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cmd/link: failed to set variable at build time if variable value is result from method call #26042

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cuonglm opened this issue Jun 25, 2018 · 5 comments
Closed

cmd/link: failed to set variable at build time if variable value is result from method call #26042

cuonglm opened this issue Jun 25, 2018 · 5 comments
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Documentation FrozenDueToAge help wanted NeedsFix The path to resolution is known, but the work has not been done.
Milestone
Go1.12

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@cuonglm
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cuonglm commented Jun 25, 2018

Please answer these questions before submitting your issue. Thanks!

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

go version go1.10.3 linux/amd64

Does this issue reproduce with the latest release?

yes

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

GOARCH="amd64"
GOBIN=""
GOCACHE="/home/cuonglm/.cache/go-build"
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/cuonglm/go"
GORACE=""
GOROOT="/home/cuonglm/sources/go"
GOTMPDIR=""
GOTOOLDIR="/home/cuonglm/sources/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"
PKG_CONFIG="pkg-config"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build755255777=/tmp/go-build -gno-record-gcc-switches"

What did you do?

https://play.golang.org/p/M9xGmognyrl

run as:

go run -ldflags "-X main.X=y -X main.MyX=myY" main.go

What did you expect to see?

y
myY

What did you see instead?

y
myX
@ianlancetaylor
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I think that fixing this would require a completely different and less efficient approach to the implementation of the linker's -X option. I'm inclined to just make this a documented limitation.

@ianlancetaylor ianlancetaylor added Documentation help wanted NeedsFix The path to resolution is known, but the work has not been done. labels Jun 25, 2018
@ianlancetaylor ianlancetaylor added this to the Go1.12 milestone Jun 25, 2018
@cuonglm
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cuonglm commented Jun 25, 2018

@ianlancetaylor Can you please explain more details about current implementation? Or just give me link to linker code which cause this problem.

@ianlancetaylor
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Every global variable has memory allocated in the data section of the program. The contents of the data section are set to the initial value of the variable. For a variable initialized with a function call, the variable's value is set by an initialization function. The -X option simply changes the initial value stored in the data section, but does not affect the initialization function.

The implementation of -X is the function addstrdata in cmd/link/internal/ld/data.go, which sets the global variable strdata, and then dostrdata which calls addstrdata for each element of strdata.

@cuonglm
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cuonglm commented Jun 25, 2018

@ianlancetaylor Thanks. I went through those codes already but don't fully understand them.

/me will send the CL this morning

@gopherbot
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Change https://golang.org/cl/120935 mentions this issue: cmd/link: document limitation of -X

@golang golang locked and limited conversation to collaborators Jun 27, 2019
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Documentation FrozenDueToAge help wanted NeedsFix The path to resolution is known, but the work has not been done.
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@ianlancetaylor @cuonglm @gopherbot