95 lines
3.9 KiB
Markdown
95 lines
3.9 KiB
Markdown
---
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title: gopreload: LD_PRELOAD for the Gopher crowd
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date: 207-03-25
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---
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gopreload: LD_PRELOAD for the Gopher crowd
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==========================================
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A common pattern in Go libraries is to take advantage of [init functions][initf]
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to do things like settings up defaults in loggers, automatic metrics instrumentation,
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flag values, [debugging tools][manhole] or database drivers. With monorepo culture
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prevalent in larger microservices based projects, this can lead to a few easily
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preventable problems:
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- Forgetting to set up a logger default or metrics submission, making operations
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teams blind to the performance of the app and developer teams blind to errors
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that come up during execution.
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- The requirement to make code changes to add things like metrics or HTTP routing
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extensions.
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There is an environment variable in Linux libc's called `LD_PRELOAD` that will
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load arbitrary shared objects into ram before anything else is started. This
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has been used for [good][good-ld-preload] and [evil][evil-ld-preload], but the
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behavior is the same basic idea as [underscore imports in Go][underscore-import].
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My solution for this is [gopreload][gopreload]. It emulates the behavior of
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`LD_PRELOAD` but with [Go plugins][go-plugins]. This allows users to explicitly
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automatically load arbitrary Go code into ram while the process starts.
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## Usage
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To use this, add `gopreload` to your application's imports:
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```go
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// gopreload.go
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package main
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/*
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This file is separate to make it very easy to both add into an application, but
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also very easy to remove.
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*/
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import _ "github.com/Xe/gopreload"
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```
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and then compile `manhole`:
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```console
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$ go get -d github.com/Xe/gopreload/manhole
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$ go build -buildmode plugin -o $GOPATH/manhole.so github.com/Xe/gopreload/manhole
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```
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then run your program with `GO_PRELOAD` set to the path of `manhole.so`:
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```console
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$ export GO_PRELOAD=$GOPATH/manhole.so
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$ go run *.go
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2017/03/25 10:56:22 gopreload: trying to open: /home/xena/go/manhole.so
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2017/03/25 10:56:22 manhole: Now listening on http://127.0.0.2:37588
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```
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That endpoint has pprof and a few other [fun tools][manhole-tools] set up, making
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it a good stopgap "manhole" into the performance of a service.
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## Security Implications
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This package assumes that programs run using it are never started with environment
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variables that are set by unauthenticated users. Any errors in loading the plugins
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will be logged using the standard library logger `log` and ignored.
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This has about the same security implications as [`LD_PRELOAD`][ld-preload] does in most
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Linux distributions, but the risk is minimal compared to the massive benefit for
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being able to have arbitrary background services all be able to be dug into using
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the same tooling or being able to have metric submission be completely separated
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from the backend metric creation. Common logging setup processes can be _always_
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loaded, making the default logger settings into the correct settings.
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## Feedback
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To give feedback about gopreload, please contact me on [twitter][twitter-addr] or
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on the Gophers slack (I'm `@xena` there). For issues with gopreload please file
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[an issue on Github][gopreload-issues].
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[initf]: https://golang.org/doc/effective_go.html#init
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[manhole]: https://github.com/Xe/gopreload/tree/master/manhole
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[good-ld-preload]: http://www.logix.cz/michal/devel/faketime/
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[evil-ld-preload]: https://rafalcieslak.wordpress.com/2013/04/02/dynamic-linker-tricks-using-ld_preload-to-cheat-inject-features-and-investigate-programs/
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[underscore-import]: https://golang.org/doc/effective_go.html#blank
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[gopreload]: https://github.com/Xe/gopreload
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[go-plugins]: https://golang.org/pkg/plugin/
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[manhole-tools]: https://github.com/Xe/gopreload/blob/master/manhole/server.go
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[ld-preload]: https://rafalcieslak.wordpress.com/2013/04/02/dynamic-linker-tricks-using-ld_preload-to-cheat-inject-features-and-investigate-programs/
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[twitter-addr]: https://twitter.com/theprincessxena
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[gopreload-issues]: https://github.com/Xe/gopreload/issues/new
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