Add utility function create shell command from uid and SELinux context

The shell command is normally just "sh" or "su", but sometimes you need
to switch to a specific uid or SELinux context. The Shell.SU.shell(...)
method will construct a string in this format: "su[ --context
<context>][ <uid>]". 

Currently only SuperSU v1.90 supports the context parameter, so the code
auto-detects if that is the su binary currently being used. Update the
command format for other su binaries if/when that feature becomes
available. 

To reduce the chance of issues, the uid is only passed if not 0 (root,
this is the default case), and the SELinux context is only passed if
SELinux is detected to be in enforcing mode (otherwise it may be useful
for development/logging purposes only, but not end-user)

libsuperuser/src/eu/chainfire/libsuperuser/Shell.java

@@ -17,7 +17,10 @@
 package eu.chainfire.libsuperuser;
 
 import java.io.DataOutputStream;
+import java.io.File;
+import java.io.FileInputStream;
 import java.io.IOException;
+import java.io.InputStream;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.HashMap;
@@ -337,6 +340,72 @@ public static boolean isSU(String shell) {
 
 	        return shell.equals("su");
 	    }
+
+        /**
+         * Constructs a shell command to start a su shell using the supplied
+         * uid and SELinux context. This is can be an expensive operation, 
+         * consider caching the result.
+         * 
+         * @param uid Uid to use (0 == root)
+         * @param context (SELinux) context name to use or null
+         * @return Shell command
+         */
+        public static String shell(int uid, String context) {
+            // su[ --context <context>][ <uid>]
+            String shell = "su";
+
+            // First known firmware with SELinux built-in was a 4.2 (17) leak 
+            if ((context != null) && (android.os.Build.VERSION.SDK_INT >= 17)) {
+                Boolean enforcing = null;
+
+                // Detect enforcing through sysfs, not always present
+                if (enforcing == null) {
+                    File f = new File("/sys/fs/selinux/enforce");
+                    if (f.exists()) {
+                        try {
+                            InputStream is = new FileInputStream("/sys/fs/selinux/enforce");
+                            try {
+                                enforcing = (is.read() == '1');                            
+                            } finally {
+                                is.close();
+                            }
+                        } catch (Exception e) {
+                        }
+                    }
+                }
+
+                // 4.4+ builds are enforcing by default, take the gamble
+                if (enforcing == null) {
+                    enforcing = (android.os.Build.VERSION.SDK_INT >= 19);
+                }
+
+                // Switching to a context in permissive mode is not generally 
+                // useful aside from audit testing, but we want to avoid
+                // switching the context due to the increased chance of issues. 
+                if (enforcing) {
+                    String display = version(false);
+                    String internal = version(true);
+
+                    // We only know the format for SuperSU v1.90+ right now
+                    if (
+                            (display != null) && 
+                            (internal != null) && 
+                            (display.endsWith("SUPERSU")) && 
+                            (Integer.valueOf(internal) >= 190)
+                    ) {
+                        shell = String.format(Locale.ENGLISH, "%s --context %s", shell, context);
+                    }
+                }
+            }
+
+            // Most su binaries support the "su <uid>" format, but in case
+            // they don't, lets skip it for the default 0 (root) case
+            if (uid > 0) {
+                shell = String.format(Locale.ENGLISH, "%s %d", shell, uid);
+            }
+
+            return shell;
+        }	    
 	}
 
 	/**