The long-overdue problem coming for some people in Go 1.23

May 24, 2024

Right now, if you try to build anything using the currently released version of github.com/quic-go/quic-go with the current development version of Go (such as this DNS query program), you will probably encounter the following error:

link: github.com/quic-go/quic-go/internal/qtls: invalid reference to crypto/tls.defaultCipherSuitesTLS13

Experienced Go developers may now be scratching their heads about how quic-go/internal/qtls is referring to crypto/tls.defaultCipherSuitesTLS13, since the latter isn't an exported identifier (in Go, all exported identifiers start with a capital letter). The simple answer is that the qtls package is cheating (in cipher_suite.go).

The official Go compiler has a number of special compiler directives. A few of them are widely known and used, for example '//go:noinline' is common in some benchmarking to stop the compiler from optimizing your test functions too much. One of the not well known ones is '//go:linkname', and for this I'll just quote from its documentation:

//go:linkname localname [importpath.name]

[...] This directive determines the object-file symbol used for a Go var or func declaration, allowing two Go symbols to alias the same object-file symbol, thereby enabling one package to access a symbol in another package even when this would violate the usual encapsulation of unexported declarations, or even type safety. For that reason, it is only enabled in files that have imported "unsafe".

Let me translate that: go:linkname allows you to access unexported variables and functions of other packages. In particular, it allows you to access unexported variables (and functions) from the Go standard library, such as crypto/tls.defaultCipherSuitesTLS13.

The Go standard library uses go:linkname internally to access various unexported things from other packages and from the core runtime, which is perfectly fair; the entire standard library is developed by the same people, and they have to be very careful and conservative with the public API. However, go:linkname has also been used by a wide assortment of third party packages to access unexported pieces of the standard library that those packages found convenient or useful (such as Go's default cipher suites for TLS 1.3). Accessing unexported things from the Go standard library isn't covered by the Go 1 compatibility guarantee, for obvious reasons, but in practice the Go developers find themselves not wanting to break too much of the Go package ecosystem even if said ecosystem is doing unsupported things.

Last week, the Go developers noticed this (I believe not for the first time) and Russ Cox filed issue #67401: cmd/link: lock down future uses of linkname, where you can find a thorough discussion of the issue. The end result is that the current development version of Go, which will become Go 1.23, is now much more restrictive about go:linkname, requiring that the target symbol opt in to this usage. Starting from Go 1.23, you will not be able to 'go:linkname' to things in the standard library that have not specifically allowed this (and the rules are probably going to get stricter in future Go versions; in a few versions I wouldn't be surprised if you couldn't go:linkname into the standard library at all from outside packages).

So this is what is happening with github.com/quic-go/quic-go. It is internally using a go:linkname to get access to crypto/tls's defaultCipherSuitesTLS13, but in Go 1.23, defaultCipherSuitesTLS13 is not one of the symbols that has opted in to this use, so the build is now failing. The quic-go package is probably far from the only package that is going to get caught out by this, now and in the future.

(The Go developers have been adding specific opt-ins for sufficiently used internal identifiers, in files generally called 'badlinkname.go' in the packages. You can see the current state for crypto/tls in its badlinkname.go file.)

Written on 24 May 2024.
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Last modified: Fri May 24 22:06:10 2024
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