plugin: "unexpected fault address 0x0" with 8+ exported functions

[fsenart]

Please answer these questions before submitting your issue. Thanks!

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

go version go1.8 linux/amd64

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

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/fsenart/projects"
GORACE=""
GOROOT="/home/fsenart/tools/go1.8"
GOTOOLDIR="/home/fsenart/tools/go1.8/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build519169107=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
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?

If possible, provide a recipe for reproducing the error.
A complete runnable program is good.
A link on play.golang.org is best.

// host.go
package main

import (
	"plugin"
)

func main() {
	p, err := plugin.Open("guest.so")
	if err != nil {
		panic(err)
	}

	f, err := p.Lookup("F")
	if err != nil {
		panic(err)
	}

	f.(func())()
}

// guest.go
package main

func F() {}

func F1() {}
func F2() {}
func F3() {}
func F4() {}
func F5() {}
func F6() {}
func F7() {}

run: build
	@for i in `seq 1 100`; do \
          ./host; \
          if [ $$? -eq 2 ] ; then \
	    echo -e "\n\n>>> Fails after $$i runs"; \
	    exit 1; \
	  fi \
	done

build:
	@go build -o host host.go
	@go build -buildmode=plugin -o guest.so guest.go

What did you expect to see?

I don't expect to see any error to happen.

What did you see instead?

Randomly, after a few runs, I encounter a fatal error.

unexpected fault address 0x0
fatal error: fault
[signal SIGSEGV: segmentation violation code=0x80 addr=0x0 pc=0x4911e4]

goroutine 1 [running]:
runtime.throw(0x4af7bd, 0x5)
	/home/fsenart/tools/go1.8/src/runtime/panic.go:596 +0x95 fp=0xc42004deb0 sp=0xc42004de90
runtime.sigpanic()
	/home/fsenart/tools/go1.8/src/runtime/signal_unix.go:297 +0x28c fp=0xc42004df00 sp=0xc42004deb0
main.main()
	/home/fsenart/projects/src/issue/host.go:18 +0xb4 fp=0xc42004df88 sp=0xc42004df00
runtime.main()
	/home/fsenart/tools/go1.8/src/runtime/proc.go:185 +0x20a fp=0xc42004dfe0 sp=0xc42004df88
runtime.goexit()
	/home/fsenart/tools/go1.8/src/runtime/asm_amd64.s:2197 +0x1 fp=0xc42004dfe8 sp=0xc42004dfe0

goroutine 17 [syscall, locked to thread]:
runtime.goexit()
	/home/fsenart/tools/go1.8/src/runtime/asm_amd64.s:2197 +0x1


>>> Fails after 6 runs
Makefile:2: recipe for target 'run' failed
make: *** [run] Error 1

Observations

• The number of exported functions in the plugin is very important.
• If I export less than 8 functions no error happens anymore.

[ianlancetaylor]

CC @crawshaw

[gopherbot]

CL https://golang.org/cl/40431 mentions this issue.