mirror of
https://github.com/superseriousbusiness/gotosocial.git
synced 2024-11-05 00:39:59 +00:00
57dc742c76
Bumps [github.com/KimMachineGun/automemlimit](https://github.com/KimMachineGun/automemlimit) from 0.2.4 to 0.2.5. - [Release notes](https://github.com/KimMachineGun/automemlimit/releases) - [Commits](https://github.com/KimMachineGun/automemlimit/compare/v0.2.4...v0.2.5) --- updated-dependencies: - dependency-name: github.com/KimMachineGun/automemlimit dependency-type: direct:production update-type: version-update:semver-patch ... Signed-off-by: dependabot[bot] <support@github.com> Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>
87 lines
3.9 KiB
Markdown
87 lines
3.9 KiB
Markdown
Architecture of the library
|
|
===
|
|
|
|
ELF -> Specifications -> Objects -> Links
|
|
|
|
ELF
|
|
---
|
|
|
|
BPF is usually produced by using Clang to compile a subset of C. Clang outputs
|
|
an ELF file which contains program byte code (aka BPF), but also metadata for
|
|
maps used by the program. The metadata follows the conventions set by libbpf
|
|
shipped with the kernel. Certain ELF sections have special meaning
|
|
and contain structures defined by libbpf. Newer versions of clang emit
|
|
additional metadata in BPF Type Format (aka BTF).
|
|
|
|
The library aims to be compatible with libbpf so that moving from a C toolchain
|
|
to a Go one creates little friction. To that end, the [ELF reader](elf_reader.go)
|
|
is tested against the Linux selftests and avoids introducing custom behaviour
|
|
if possible.
|
|
|
|
The output of the ELF reader is a `CollectionSpec` which encodes
|
|
all of the information contained in the ELF in a form that is easy to work with
|
|
in Go.
|
|
|
|
### BTF
|
|
|
|
The BPF Type Format describes more than just the types used by a BPF program. It
|
|
includes debug aids like which source line corresponds to which instructions and
|
|
what global variables are used.
|
|
|
|
[BTF parsing](internal/btf/) lives in a separate internal package since exposing
|
|
it would mean an additional maintenance burden, and because the API still
|
|
has sharp corners. The most important concept is the `btf.Type` interface, which
|
|
also describes things that aren't really types like `.rodata` or `.bss` sections.
|
|
`btf.Type`s can form cyclical graphs, which can easily lead to infinite loops if
|
|
one is not careful. Hopefully a safe pattern to work with `btf.Type` emerges as
|
|
we write more code that deals with it.
|
|
|
|
Specifications
|
|
---
|
|
|
|
`CollectionSpec`, `ProgramSpec` and `MapSpec` are blueprints for in-kernel
|
|
objects and contain everything necessary to execute the relevant `bpf(2)`
|
|
syscalls. Since the ELF reader outputs a `CollectionSpec` it's possible to
|
|
modify clang-compiled BPF code, for example to rewrite constants. At the same
|
|
time the [asm](asm/) package provides an assembler that can be used to generate
|
|
`ProgramSpec` on the fly.
|
|
|
|
Creating a spec should never require any privileges or be restricted in any way,
|
|
for example by only allowing programs in native endianness. This ensures that
|
|
the library stays flexible.
|
|
|
|
Objects
|
|
---
|
|
|
|
`Program` and `Map` are the result of loading specs into the kernel. Sometimes
|
|
loading a spec will fail because the kernel is too old, or a feature is not
|
|
enabled. There are multiple ways the library deals with that:
|
|
|
|
* Fallback: older kernels don't allow naming programs and maps. The library
|
|
automatically detects support for names, and omits them during load if
|
|
necessary. This works since name is primarily a debug aid.
|
|
|
|
* Sentinel error: sometimes it's possible to detect that a feature isn't available.
|
|
In that case the library will return an error wrapping `ErrNotSupported`.
|
|
This is also useful to skip tests that can't run on the current kernel.
|
|
|
|
Once program and map objects are loaded they expose the kernel's low-level API,
|
|
e.g. `NextKey`. Often this API is awkward to use in Go, so there are safer
|
|
wrappers on top of the low-level API, like `MapIterator`. The low-level API is
|
|
useful when our higher-level API doesn't support a particular use case.
|
|
|
|
Links
|
|
---
|
|
|
|
BPF can be attached to many different points in the kernel and newer BPF hooks
|
|
tend to use bpf_link to do so. Older hooks unfortunately use a combination of
|
|
syscalls, netlink messages, etc. Adding support for a new link type should not
|
|
pull in large dependencies like netlink, so XDP programs or tracepoints are
|
|
out of scope.
|
|
|
|
Each bpf_link_type has one corresponding Go type, e.g. `link.tracing` corresponds
|
|
to BPF_LINK_TRACING. In general, these types should be unexported as long as they
|
|
don't export methods outside of the Link interface. Each Go type may have multiple
|
|
exported constructors. For example `AttachTracing` and `AttachLSM` create a
|
|
tracing link, but are distinct functions since they may require different arguments.
|