Berkeley Packet Filter

The Berkeley Packet Filter (BPF) is a technology used in certain computer operating systems for programs that need to, among other things, analyze network traffic. It provides a raw interface to data link layers, permitting raw link-layer packets to be sent and received.^[1] In addition, if the driver for the network interface supports promiscuous mode, it allows the interface to be put into that mode so that all packets on the network can be received, even those destined to other hosts.

Berkeley Packet Filter

Developer(s)	Steven McCanne, Van Jacobson
Initial release	December 19, 1992; 30 years ago (1992-12-19)
Operating system	Unix-like (FreeBSD, OpenBSD, NetBSD, DragonFly BSD, macOS, Oracle Solaris 11 and later, AIX, Tru64, Linux, Orbis), Windows

BPF supports filtering packets, allowing a userspace process to supply a filter program that specifies which packets it wants to receive. For example, a tcpdump process may want to receive only packets that initiate a TCP connection. BPF returns only packets that pass the filter that the process supplies. This avoids copying unwanted packets from the operating system kernel to the process, greatly improving performance. The filter program is in the form of instructions for a virtual machine, which are interpreted, or compiled into machine code by a just-in-time (JIT) mechanism and executed, in the kernel.

BPF is sometimes used to refer to just the filtering mechanism, rather than to the entire interface. Some systems, such as Linux and Tru64 UNIX, provide a raw interface to the data link layer other than the BPF raw interface but use the BPF filtering mechanisms for that raw interface. The BPF filtering mechanism is available on most Unix-like operating systems.

The Linux kernel provides an extended version of the BPF filtering mechanism, called eBPF, which uses a JIT mechanism, and which is used for packet filtering, as well as for other purposes in the kernel. eBPF is also available for Microsoft Windows.^[2]

Raw data-link interface

BPF provides pseudo-devices that can be bound to a network interface; reads from the device will read buffers full of packets received on the network interface, and writes to the device will inject packets on the network interface.

In 2007, Robert Watson and Christian Peron added zero-copy buffer extensions to the BPF implementation in the FreeBSD operating system,^[3] allowing kernel packet capture in the device driver interrupt handler to write directly to user process memory in order to avoid the requirement for two copies for all packet data received via the BPF device. While one copy remains in the receipt path for user processes, this preserves the independence of different BPF device consumers, as well as allowing the packing of headers into the BPF buffer rather than copying complete packet data.^[4]

Filtering

BPF's filtering capabilities are implemented as an interpreter for a machine language for the BPF virtual machine, a 32-bit machine with fixed-length instructions, one accumulator, and one index register. Programs in that language can fetch data from the packet, perform arithmetic operations on data from the packet, and compare the results against constants or against data in the packet or test bits in the results, accepting or rejecting the packet based on the results of those tests.

BPF is often extended by "overloading" the load (ld) and store (str) instructions.

Traditional Unix-like BPF implementations can be used in userspace, despite being written for kernel-space. This is accomplished using preprocessor conditions.

Extensions and optimizations

Some projects use BPF instruction sets or execution techniques different from the originals.

Some platforms, including FreeBSD, NetBSD, and WinPcap, use a just-in-time (JIT) compiler to convert BPF instructions into native code in order to improve performance. Linux includes a BPF JIT compiler which is disabled by default.

Kernel-mode interpreters for that same virtual machine language are used in raw data link layer mechanisms in other operating systems, such as Tru64 Unix, and for socket filters in the Linux kernel and in the WinPcap and Npcap packet capture mechanism.

Since version 3.18, the Linux kernel includes an extended BPF virtual machine with ten 64-bit registers, termed extended BPF (eBPF). It can be used for non-networking purposes, such as for attaching eBPF programs to various tracepoints.^[5][6][7] Since kernel version 3.19, eBPF filters can be attached to sockets,^[8][9] and, since kernel version 4.1, to traffic control classifiers for the ingress and egress networking data path.^[10][11] The original and obsolete version has been retroactively renamed to classic BPF (cBPF). Nowadays, the Linux kernel runs eBPF only and loaded cBPF bytecode is transparently translated into an eBPF representation in the kernel before program execution.^[12] All bytecode is verified before running to prevent denial-of-service attacks. Until Linux 5.3, the verifier prohibited the use of loops, to prevent potentially unbounded execution times; loops with bounded execution time are now permitted in more recent kernels.^[13]

A user-mode interpreter for BPF is provided with the libpcap/WinPcap/Npcap implementation of the pcap API, so that, when capturing packets on systems without kernel-mode support for that filtering mechanism, packets can be filtered in user mode; code using the pcap API will work on both types of systems, although, on systems where the filtering is done in user mode, all packets, including those that will be filtered out, are copied from the kernel to user space. That interpreter can also be used when reading a file containing packets captured using pcap.

Another user-mode interpreter is uBPF, which supports JIT and eBPF (without cBPF). Its code has been reused to provide eBPF support in non-Linux systems.^[14] Microsoft's eBPF on Windows builds on uBPF and the PREVAIL formal verifier.^[15] rBPF, a Rust rewrite of uBPF, is used by the Solana blockchain platform as the execution engine.^[16]

Programming

Classic BPF is generally emitted by a program from some very high-level textual rule describing the pattern to match. One such representation is found in libpcap.^[17] Classic BPF and eBPF can also be written either directly as machine code, or using an assembly language for a textual representation. Notable assemblers include Linux kernel's bpf_asm tool (cBPF), bpfc (cBPF), and the ubpf assembler (eBPF). The bpftool command can also act as a disassembler for both flavors of BPF. The assembly languages are not necessarily compatible with each other.

eBPF bytecode has recently become a target of higher-level languages. LLVM added eBPF support in 2014, and GCC followed in 2019. Both toolkits allow compiling C and other supported languages to eBPF. A subset of P4 can also be compiled into eBPF using BCC, an LLVM-based compiler kit.^[18]

History

The original paper was written by Steven McCanne and Van Jacobson in 1992 while at Lawrence Berkeley Laboratory.^[1][19]

In August 2003, SCO Group publicly claimed that the Linux kernel was infringing Unix code which they owned.^[20] Programmers quickly discovered that one example they gave was the Berkeley Packet Filter, which in fact SCO never owned.^[21] SCO has not explained or acknowledged the mistake but the ongoing legal action may eventually force an answer.^{[22][needs update?]}

Security concerns

The Spectre attack could leverage the Linux kernel's eBPF interpreter or JIT compiler to extract data from other kernel processes.^[23] A JIT hardening feature in the kernel mitigates this vulnerability.^[24]

Chinese computer security group Pangu Lab said the NSA used BPF to conceal network communications as part of a complex Linux backdoor.^[25]

See also

• eBPF
• Data link layer
• Proof-carrying code
• Express Data Path

References

1. McCanne, Steven; Jacobson, Van (1992-12-19). "The BSD Packet Filter: A New Architecture for User-level Packet Capture" (PDF).
2. "Microsoft embraces Linux kernel's eBPF super-tool, extends it for Windows". The Register. 2021-05-11. from the original on 2021-05-11.
3. "bpf(4) Berkeley Packet Filter". FreeBSD. 2010-06-15.
4. Watson, Robert N. M.; Peron, Christian S. J. (2007-03-09). "Zero-Copy BPF" (PDF).
5. "Linux kernel 3.18, Section 1.3. bpf() syscall for eBFP virtual machine programs". kernelnewbies.org. December 7, 2014. Retrieved September 6, 2019.
6. Jonathan Corbet (September 24, 2014). "The BPF system call API, version 14". LWN.net. Retrieved January 19, 2015.
7. Jonathan Corbet (July 2, 2014). "Extending extended BPF". LWN.net. Retrieved January 19, 2015.
8. "Linux kernel 3.19, Section 11. Networking". kernelnewbies.org. February 8, 2015. Retrieved February 13, 2015.
9. Jonathan Corbet (December 10, 2014). "Attaching eBPF programs to sockets". LWN.net. Retrieved February 13, 2015.
10. "Linux kernel 4.1, Section 11. Networking". kernelnewbies.org. June 21, 2015. Retrieved October 17, 2015.
11. "BPF and XDP Reference Guide". cilium.readthedocs.io. April 24, 2017. Retrieved April 23, 2018.
12. "BPF and XDP Reference Guide — Cilium 1.6.5 documentation". docs.cilium.io. Retrieved 2019-12-18.
13. "bpf: introduce bounded loops". git.kernel.org. June 19, 2019. Retrieved August 19, 2022.
14. "generic-ebpf/generic-ebpf". GitHub. 28 April 2022.
15. "microsoft/ebpf-for-windows: eBPF implementation that runs on top of Windows". GitHub. Microsoft. 11 May 2021.
16. "Overview | Solana Docs".
17. "BPF syntax". biot.com.
18. "Dive into BPF: a list of reading material". qmonnet.github.io.
19. McCanne, Steven; Jacobson, Van (January 1993). "The BSD Packet Filter: A New Architecture for User-level Packet Capture". USENIX.
20. . 15 Obfuscated Copying. Archived from the original on August 25, 2003. Retrieved September 5, 2019.
21. Bruce Perens. . Archived from the original on February 17, 2009.
22. Moglen, Eben (November 24, 2003). "SCO: Without Fear and Without Research". GNU Operating System. The Free Software Foundation. Retrieved September 5, 2019.
23. "Reading privileged memory with a side-channel". Project Zero team at Google. January 3, 2018. Retrieved January 20, 2018.
24. "bpf: introduce BPF_JIT_ALWAYS_ON config". git.kernel.org. from the original on 2020-10-19. Retrieved 2021-09-20.
25. "Anatomy of suspected top-tier decade-hidden NSA backdoor". The Register. February 23, 2022. Retrieved February 24, 2022.

Further reading

• McCanne, Steven; Jacobson, Van (1992-12-19). "The BSD Packet Filter: A New Architecture for User-level Packet Capture" (PDF).

External links

• bpf(4) – FreeBSD Kernel Interfaces Manual – an example of conventional BPF
• eBPF.io - Introduction, Tutorials & Community Resources
• bpfc, a Berkeley Packet Filter compiler, Linux BPF JIT disassembler (part of netsniff-ng)
• BPF Documentation, for Linux kernel
• Linux filter documentation, for both cBPF and eBPF bytecode formats
• ebpf-for-windows on GitHub