libcontainer: remove dependency on libapparmor

libapparmor is integrated in libcontainer using cgo but is only used to
call a single function: aa_change_onexec. It turns out this function is
simple enough (writing a string to a file in /proc/<n>/attr/...) to be
re-implemented locally in libcontainer in plain Go.

This allows to drop the dependency on libapparmor and the corresponding
cgo integration.

Fixes #1674

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>

libcontainer/apparmor/apparmor.go

@@ -2,15 +2,10 @@
 
 package apparmor
 
-// #cgo LDFLAGS: -lapparmor
-// #include <sys/apparmor.h>
-// #include <stdlib.h>
-import "C"
 import (
 	"fmt"
 	"io/ioutil"
 	"os"
-	"unsafe"
 )
 
 // IsEnabled returns true if apparmor is enabled for the host.
@@ -24,16 +19,36 @@ func IsEnabled() bool {
 	return false
 }
 
+func setprocattr(attr, value string) error {
+	// Under AppArmor you can only change your own attr, so use /proc/self/
+	// instead of /proc/<tid>/ like libapparmor does
+	path := fmt.Sprintf("/proc/self/attr/%s", attr)
+
+	f, err := os.OpenFile(path, os.O_WRONLY, 0)
+	if err != nil {
+		return err
+	}
+	defer f.Close()
+
+	_, err = fmt.Fprintf(f, "%s", value)
+	return err
+}
+
+// changeOnExec reimplements aa_change_onexec from libapparmor in Go
+func changeOnExec(name string) error {
+	value := "exec " + name
+	if err := setprocattr("exec", value); err != nil {
+		return fmt.Errorf("apparmor failed to apply profile: %s", err)
+	}
+	return nil
+}
+
 // ApplyProfile will apply the profile with the specified name to the process after
 // the next exec.
 func ApplyProfile(name string) error {
 	if name == "" {
 		return nil
 	}
-	cName := C.CString(name)
-	defer C.free(unsafe.Pointer(cName))
-	if _, err := C.aa_change_onexec(cName); err != nil {
-		return fmt.Errorf("apparmor failed to apply profile: %s", err)
-	}
-	return nil
+
+	return changeOnExec(name)
 }