ARCHITECTURE.md (3989B)
1 Architecture of the library 2 === 3 4 ELF -> Specifications -> Objects -> Links 5 6 ELF 7 --- 8 9 BPF is usually produced by using Clang to compile a subset of C. Clang outputs 10 an ELF file which contains program byte code (aka BPF), but also metadata for 11 maps used by the program. The metadata follows the conventions set by libbpf 12 shipped with the kernel. Certain ELF sections have special meaning 13 and contain structures defined by libbpf. Newer versions of clang emit 14 additional metadata in BPF Type Format (aka BTF). 15 16 The library aims to be compatible with libbpf so that moving from a C toolchain 17 to a Go one creates little friction. To that end, the [ELF reader](elf_reader.go) 18 is tested against the Linux selftests and avoids introducing custom behaviour 19 if possible. 20 21 The output of the ELF reader is a `CollectionSpec` which encodes 22 all of the information contained in the ELF in a form that is easy to work with 23 in Go. 24 25 ### BTF 26 27 The BPF Type Format describes more than just the types used by a BPF program. It 28 includes debug aids like which source line corresponds to which instructions and 29 what global variables are used. 30 31 [BTF parsing](internal/btf/) lives in a separate internal package since exposing 32 it would mean an additional maintenance burden, and because the API still 33 has sharp corners. The most important concept is the `btf.Type` interface, which 34 also describes things that aren't really types like `.rodata` or `.bss` sections. 35 `btf.Type`s can form cyclical graphs, which can easily lead to infinite loops if 36 one is not careful. Hopefully a safe pattern to work with `btf.Type` emerges as 37 we write more code that deals with it. 38 39 Specifications 40 --- 41 42 `CollectionSpec`, `ProgramSpec` and `MapSpec` are blueprints for in-kernel 43 objects and contain everything necessary to execute the relevant `bpf(2)` 44 syscalls. Since the ELF reader outputs a `CollectionSpec` it's possible to 45 modify clang-compiled BPF code, for example to rewrite constants. At the same 46 time the [asm](asm/) package provides an assembler that can be used to generate 47 `ProgramSpec` on the fly. 48 49 Creating a spec should never require any privileges or be restricted in any way, 50 for example by only allowing programs in native endianness. This ensures that 51 the library stays flexible. 52 53 Objects 54 --- 55 56 `Program` and `Map` are the result of loading specs into the kernel. Sometimes 57 loading a spec will fail because the kernel is too old, or a feature is not 58 enabled. There are multiple ways the library deals with that: 59 60 * Fallback: older kernels don't allow naming programs and maps. The library 61 automatically detects support for names, and omits them during load if 62 necessary. This works since name is primarily a debug aid. 63 64 * Sentinel error: sometimes it's possible to detect that a feature isn't available. 65 In that case the library will return an error wrapping `ErrNotSupported`. 66 This is also useful to skip tests that can't run on the current kernel. 67 68 Once program and map objects are loaded they expose the kernel's low-level API, 69 e.g. `NextKey`. Often this API is awkward to use in Go, so there are safer 70 wrappers on top of the low-level API, like `MapIterator`. The low-level API is 71 useful when our higher-level API doesn't support a particular use case. 72 73 Links 74 --- 75 76 BPF can be attached to many different points in the kernel and newer BPF hooks 77 tend to use bpf_link to do so. Older hooks unfortunately use a combination of 78 syscalls, netlink messages, etc. Adding support for a new link type should not 79 pull in large dependencies like netlink, so XDP programs or tracepoints are 80 out of scope. 81 82 Each bpf_link_type has one corresponding Go type, e.g. `link.tracing` corresponds 83 to BPF_LINK_TRACING. In general, these types should be unexported as long as they 84 don't export methods outside of the Link interface. Each Go type may have multiple 85 exported constructors. For example `AttachTracing` and `AttachLSM` create a 86 tracing link, but are distinct functions since they may require different arguments.