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# Past garble versions might not properly patch cmd/link with "git apply"
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# when running inside a git repository. Skip the extra check with -short.
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[!short] [exec:git] exec git init -q
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[!short] [exec:git] env GARBLE_CACHE_DIR=$WORK/linker-cache
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patch and rebuild cmd/link to modify the magic value in pclntab
This value is hard-coded in the linker and written in a header.
We could rewrite the final binary, like we used to do with import paths,
but that would require once again maintaining libraries to do so.
Instead, we're now modifying the linker to do what we want.
It's not particularly hard, as every Go install has its source code,
and rebuilding a slightly modified linker only takes a few seconds at most.
Thanks to `go build -overlay`, we only need to copy the files we modify,
and right now we're just modifying one file in the toolchain.
We use a git patch, as the change is fairly static and small,
and the patch is easier to understand and maintain.
The other side of this change is in the runtime,
as it also hard-codes the magic value when loading information.
We modify the code via syntax trees in that case, like `-tiny` does,
because the change is tiny (one literal) and the affected lines of code
are modified regularly between major Go releases.
Since rebuilding a slightly modified linker can take a few seconds,
and Go's build cache does not cache linked binaries,
we keep our own cached version of the rebuilt binary in `os.UserCacheDir`.
The feature isn't perfect, and will be improved in the future.
See the TODOs about the added dependency on `git`,
or how we are currently only able to cache one linker binary at once.
Fixes #622.
2 years ago
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garble build
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exec ./main
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! cmp stderr main.stderr
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[short] stop # no need to verify this with -short
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go build
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exec ./main
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cmp stderr main.stderr
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-- go.mod --
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module test/main
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go 1.20
|
patch and rebuild cmd/link to modify the magic value in pclntab
This value is hard-coded in the linker and written in a header.
We could rewrite the final binary, like we used to do with import paths,
but that would require once again maintaining libraries to do so.
Instead, we're now modifying the linker to do what we want.
It's not particularly hard, as every Go install has its source code,
and rebuilding a slightly modified linker only takes a few seconds at most.
Thanks to `go build -overlay`, we only need to copy the files we modify,
and right now we're just modifying one file in the toolchain.
We use a git patch, as the change is fairly static and small,
and the patch is easier to understand and maintain.
The other side of this change is in the runtime,
as it also hard-codes the magic value when loading information.
We modify the code via syntax trees in that case, like `-tiny` does,
because the change is tiny (one literal) and the affected lines of code
are modified regularly between major Go releases.
Since rebuilding a slightly modified linker can take a few seconds,
and Go's build cache does not cache linked binaries,
we keep our own cached version of the rebuilt binary in `os.UserCacheDir`.
The feature isn't perfect, and will be improved in the future.
See the TODOs about the added dependency on `git`,
or how we are currently only able to cache one linker binary at once.
Fixes #622.
2 years ago
|
|
|
-- main.go --
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package main
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import (
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"strconv"
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"strings"
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_ "unsafe"
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)
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type fakeModuleData struct {
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pcHeader *struct {
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magic uint32
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}
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}
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//go:linkname activeModules runtime.activeModules
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func activeModules() []*fakeModuleData
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// genericMagicValue returns magic value without last digit
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func genericMagicValue() string {
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|
mod := activeModules()[0]
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|
magicValHex := strings.ToUpper(strconv.FormatUint(uint64(mod.pcHeader.magic), 16))
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return "0x" + magicValHex[:len(magicValHex)-1] + "?"
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}
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func main() {
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println(genericMagicValue())
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}
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-- main.stderr --
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0xFFFFFFF?
|
