 = 0 and exit with
opcode 0x95.

52.
http://www.brendangregg.com/ebpf.html
This is our map YOU ARE HERE

53.
eBPF Maps

54.
Helper Functions
● https://github.com/torvalds/linux/blob/master/include/uapi/linux/bpf.h
● https://github.com/iovisor/bcc/blob/master/docs/reference_guide.md
● int bpf_probe_read(void *dst, int size, const void *src) ← all reads must call
● int bpf_probe_read_str(void *dst, int size, const void *src)
● u64 bpf_ktime_get_ns(void)
● u64 bpf_get_current_pid_tgid(void)
● bpf_get_current_comm(char *buf, int size_of_buf)
● BPF_PERF_OUTPUT(name)
● int perf_submit((void *)ctx, (void *)data, u32 data_size)
● Map Functions
○ *val map.lookup(&key), val lookup_or_init(&key, &zero), delete(&key),
update(&key, &val), map.increment(key[, increment_amount])

55.
Segment Use Cases

56.
segmentio/netsniff - tw: @julien_fabre / gh: @pryz

57.
segmentio/ebpf
● Golang eBPF “Collectors”.
● CLI + ebpfd agent processes configuration and starts
eBPF programs.
● Stats aggregation, publishing to observers, 3rd party
stats forwarding (datadog etc.).
● Docker / pid -> container/service resolution.

58.
segmentio/ebpf

59.
Thank You! Questions?

60.
References
● https://lwn.net/Articles/740157/ - A thorough introduction to eBPF
● https://lwn.net/Articles/599755/ - BPF: the universal in-kernel virtual machine
● https://www.collabora.com/news-and-blog/blog/2019/04/15/an-ebpf-overview-part-2-machine-and-bytecode/
● https://www.youtube.com/watch?v=2lbtr85Yrs4 - Kernel Tracing with eBPF
● https://www.kernel.org/doc/Documentation/networking/filter.txt - Linux Socket Filtering aka Berkeley Packet Filter
● http://www.brendangregg.com/ebpf.html - Linux Extended BPF (eBPF) Tracing Tools
● https://www.slideshare.net/vh21/meet-cutebetweenebpfandtracing - Meet cute between eBPF and tracing
● https://blog.cloudflare.com/bpf-the-forgotten-bytecode/ - BPF the forgotten bytecode
● https://www.oreilly.com/learning/using-linux-tracing-tools - Modern Linux Tracing Landscape
● https://lwn.net/Articles/742082/ - An introduction to the BPF Compiler Collection
● https://bolinfest.github.io/opensnoop-native/ - How I ended up writing opensnoop in pure C using eBPF
● https://lwn.net/Articles/753601/ - Using user-space tracepoints with BPF
● http://brendangregg.com/perf.html - Perf Examples