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diff --git a/Documentation/bpf/bpf_licensing.rst b/Documentation/bpf/bpf_licensing.rst new file mode 100644 index 000000000000..b19c433f41d2 --- /dev/null +++ b/Documentation/bpf/bpf_licensing.rst @@ -0,0 +1,92 @@ +============= +BPF licensing +============= + +Background +========== + +* Classic BPF was BSD licensed + +"BPF" was originally introduced as BSD Packet Filter in +http://www.tcpdump.org/papers/bpf-usenix93.pdf. The corresponding instruction +set and its implementation came from BSD with BSD license. That original +instruction set is now known as "classic BPF". + +However an instruction set is a specification for machine-language interaction, +similar to a programming language. It is not a code. Therefore, the +application of a BSD license may be misleading in a certain context, as the +instruction set may enjoy no copyright protection. + +* eBPF (extended BPF) instruction set continues to be BSD + +In 2014, the classic BPF instruction set was significantly extended. We +typically refer to this instruction set as eBPF to disambiguate it from cBPF. +The eBPF instruction set is still BSD licensed. + +Implementations of eBPF +======================= + +Using the eBPF instruction set requires implementing code in both kernel space +and user space. + +In Linux Kernel +--------------- + +The reference implementations of the eBPF interpreter and various just-in-time +compilers are part of Linux and are GPLv2 licensed. The implementation of +eBPF helper functions is also GPLv2 licensed. Interpreters, JITs, helpers, +and verifiers are called eBPF runtime. + +In User Space +------------- + +There are also implementations of eBPF runtime (interpreter, JITs, helper +functions) under +Apache2 (https://github.com/iovisor/ubpf), +MIT (https://github.com/qmonnet/rbpf), and +BSD (https://github.com/DPDK/dpdk/blob/main/lib/librte_bpf). + +In HW +----- + +The HW can choose to execute eBPF instruction natively and provide eBPF runtime +in HW or via the use of implementing firmware with a proprietary license. + +In other operating systems +-------------------------- + +Other kernels or user space implementations of eBPF instruction set and runtime +can have proprietary licenses. + +Using BPF programs in the Linux kernel +====================================== + +Linux Kernel (while being GPLv2) allows linking of proprietary kernel modules +under these rules: +Documentation/process/license-rules.rst + +When a kernel module is loaded, the linux kernel checks which functions it +intends to use. If any function is marked as "GPL only," the corresponding +module or program has to have GPL compatible license. + +Loading BPF program into the Linux kernel is similar to loading a kernel +module. BPF is loaded at run time and not statically linked to the Linux +kernel. BPF program loading follows the same license checking rules as kernel +modules. BPF programs can be proprietary if they don't use "GPL only" BPF +helper functions. + +Further, some BPF program types - Linux Security Modules (LSM) and TCP +Congestion Control (struct_ops), as of Aug 2021 - are required to be GPL +compatible even if they don't use "GPL only" helper functions directly. The +registration step of LSM and TCP congestion control modules of the Linux +kernel is done through EXPORT_SYMBOL_GPL kernel functions. In that sense LSM +and struct_ops BPF programs are implicitly calling "GPL only" functions. +The same restriction applies to BPF programs that call kernel functions +directly via unstable interface also known as "kfunc". + +Packaging BPF programs with user space applications +==================================================== + +Generally, proprietary-licensed applications and GPL licensed BPF programs +written for the Linux kernel in the same package can co-exist because they are +separate executable processes. This applies to both cBPF and eBPF programs. |