cmd/link: -a doesn't work with external linkmode

[Helflym]

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

$ go version
go version devel +c077c74312 Fri Jan 18 03:50:05 2019 +0000 linux/amd64

Does this issue reproduce with the latest release?

Yes, with go1.10.4 linux/amd64

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

go env Output

$ go env
GOARCH="amd64"
GOBIN=""
GOCACHE="/home/chigotc/.cache/go-build"
GOEXE=""
GOFLAGS=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/chigotc/go/gopath"
GOPROXY=""
GORACE=""
GOROOT="/home/chigotc/go/goroot"
GOTMPDIR=""
GOTOOLDIR="/home/chigotc/go/goroot/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
CXX="g++"
CGO_ENABLED="1"
GOMOD=""
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-build080404221=/tmp/go-build -gno-record-gcc-switches"

What did you do?

go build -ldflags='-a -linkmode=external' helloworld.go

What did you see instead?

...
type.runtime.gclinkptr
	0045dd60| 08 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
	0045dd70| a8 65 c5 1a 07 08 08 8c 00 00 00 00 00 00 00 00
	0045dd80| 00 00 00 00 00 00 00 00 ff 4b 00 00 00 e8 00 00
	0045dd90| 68 10 00 00 01 00 00 00 10 00 00 00 00 00 00 00
	0045dda0| fe 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00
	reloc 0045dd78/8 addr runtime.algarray+0x50 [0x4bff30]
	reloc 0045dd80/8 addr runtime.gcbits.+0x0 [0x47b598]
	reloc 0045dd88/4 ? type..namedata.*runtime.gclinkptr-+0x0 [0x4546df]
	reloc 0045dd8c/4 ? type.*runtime.gclinkptr+0x0 [0x45e2e0]
	reloc 0045dd90/4 ? type..importpath.runtime.+0x0 [0x450b48]
	reloc 0045dda0/4 ? type..namedata.ptr-+0x0 [0x44fcde]
panic: runtime error: invalid memory address or nil pointer dereference
[signal SIGSEGV: segmentation violation code=0x1 addr=0x28 pc=0x54a9ef]

goroutine 1 [running]:
cmd/link/internal/ld.Datblk(0xc0006be000, 0x45ddb0, 0x2eb27)
	/home/chigotc/go/goroot/src/cmd/link/internal/ld/data.go:839 +0x5ff
cmd/link/internal/amd64.asmb(0xc0006be000)
	/home/chigotc/go/goroot/src/cmd/link/internal/amd64/asm.go:678 +0x196
cmd/link/internal/ld.Main(0x82ad20, 0x10, 0x20, 0x1, 0x7, 0x10, 0x67739a, 0x1b, 0x673f73, 0x14, ...)
	/home/chigotc/go/goroot/src/cmd/link/internal/ld/main.go:244 +0xd36
main.main()
	/home/chigotc/go/goroot/src/cmd/link/main.go:65 +0x1d0

This error comes from data.go where r.Sym.Value should use the same way r.Sym.Name is done.

However, while doing it locally, I've found that these relocations are quite unusual:

type.runtime.gclinkptr
        0045dd60| 08 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
        0045dd70| a8 65 c5 1a 07 08 08 8c 00 00 00 00 00 00 00 00
        0045dd80| 00 00 00 00 00 00 00 00 ff 4b 00 00 00 e8 00 00
        0045dd90| 68 10 00 00 01 00 00 00 10 00 00 00 00 00 00 00
        0045dda0| fe 01 00 00 00 00 00 00 00 00 00 00 00 00 00 00
        reloc 0045dd78/8 addr runtime.algarray+0x50 [0x4bff30]
        reloc 0045dd80/8 addr runtime.gcbits.+0x0 [0x47b598]
        reloc 0045dd88/4 ? type..namedata.*runtime.gclinkptr-+0x0 [0x4546df]
        reloc 0045dd8c/4 ? type.*runtime.gclinkptr+0x0 [0x45e2e0]
        reloc 0045dd90/4 ? type..importpath.runtime.+0x0 [0x450b48]
        reloc 0045dda0/4 ? type..namedata.ptr-+0x0 [0x44fcde]
        reloc 0045dda4/0 ? +0x0 [-0x1]
        reloc 0045dda8/0 ? +0x0 [-0x1]
        reloc 0045ddac/0 ? +0x0 [-0x1]
        0045ddb0| 00 ...

Does anyone know why these relocations are generated in a first instance ? I didn't manage to find where they come from.

[ianlancetaylor]

That looks like a type descriptor. The relocs are being generated by dcommontype in cmd/compile/internal/gc/reflect.go.

[gopherbot]

Change https://golang.org/cl/158998 mentions this issue: cmd/link: fix -a with external linker