libcontainer: set cgroup config late

Due to the fact that the init is implemented in Go (which seemingly
randomly spawns new processes and loves eating memory), most cgroup
configurations are required to have an arbitrary minimum dictated by the
init. This confuses users and makes configuration more annoying than it
should. An example of this is pids.max, where Go spawns multiple
processes that then cause init to violate the pids cgroup constraint
before the container can even start.

Solve this problem by setting the cgroup configurations as late as
possible, to avoid hitting as many of the resources hogged by the Go
init as possible. This has to be done before seccomp rules are applied,
as the parent and child must synchronise in order for the parent to
correctly set the configurations (and writes might be blocked by seccomp).

Signed-off-by: Aleksa Sarai <asarai@suse.com>

libcontainer/process_linux.go

@@ -5,6 +5,7 @@ package libcontainer
 import (
 	"encoding/json"
 	"errors"
+	"fmt"
 	"io"
 	"os"
 	"os/exec"
@@ -86,6 +87,7 @@ func (p *setnsProcess) start() (err error) {
 	if err := json.NewEncoder(p.parentPipe).Encode(p.config); err != nil {
 		return newSystemError(err)
 	}
+
 	if err := syscall.Shutdown(int(p.parentPipe.Fd()), syscall.SHUT_WR); err != nil {
 		return newSystemError(err)
 	}
@@ -95,6 +97,7 @@ func (p *setnsProcess) start() (err error) {
 	if err := json.NewDecoder(p.parentPipe).Decode(&ierr); err != nil && err != io.EOF {
 		return newSystemError(err)
 	}
+	// Must be done after Shutdown so the child will exit and we can wait for it.
 	if ierr != nil {
 		p.wait()
 		return newSystemError(ierr)
@@ -198,15 +201,11 @@ func (p *initProcess) start() (err error) {
 		return newSystemError(err)
 	}
 	p.setExternalDescriptors(fds)
-
 	// Do this before syncing with child so that no children
 	// can escape the cgroup
 	if err := p.manager.Apply(p.pid()); err != nil {
 		return newSystemError(err)
 	}
-	if err := p.manager.Set(p.config.Config); err != nil {
-		return newSystemError(err)
-	}
 	defer func() {
 		if err != nil {
 			// TODO: should not be the responsibility to call here
@@ -232,13 +231,58 @@ func (p *initProcess) start() (err error) {
 	if err := p.sendConfig(); err != nil {
 		return newSystemError(err)
 	}
-	// wait for the child process to fully complete and receive an error message
-	// if one was encoutered
-	var ierr *genericError
-	if err := json.NewDecoder(p.parentPipe).Decode(&ierr); err != nil && err != io.EOF {
+
+	var (
+		procSync syncType
+		sentRun  bool
+		ierr     *genericError
+	)
+
+loop:
+	for {
+		if err := json.NewDecoder(p.parentPipe).Decode(&procSync); err != nil {
+			if err == io.EOF {
+				break loop
+			}
+			return newSystemError(err)
+		}
+
+		switch procSync {
+		case procStart:
+			break loop
+		case procReady:
+			if err := p.manager.Set(p.config.Config); err != nil {
+				return newSystemError(err)
+			}
+			// Sync with child.
+			if err := json.NewEncoder(p.parentPipe).Encode(procRun); err != nil {
+				return newSystemError(err)
+			}
+			sentRun = true
+		case procError:
+			// wait for the child process to fully complete and receive an error message
+			// if one was encoutered
+			if err := json.NewDecoder(p.parentPipe).Decode(&ierr); err != nil && err != io.EOF {
+				return newSystemError(err)
+			}
+			if ierr != nil {
+				break loop
+			}
+			// Programmer error.
+			panic("No error following JSON procError payload.")
+		default:
+			return newSystemError(fmt.Errorf("invalid JSON synchronisation payload from child"))
+		}
+	}
+	if !sentRun {
+		return newSystemError(fmt.Errorf("could not synchronise with container process"))
+	}
+	if err := syscall.Shutdown(int(p.parentPipe.Fd()), syscall.SHUT_WR); err != nil {
 		return newSystemError(err)
 	}
+	// Must be done after Shutdown so the child will exit and we can wait for it.
 	if ierr != nil {
+		p.wait()
 		return newSystemError(ierr)
 	}
 	return nil
@@ -276,8 +320,7 @@ func (p *initProcess) sendConfig() error {
 	if err := json.NewEncoder(p.parentPipe).Encode(p.config); err != nil {
 		return err
 	}
-	// shutdown writes for the parent side of the pipe
-	return syscall.Shutdown(int(p.parentPipe.Fd()), syscall.SHUT_WR)
+	return nil
 }
 
 func (p *initProcess) createNetworkInterfaces() error {