Last week, Go 1.23 entered the freeze period, meaning no new features will be added, and any already added features are unlikely to be removed. This is a great opportunity to preview the upcoming changes.
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Today, we will discuss the changes to the //go:linkname
directive in Go 1.23.
Related issue: GitHub Issue #67401
TL;DR: The //go:linkname
directive is not officially recommended and does not guarantee any forward or backward compatibility. It is wise to avoid using it whenever possible.
With that in mind, let’s dive into the new changes and see how they relate to us.
What Does the linkname
Directive Do?
In simple terms, the linkname
directive is used to pass information to the compiler and linker. Depending on its usage, it can be divided into three categories:
1. Pull
The “pull” usage is as follows:
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This directive format is //go:linkname <local function or package-level variable> <fully defined function or variable in this or another package>
. It tells the compiler and linker that my_func
should directly use fmt.Println
, making my_func
an alias for fmt.Println
.
This usage allows ignoring whether functions or variables are exported, pulling even package-private elements into use. However, this method is risky and can lead to panics if there’s a type mismatch.
2. Push
The “push” usage looks like this:
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Here, you only need to pass the function or variable name as the first parameter to the directive, specifying the package name where it should be used. This usage signifies that the function or variable will be named xxx.yyy
.
3. Handshake
The “handshake” usage combines both methods:
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The pull side remains unchanged, but the push side doesn’t need to specify the package name. This usage implies a handshake between the two ends, clearly marking which function or variable should be linked.
Risks of linkname
The primary risk is the ability to use package-private functions or variables without the package’s knowledge. For example:
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Normally, uintPow
shouldn’t be accessible outside its package. But linkname
bypasses this restriction, which can lead to severe type-related memory errors or runtime panics.
Positive Aspects of linkname
Despite its risks, linkname
exists for valid reasons, such as during the startup of Go programs. For example, in Go’s runtime:
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Here, linkname
allows the runtime to call the user-defined main
function.
Changes to linkname
in Go 1.23
Given the risks, the Go core team has decided to limit linkname
usage:
- New standard library packages will prohibit
linkname
. - A new ldflag,
-checklinkname=1
, has been added to enforce restrictions. It defaults to 0 but will be set to 1 in the final release of 1.23. - Pull-only
linkname
will be prohibited for the standard library, allowing only the handshake mode.
For instance, the following code will no longer compile in 1.23:
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Future of linkname
The long-term goal is to only allow handshake mode. As developers, we should:
- Use
-checklinkname=1
to audit and remove unnecessarylinkname
usage. - Propose to make private APIs public if necessary.
- As a last resort, disable the restriction with
-ldflags=-checklinkname=0
.
Conclusion
In summary, avoid using //go:linkname
to prevent unforeseen issues.