syscall: add mechanism to steal the controlling terminal from a different session group on Linux

[devimc]

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

go version devel +5e79787 Mon May 22 13:44:12 2017 +0000 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/X/gopath/"
GORACE=""
GOROOT="/home/X/gopath/src/github.com/golang/go-linux-amd64-bootstrap"
GOTOOLDIR="/home/X/gopath/src/github.com/golang/go-linux-amd64-bootstrap/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build111441286=/tmp/go-build -gno-record-gcc-switches"
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"

What did you do?

cmd := exec.Command(config.Path)
cmd.Env = os.Environ()
f, err = os.OpenFile(params.Console, os.O_RDWR, 0660)
if err != nil {
	return -1, err
}
cmd.Stdin = f
cmd.Stdout = f
cmd.Stderr = f
cmd.SysProcAttr = &syscall.SysProcAttr{
	// Create Session
	Setsid: true,
	// Set Controlling terminal to Ctty
	Setctty: true,
	Ctty:    int(f.Fd()),
}
if err := cmd.Start(); err != nil {
	return -1, err
}

What did you expect to see?

A processes running using the assigned terminal

What did you see instead?

fork/exec X: operation not permitted

Solution

Allow spawned processes steal the terminal

[devimc]

proposed fix devimc@559e40c

[bradfitz]

To @ianlancetaylor for decision.

@devimc, we can't review code outside of Gerrit, which enforces that a CLA has been submitted. See https://golang.org/doc/contribute.html

But you can wait until Ian decides whether this is something we want.

[devimc]

@bradfitz great! thanks

[ianlancetaylor]

Being able to steal the controlling TTY makes sense.

I think the only thing that really needs to be decided is whether we should always steal the TTY, or whether we should have a new field in syscall.SysProcAttr saying whether to steal the TTY. My inclination is to always steal the TTY if possible--that is, always pass 1 as the argument to TIOCSCTTY.

[devimc]

I think we can't always pass 1 as the argument to TIOCSCTTY because if the process has not the capability to steal the tty (CAP_SYS_ADMIN) this call will fail, for that reason I think the best way to address this is allowing user/developer to decide when steal or not a tty

[ianlancetaylor]

As far as I can see in the Linux kernel sources, the argument to TIOCSCTTY is only checked if the tty is already the controlling terminal of some other session. Do you see something else?

[gopherbot]

CL https://golang.org/cl/44343 mentions this issue.

[devimc]

Hi @ianlancetaylor you're right, I sent a PR https://golang.org/cl/44343
thanks