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cmd/go: go build should error if ldflags -X pointed at const #20649

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icio opened this issue Jun 12, 2017 · 3 comments
Closed

cmd/go: go build should error if ldflags -X pointed at const #20649

icio opened this issue Jun 12, 2017 · 3 comments
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@icio
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icio commented Jun 12, 2017 •
edited

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.8.3 darwin/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="darwin"
GOOS="darwin"
GOPATH="/Users/icio/go"
GORACE=""
GOROOT="/usr/local/go"
GOTOOLDIR="/usr/local/go/pkg/tool/darwin_amd64"
GCCGO="gccgo"
CC="clang"
GOGCCFLAGS="-fPIC -m64 -pthread -fno-caret-diagnostics -Qunused-arguments -fmessage-length=0 -fdebug-prefix-map=/var/folders/87/jm5nnqqs2_xcxr3bfpkg73tw0000gn/T/go-build323188801=/tmp/go-build -gno-record-gcc-switches -fno-common"
CXX="clang++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

Created a file main.go:

package main

var Forename = "Paul"
const Surname = "Scott"

func main() {
	println("Hello, " + Forename + " " + Surname)
}

And ran the following command:

go run -ldflags '-X main.Forename=Bob -X main.Surname=Smith' main.go

What did you expect to see?

An error that main.Surname is const and could not be overridden, or

Hello, Bob Smith

What did you see instead?

Hello, Bob Scott
@icio icio changed the title cmd/go: go build should error if -X pointed at const cmd/go: go build should error if ldflags -X pointed at const Jun 12, 2017
@ianlancetaylor
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I'm fine with the idea in principle but I don't see how it is possible in practice. We decided a long time ago that the linker should accept a -X option for an unknown symbol, so that people could use a single set of -X options to build multiple programs. In your test Surname is a constant, so the linker never sees it. The effect is that the linker thinks you are using -X with an unknown symbol, so it doesn't do anything.

I'll leave this open for now in case someone else thinks of a way to implement it.

@icio
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icio commented Jun 12, 2017

That's reasonable. I wondered whether this was a result of the const disappearing before the -X was considered, but I'm not familiar with the full steps involved. Thanks for looking.

@rsc
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rsc commented Jun 12, 2017

I can't think of a way to do it, and worse I can think of migration paths that it might break (if we think it's important to support -X in the transition from builds with no var that add a var, it seems reasonable to think it's also important to support -X in the transition from builds with a const to builds with a var).

@rsc rsc closed this as completed Oct 25, 2017
@golang golang locked and limited conversation to collaborators Oct 25, 2018
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@rsc @icio @ianlancetaylor @gopherbot