[Metalava] Add "strict input" mode am: 1725ea84ae

Original change: https://googleplex-android-review.googlesource.com/c/platform/tools/metalava/+/11832628

Change-Id: Iff527da0b6db035891f40da86728b13102ec2233

5 files changed, 687 insertions, 31 deletions

diff --git a/src/main/java/com/android/tools/metalava/Driver.kt b/src/main/java/com/android/tools/metalava/Driver.kt
index 8de2bc6..9626d7f 100644
--- a/src/main/java/com/android/tools/metalava/Driver.kt
+++ b/src/main/java/com/android/tools/metalava/Driver.kt
@@ -86,6 +86,8 @@ fun main(args: Array<String>) {
     run(args, setExitCode = true)
 }
 
+internal var hasFileReadViolations = false
+
 /**
  * The metadata driver is a command line interface to extracting various metadata
  * from a source tree (or existing signature files etc). Run with --help to see
@@ -111,11 +113,17 @@ fun run(
         compatibility = Compatibility(compat = Options.useCompatMode(modifiedArgs))
         options = Options(modifiedArgs, stdout, stderr)
 
+        maybeActivateSandbox()
+
         processFlags()
 
         if (options.allReporters.any { it.hasErrors() } && !options.passBaselineUpdates) {
             exitCode = -1
         }
+        if (hasFileReadViolations) {
+            stderr.println("$PROGRAM_NAME detected access to files that are not explicitly specified. See ${options.strictInputViolationsFile} for details.")
+            exitCode = -1
+        }
     } catch (e: DriverException) {
         stdout.flush()
         stderr.flush()
@@ -143,6 +151,7 @@ fun run(
     }
 
     options.reportEvenIfSuppressedWriter?.close()
+    options.strictInputViolationsPrintWriter?.close()
 
     // Show failure messages, if any.
     options.allReporters.forEach {
@@ -168,6 +177,35 @@ private fun exit(exitCode: Int = 0) {
     System.exit(exitCode)
 }
 
+private fun maybeActivateSandbox() {
+    // Set up a sandbox to detect access to files that are not explicitly specified.
+    if (options.strictInputFiles == Options.StrictInputFileMode.PERMISSIVE) {
+        return
+    }
+
+    val writer = options.strictInputViolationsPrintWriter!!
+
+    // Writes all violations to [Options.strictInputFiles].
+    // Note violation reads on directories are logged, but is considered to be a "warning" and
+    // doesn't affect the exit code. See [FileReadSandbox] for the details.
+    FileReadSandbox.activate(object : FileReadSandbox.Listener {
+        var seen = mutableSetOf<String>()
+        override fun onViolation(absolutePath: String, isDirectory: Boolean) {
+            if (!seen.contains(absolutePath)) {
+                val suffix = if (isDirectory) "/" else ""
+                writer.println("Sandbox violation: $absolutePath$suffix")
+                if (options.strictInputFiles == Options.StrictInputFileMode.STRICT_WITH_STACK) {
+                    Throwable().printStackTrace(writer)
+                }
+                seen.add(absolutePath)
+                if (!isDirectory) {
+                    hasFileReadViolations = true
+                }
+            }
+        }
+    })
+}
+
 private fun processFlags() {
     val stopwatch = Stopwatch.createStarted()
 
@@ -863,10 +901,13 @@ internal fun parseSources(
     val joined = mutableListOf<File>()
     joined.addAll(sourcePath.mapNotNull { if (it.path.isNotBlank()) it.absoluteFile else null })
     joined.addAll(classpath.map { it.absoluteFile })
+
     // Add in source roots implied by the source files
     val sourceRoots = mutableListOf<File>()
-    extractRoots(sources, sourceRoots)
-    joined.addAll(sourceRoots)
+    if (options.allowImplicitRoot) {
+        extractRoots(sources, sourceRoots)
+        joined.addAll(sourceRoots)
+    }
 
     // Create project environment with those paths
     projectEnvironment.registerPaths(joined)
diff --git a/src/main/java/com/android/tools/metalava/FileReadSandbox.kt b/src/main/java/com/android/tools/metalava/FileReadSandbox.kt
new file mode 100644
index 0000000..02bbae2
--- /dev/null
+++ b/src/main/java/com/android/tools/metalava/FileReadSandbox.kt
@@ -0,0 +1,315 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.android.tools.metalava
+
+import java.io.File
+import java.io.FileDescriptor
+import java.net.InetAddress
+import java.security.Permission
+import kotlin.concurrent.getOrSet
+
+/**
+ * Detect access to files that not explicitly specified in the command line.
+ *
+ * This class detects reads on both files and directories. Directory accesses are logged by
+ * [Driver], which only logs it, but doesn't consider it an error.
+ *
+ * We do not prevent reads on directories that are not explicitly listed in the command line because
+ * metalava (or JVM, really) seems to read some system directories such as /usr/, etc., but this
+ * behavior may be JVM dependent so we do not want to have to explicitly whitelist them.
+ * (Because, otherwise, when we update JVM, it may access different directories and we end up
+ * having to update the implicit whitelist.) As long as we don't read files, reading directories
+ * shouldn't (normally) affect the result, so we simply allow any directory reads.
+ */
+internal object FileReadSandbox {
+    private var installed = false
+
+    private var previousSecurityManager: SecurityManager? = null
+    private val mySecurityManager = MySecurityManager()
+
+    /**
+     * Set of paths that are allowed to access. This is used with an exact match.
+     */
+    private var allowedPaths = mutableSetOf<String>()
+
+    /**
+     * Set of path prefixes that are allowed to access.
+     */
+    private var allowedPathPrefixes = mutableSetOf<String>()
+
+    interface Listener {
+        /** Called when a violation is detected with the absolute path. */
+        fun onViolation(absolutePath: String, isDirectory: Boolean)
+    }
+
+    private var listener: Listener? = null
+
+    init {
+        initialize()
+    }
+
+    private fun initialize() {
+        allowedPaths.clear()
+        allowedPathPrefixes.clear()
+        listener = null
+
+        // Allow certain directories by default.
+        // The sandbox detects all file accessed done by JVM implicitly too (e.g. JVM seems to
+        // access /dev/urandom on linux), so we need to allow them.
+        // At least on linux, there doesn't seem to be any access to files under /proc/ or /sys/,
+        // but they should be allowed anyway.
+        listOf("/dev", "/proc", "/sys").forEach {
+            allowAccess(File(it))
+        }
+        // Allow access to the directory containing the metalava's jar itself.
+        // (so even if metalava loads other jar files in the same directory, that wouldn't be a
+        // violation.)
+        FileReadSandbox::class.java.protectionDomain?.codeSource?.location?.toURI()?.path?.let {
+            allowAccess(File(it).parentFile)
+        }
+
+        // Allow access to $JAVA_HOME.
+        // We also allow $ANDROID_JAVA_HOME, which is used in the Android platform build.
+        // (which is normally $ANDROID_BUILD_TOP + "/prebuilts/jdk/jdk11/linux-x86" as of writing.)
+        listOf(
+            "JAVA_HOME",
+            "ANDROID_JAVA_HOME"
+        ).forEach {
+            System.getenv(it)?.let {
+                allowAccess(File(it))
+            }
+        }
+        // JVM seems to use ~/.cache/
+        System.getenv("HOME")?.let {
+            allowAccess(File("$it/.cache"))
+        }
+    }
+
+    /** Activate the sandbox. */
+    fun activate(listener: Listener) {
+        if (installed) {
+            throw IllegalStateException("Already activated")
+        }
+        previousSecurityManager = System.getSecurityManager()
+        System.setSecurityManager(mySecurityManager)
+        installed = true
+        this.listener = listener
+    }
+
+    /** Deactivate the sandbox. This also resets [violationCount]. */
+    fun deactivate() {
+        if (!installed) {
+            throw IllegalStateException("Not activated")
+        }
+        System.setSecurityManager(previousSecurityManager)
+        previousSecurityManager = null
+
+        installed = false
+    }
+
+    /** Reset all the state and re-initialize the sandbox. Only callable when not activated. */
+    fun reset() {
+        if (installed) {
+            throw IllegalStateException("Can't reset. Already activated")
+        }
+        initialize()
+    }
+
+    /** Allows access to files or directories. */
+    fun allowAccess(files: List<File>): List<File> {
+        files.forEach { allowAccess(it) }
+        return files
+    }
+
+    /** Allows access to a file or directory. */
+    fun <T : File?> allowAccess(file: T): T {
+        if (file == null) {
+            return file
+        }
+        val path = file.absolutePath
+        val added = allowedPaths.add(path)
+        if (file.isDirectory) {
+            // Even if it had already been added to allowedPaths (i.e. added == false),
+            // the directory might have just been created, so we still do it.
+            allowedPathPrefixes.add("$path/")
+        }
+        if (!added) {
+            // Already added; bail early.
+            return file
+        }
+
+        // Whitelist all parent directories. But don't allow prefix accesses (== access to the
+        // directory itself is okay, but don't grant access to any files/directories under it).
+        var parent = file.parentFile
+        while (true) {
+            if (parent == null) {
+                break
+            }
+            @Suppress("NAME_SHADOWING")
+            val path = parent.absolutePath
+            if (!allowedPaths.add(path)) {
+                break // already added.
+            }
+            if (path == "/") {
+                break
+            }
+            parent = parent.parentFile
+        }
+        return file
+    }
+
+    fun isAccessAllowed(file: File): Boolean {
+        val absPath = file.absolutePath
+
+        if (allowedPaths.contains(absPath)) {
+            return true
+        }
+        allowedPathPrefixes.forEach {
+            if (absPath.startsWith(it)) {
+                return true
+            }
+        }
+        return false
+    }
+
+    /** Used to skip all checks on any filesystem access made within the [check] method. */
+    private val temporaryExempt = ThreadLocal<Boolean>()
+
+    private fun check(path: String) {
+        if (temporaryExempt.getOrSet { false }) {
+            return
+        }
+
+        try {
+            temporaryExempt.set(true)
+
+            val file = File(path)
+
+            if (!isAccessAllowed(file)) {
+                listener?.onViolation(file.absolutePath, file.isDirectory)
+            }
+        } finally {
+            temporaryExempt.set(false)
+        }
+    }
+
+    /**
+     * Reading files that are created by metalava should be allowed, so we detect file writes to
+     * new files, and whitelist it.
+     */
+    private fun writeDetected(origPath: String?) {
+        if (temporaryExempt.getOrSet { false }) {
+            return
+        }
+
+        try {
+            temporaryExempt.set(true)
+
+            val file = File(origPath)
+            if (file.exists()) {
+                return // Ignore writes to an existing file / directory.
+            }
+            allowedPaths.add(file.absolutePath)
+        } finally {
+            temporaryExempt.set(false)
+        }
+    }
+
+    private class MySecurityManager : SecurityManager() {
+        override fun checkRead(file: String) {
+            check(file)
+        }
+
+        override fun checkRead(file: String, context: Any?) {
+            check(file)
+        }
+        override fun checkRead(p0: FileDescriptor?) {
+        }
+
+        override fun checkDelete(p0: String?) {
+        }
+
+        override fun checkPropertiesAccess() {
+        }
+
+        override fun checkAccess(p0: Thread?) {
+        }
+
+        override fun checkAccess(p0: ThreadGroup?) {
+        }
+
+        override fun checkExec(p0: String?) {
+        }
+
+        override fun checkListen(p0: Int) {
+        }
+
+        override fun checkExit(p0: Int) {
+        }
+
+        override fun checkLink(p0: String?) {
+        }
+
+        override fun checkPropertyAccess(p0: String?) {
+        }
+
+        override fun checkPackageDefinition(p0: String?) {
+        }
+
+        override fun checkMulticast(p0: InetAddress?) {
+        }
+
+        override fun checkMulticast(p0: InetAddress?, p1: Byte) {
+        }
+
+        override fun checkPermission(p0: Permission?) {
+        }
+
+        override fun checkPermission(p0: Permission?, p1: Any?) {
+        }
+
+        override fun checkPackageAccess(p0: String?) {
+        }
+
+        override fun checkAccept(p0: String?, p1: Int) {
+        }
+
+        override fun checkSecurityAccess(p0: String?) {
+        }
+
+        override fun checkWrite(p0: FileDescriptor?) {
+        }
+
+        override fun checkWrite(file: String?) {
+            writeDetected(file)
+        }
+
+        override fun checkPrintJobAccess() {
+        }
+
+        override fun checkCreateClassLoader() {
+        }
+
+        override fun checkConnect(p0: String?, p1: Int) {
+        }
+
+        override fun checkConnect(p0: String?, p1: Int, p2: Any?) {
+        }
+
+        override fun checkSetFactory() {
+        }
+    }
+}
\ No newline at end of file
diff --git a/src/main/java/com/android/tools/metalava/Options.kt b/src/main/java/com/android/tools/metalava/Options.kt
index 5ffa7ce..7e0a888 100644
--- a/src/main/java/com/android/tools/metalava/Options.kt
+++ b/src/main/java/com/android/tools/metalava/Options.kt
@@ -166,6 +166,10 @@ const val ARG_IGNORE_CLASSES_ON_CLASSPATH = "--ignore-classes-on-classpath"
 const val ARG_ERROR_MESSAGE_API_LINT = "--error-message:api-lint"
 const val ARG_ERROR_MESSAGE_CHECK_COMPATIBILITY_RELEASED = "--error-message:compatibility:released"
 const val ARG_ERROR_MESSAGE_CHECK_COMPATIBILITY_CURRENT = "--error-message:compatibility:current"
+const val ARG_NO_IMPLICIT_ROOT = "--no-implicit-root"
+const val ARG_STRICT_INPUT_FILES = "--strict-input-files"
+const val ARG_STRICT_INPUT_FILES_STACK = "--strict-input-files:stack"
+const val ARG_STRICT_INPUT_FILES_EXEMPT = "--strict-input-files-exempt"
 
 class Options(
     private val args: Array<String>,
@@ -657,6 +661,34 @@ class Options(
     /** How to handle typedef annotations in signature files; corresponds to $ARG_TYPEDEFS_IN_SIGNATURES */
     var typedefMode = TypedefMode.NONE
 
+    /** Allow implicit root detection (which is the default behavior). See [ARG_NO_IMPLICIT_ROOT] */
+    var allowImplicitRoot = true
+
+    enum class StrictInputFileMode {
+        PERMISSIVE,
+        STRICT,
+        STRICT_WITH_STACK;
+
+        companion object {
+            fun fromArgument(arg: String): StrictInputFileMode {
+                return when (arg) {
+                    ARG_STRICT_INPUT_FILES -> STRICT
+                    ARG_STRICT_INPUT_FILES_STACK -> STRICT_WITH_STACK
+                    else -> PERMISSIVE
+                }
+            }
+        }
+    }
+
+    /**
+     * Whether we should allow metalava to read files that are not explicitly specified in the
+     * command line. See [ARG_STRICT_INPUT_FILES] and [ARG_STRICT_INPUT_FILES_STACK]
+     */
+    var strictInputFiles = StrictInputFileMode.PERMISSIVE
+
+    var strictInputViolationsFile: File? = null
+    var strictInputViolationsPrintWriter: PrintWriter? = null
+
     /** File conversion tasks */
     data class ConvertFile(
         val fromApiFile: File,
@@ -974,7 +1006,7 @@ class Options(
                     if (index < args.size - 1) {
                         val nextArg = args[index + 1]
                         if (!nextArg.startsWith("-")) {
-                            val file = fileForPath(nextArg)
+                            val file = stringToExistingFile(nextArg)
                             if (file.isFile) {
                                 index++
                                 migrateNullsFrom = file
@@ -1005,7 +1037,7 @@ class Options(
                     if (index < args.size - 1) {
                         val nextArg = args[index + 1]
                         if (!nextArg.startsWith("-")) {
-                            val file = fileForPath(nextArg)
+                            val file = stringToExistingFile(nextArg)
                             if (file.isFile) {
                                 index++
                                 mutableCompatibilityChecks.add(CheckRequest(file, ApiType.PUBLIC_API, ReleaseType.DEV))
@@ -1297,6 +1329,28 @@ class Options(
                     }
                 }
 
+                ARG_NO_IMPLICIT_ROOT -> {
+                    allowImplicitRoot = false
+                }
+
+                ARG_STRICT_INPUT_FILES, ARG_STRICT_INPUT_FILES_STACK -> {
+                    if (strictInputViolationsFile != null) {
+                        throw DriverException("$ARG_STRICT_INPUT_FILES and $ARG_STRICT_INPUT_FILES_STACK may be specified only once")
+                    }
+                    strictInputFiles = StrictInputFileMode.fromArgument(arg)
+
+                    val file = stringToNewOrExistingFile(getValue(args, ++index))
+                    strictInputViolationsFile = file
+                    strictInputViolationsPrintWriter = file.printWriter()
+                }
+                ARG_STRICT_INPUT_FILES_EXEMPT -> {
+                    val listString = getValue(args, ++index)
+                    listString.split(File.pathSeparatorChar).forEach { path ->
+                        // Throw away the result; just let the function add the files to the whitelist.
+                        stringToExistingFilesOrDirs(path)
+                    }
+                }
+
                 "--temp-folder" -> {
                     tempFolder = stringToNewOrExistingDir(getValue(args, ++index))
                 }
@@ -1689,6 +1743,10 @@ class Options(
      * Produce a default file name for the baseline. It's normally "baseline.txt", but can
      * be prefixed by show annotations; e.g. @TestApi -> test-baseline.txt, @SystemApi -> system-baseline.txt,
      * etc.
+     *
+     * Note because the default baseline file is not explicitly set in the command line,
+     * this file would trigger a --strict-input-files violation. To avoid that, always explicitly
+     * pass a baseline file.
      */
     private fun getDefaultBaselineFile(): File? {
         if (sourcePath.isNotEmpty() && sourcePath[0].path.isNotBlank()) {
@@ -1712,6 +1770,13 @@ class Options(
         }
     }
 
+    /**
+     * Find an android stub jar that matches the given criteria.
+     *
+     * Note because the default baseline file is not explicitly set in the command line,
+     * this file would trigger a --strict-input-files violation. To avoid that, use
+     * --strict-input-files-exempt to exempt the jar directory.
+     */
     private fun findAndroidJars(
         androidJarPatterns: List<String>,
         currentApiLevel: Int,
@@ -1774,8 +1839,9 @@ class Options(
     }
 
     private fun getAndroidJarFile(apiLevel: Int, patterns: List<String>): File? {
+        // Note this method doesn't register the result to [FileReadSandbox]
         return patterns
-            .map { fileForPath(it.replace("%", Integer.toString(apiLevel))) }
+            .map { fileForPathInner(it.replace("%", Integer.toString(apiLevel))) }
             .firstOrNull { it.isFile }
     }
 
@@ -1854,78 +1920,86 @@ class Options(
     }
 
     private fun stringToExistingDir(value: String): File {
-        val file = fileForPath(value)
+        val file = fileForPathInner(value)
         if (!file.isDirectory) {
             throw DriverException("$file is not a directory")
         }
-        return file
+        return FileReadSandbox.allowAccess(file)
     }
 
     @Suppress("unused")
     private fun stringToExistingDirs(value: String): List<File> {
         val files = mutableListOf<File>()
         for (path in value.split(File.pathSeparatorChar)) {
-            val file = fileForPath(path)
+            val file = fileForPathInner(path)
             if (!file.isDirectory) {
                 throw DriverException("$file is not a directory")
             }
             files.add(file)
         }
-        return files
+        return FileReadSandbox.allowAccess(files)
     }
 
     private fun stringToExistingDirsOrJars(value: String): List<File> {
         val files = mutableListOf<File>()
         for (path in value.split(File.pathSeparatorChar)) {
-            val file = fileForPath(path)
+            val file = fileForPathInner(path)
             if (!file.isDirectory && !(file.path.endsWith(SdkConstants.DOT_JAR) && file.isFile)) {
                 throw DriverException("$file is not a jar or directory")
             }
             files.add(file)
         }
-        return files
+        return FileReadSandbox.allowAccess(files)
     }
 
     private fun stringToExistingDirsOrFiles(value: String): List<File> {
         val files = mutableListOf<File>()
         for (path in value.split(File.pathSeparatorChar)) {
-            val file = fileForPath(path)
+            val file = fileForPathInner(path)
             if (!file.exists()) {
                 throw DriverException("$file does not exist")
             }
             files.add(file)
         }
-        return files
+        return FileReadSandbox.allowAccess(files)
     }
 
     private fun stringToExistingFile(value: String): File {
-        val file = fileForPath(value)
+        val file = fileForPathInner(value)
         if (!file.isFile) {
             throw DriverException("$file is not a file")
         }
-        return file
+        return FileReadSandbox.allowAccess(file)
     }
 
     @Suppress("unused")
     private fun stringToExistingFileOrDir(value: String): File {
-        val file = fileForPath(value)
+        val file = fileForPathInner(value)
         if (!file.exists()) {
             throw DriverException("$file is not a file or directory")
         }
-        return file
+        return FileReadSandbox.allowAccess(file)
     }
 
     private fun stringToExistingFiles(value: String): List<File> {
+        return stringToExistingFilesOrDirsInternal(value, false)
+    }
+
+    private fun stringToExistingFilesOrDirs(value: String): List<File> {
+        return stringToExistingFilesOrDirsInternal(value, true)
+    }
+
+    private fun stringToExistingFilesOrDirsInternal(value: String, allowDirs: Boolean): List<File> {
         val files = mutableListOf<File>()
         value.split(File.pathSeparatorChar)
-            .map { fileForPath(it) }
+            .map { fileForPathInner(it) }
             .forEach { file ->
                 if (file.path.startsWith("@")) {
                     // File list; files to be read are stored inside. SHOULD have been one per line
                     // but sadly often uses spaces for separation too (so we split by whitespace,
                     // which means you can't point to files in paths with spaces)
                     val listFile = File(file.path.substring(1))
-                    if (!listFile.isFile) {
+                    if (!allowDirs && !listFile.isFile) {
                         throw DriverException("$listFile is not a file")
                     }
                     val contents = Files.asCharSource(listFile, UTF_8).read()
@@ -1933,23 +2007,23 @@ class Options(
                         contents
                     )
                     pathList.asSequence().map { File(it) }.forEach {
-                        if (!it.isFile) {
+                        if (!allowDirs && !it.isFile) {
                             throw DriverException("$it is not a file")
                         }
                         files.add(it)
                     }
                 } else {
-                    if (!file.isFile) {
+                    if (!allowDirs && !file.isFile) {
                         throw DriverException("$file is not a file")
                     }
                     files.add(file)
                 }
             }
-        return files
+        return FileReadSandbox.allowAccess(files)
     }
 
     private fun stringToNewFile(value: String): File {
-        val output = fileForPath(value)
+        val output = fileForPathInner(value)
 
         if (output.exists()) {
             if (output.isDirectory) {
@@ -1966,22 +2040,22 @@ class Options(
             }
         }
 
-        return output
+        return FileReadSandbox.allowAccess(output)
     }
 
     private fun stringToNewOrExistingDir(value: String): File {
-        val dir = fileForPath(value)
+        val dir = fileForPathInner(value)
         if (!dir.isDirectory) {
             val ok = dir.mkdirs()
             if (!ok) {
                 throw DriverException("Could not create $dir")
             }
         }
-        return dir
+        return FileReadSandbox.allowAccess(dir)
     }
 
     private fun stringToNewOrExistingFile(value: String): File {
-        val file = fileForPath(value)
+        val file = fileForPathInner(value)
         if (!file.exists()) {
             val parentFile = file.parentFile
             if (parentFile != null && !parentFile.isDirectory) {
@@ -1991,11 +2065,11 @@ class Options(
                 }
             }
         }
-        return file
+        return FileReadSandbox.allowAccess(file)
     }
 
     private fun stringToNewDir(value: String): File {
-        val output = fileForPath(value)
+        val output = fileForPathInner(value)
         val ok =
             if (output.exists()) {
                 if (output.isDirectory) {
@@ -2013,10 +2087,19 @@ class Options(
             throw DriverException("Could not create $output")
         }
 
-        return output
+        return FileReadSandbox.allowAccess(output)
     }
 
-    private fun fileForPath(path: String): File {
+    /**
+     * Converts a path to a [File] that represents the absolute path, with the following special
+     * behavior:
+     * - "~" will be expanded into the home directory path.
+     * - If the given path starts with "@", it'll be converted into "@" + [file's absolute path]
+     *
+     * Note, unlike the other "stringToXxx" methods, this method won't register the given path
+     * to [FileReadSandbox].
+     */
+    private fun fileForPathInner(path: String): File {
         // java.io.File doesn't automatically handle ~/ -> home directory expansion.
         // This isn't necessary when metalava is run via the command line driver
         // (since shells will perform this expansion) but when metalava is run
@@ -2310,6 +2393,18 @@ class Options(
             ARG_CURRENT_CODENAME, "Sets the code name for the current source code",
             ARG_CURRENT_JAR, "Points to the current API jar, if any",
 
+            "", "\nSandboxing:",
+            ARG_NO_IMPLICIT_ROOT, "Disable implicit root directory detection. " +
+                "Otherwise, $PROGRAM_NAME adds in source roots implied by the source files",
+            "$ARG_STRICT_INPUT_FILES <file>", "Do not read files that are not explicitly specified in the command line. " +
+                "All violations are written to the given file. Reads on directories are always allowed, but " +
+                "$PROGRAM_NAME still trackes reads on directories that are not specified in the command line, " +
+                "and write them to the file.",
+            "$ARG_STRICT_INPUT_FILES_STACK <file>", "Same as $ARG_STRICT_INPUT_FILES but also print stacktraces.",
+            "$ARG_STRICT_INPUT_FILES_EXEMPT <files or dirs>", "Used with $ARG_STRICT_INPUT_FILES. Explicitly allow " +
+                "access to files and/or directories (separated by `${File.pathSeparator}). Can also be " +
+                "@ followed by a path to a text file containing paths to the full set of files and/or directories.",
+
             "", "\nEnvironment Variables:",
             ENV_VAR_METALAVA_DUMP_ARGV, "Set to true to have metalava emit all the arguments it was invoked with. " +
                 "Helpful when debugging or reproducing under a debugger what the build system is doing.",
diff --git a/src/test/java/com/android/tools/metalava/FileReadSandboxTest.kt b/src/test/java/com/android/tools/metalava/FileReadSandboxTest.kt
new file mode 100644
index 0000000..8341dfd
--- /dev/null
+++ b/src/test/java/com/android/tools/metalava/FileReadSandboxTest.kt
@@ -0,0 +1,159 @@
+/*
+ * Copyright (C) 2020 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.android.tools.metalava
+
+import org.junit.Rule
+import org.junit.Test
+import org.junit.rules.TemporaryFolder
+import java.io.File
+import kotlin.test.assertEquals
+import kotlin.test.assertTrue
+
+class FileReadSandboxTest {
+    @get:Rule
+    var temporaryFolder = TemporaryFolder()
+
+    @Test
+    fun `Test sandbox`() {
+        // Create target files.
+        val root = temporaryFolder.newFolder()
+
+        val goodFile = File(root, "goodFile").apply { createNewFile() }
+        val badFile = File(root, "badFile").apply { createNewFile() }
+
+        val goodDir = File(root, "goodDir").apply { mkdirs() }
+        val goodDirFile = File(goodDir, "file").apply { createNewFile() }
+
+        val badDir = File(root, "badDir").apply { mkdirs() }
+        val badDirFile = File(badDir, "file").apply { createNewFile() }
+
+        val subDir = File(root, "subdir").apply { mkdirs() }
+        val subSubDir = File(subDir, "subdir").apply { mkdirs() }
+        val subSubDir2 = File(subDir, "subdir2").apply { mkdirs() }
+        val subSubDirGoodFile = File(subSubDir, "good").apply { createNewFile() }
+        val subSubDirBadFile = File(subSubDir, "bad").apply { createNewFile() }
+
+        // Structure:
+        //   - *  Explicitly allowed
+        //   - ** Implicitly allowed (parent dirctories of whitelisted paths)
+        //   - @  Not allowed
+        // root        **
+        //  |-goodFile *
+        //  |
+        //  |-badFile  @
+        //  |
+        //  |-goodDir  *
+        //  |  |-goodDirFile **
+        //  |
+        //  |-badDir         @
+        //  |  |-badDirFile  @
+        //  |
+        //  |-subDir **
+        //     |-subSubDir **
+        //     |  |-subSubDirGoodFile *
+        //     |  |-subSubDirBadFile  @
+        //     |-subSubDir2           @
+
+        // Set up the sandbox.
+        FileReadSandbox.allowAccess(goodFile)
+        FileReadSandbox.allowAccess(goodDir)
+        FileReadSandbox.allowAccess(subSubDirGoodFile)
+
+        var allowedSet = mutableSetOf(root, goodFile, goodDir, goodDirFile, subDir, subSubDir, subSubDirGoodFile).map { it.absolutePath }
+        var emptySet = setOf<String>()
+
+        var violations = mutableSetOf<String>()
+
+        // Make sure whitelisted files are not in the violation list.
+        fun assertWhitelistedFilesNotReported() {
+            assertEquals(emptySet, violations.intersect(allowedSet))
+        }
+
+        // Assert given files are reported as violations.
+        fun assertViolationReported(vararg files: File) {
+            val fileSet = files.map { it.absolutePath }.toSet()
+            assertEquals(fileSet, violations.intersect(fileSet))
+        }
+
+        // Assert given files are *not* reported as violations.
+        fun assertViolationNotReported(vararg files: File) {
+            val fileSet = files.map { it.absolutePath }.toSet()
+            assertEquals(emptySet, violations.intersect(fileSet))
+        }
+
+        val listener = object : FileReadSandbox.Listener {
+            override fun onViolation(absolutePath: String, isDirectory: Boolean) {
+                violations.add(absolutePath)
+            }
+        }
+
+        // Activate the sandbox.
+        FileReadSandbox.activate(listener)
+        try {
+            // Access "good" files, which should be allowed.
+            goodFile.readBytes()
+            subSubDirGoodFile.readBytes()
+
+            // If a file is created by metalava, then it can be read.
+            val newFile1 = File(root, "newFile1").apply { createNewFile() }
+            newFile1.readBytes()
+
+            assertWhitelistedFilesNotReported()
+            assertViolationNotReported(badFile, badDir, badDirFile, subSubDirBadFile)
+
+            // Access "bad" files.
+            badFile.readBytes()
+            badDirFile.readBytes()
+
+            assertWhitelistedFilesNotReported()
+            assertViolationReported(badFile, badDirFile)
+            assertViolationNotReported(subSubDirBadFile)
+
+            // Clear the violations, and access badDir.
+            violations.clear()
+            badDir.listFiles()
+            root.listFiles()
+
+            assertWhitelistedFilesNotReported()
+            assertViolationReported(badDir)
+            assertViolationNotReported(badFile, badDirFile, subSubDirBadFile)
+
+            // Check "implicitly allowed access" directories.
+            violations.clear()
+            subDir.listFiles() // Implicitly allowed.
+            subSubDir.listFiles() // Implicitly allowed.
+            subSubDir2.listFiles() // *Not* implicitly allowed.
+            subSubDirGoodFile.readBytes()
+            subSubDirBadFile.readBytes() // *Not* allowed.
+            root.listFiles()
+
+            assertWhitelistedFilesNotReported()
+
+            assertViolationReported(subSubDir2, subSubDirBadFile) // These are not allowed to read.
+        } finally {
+            // Deactivate the sandbox.
+            FileReadSandbox.deactivate()
+            FileReadSandbox.reset()
+        }
+        violations.clear()
+
+        // This shouldn't trigger the listener.
+        badFile.readBytes()
+
+        assertTrue(violations.isEmpty())
+    }
+}
diff --git a/src/test/java/com/android/tools/metalava/OptionsTest.kt b/src/test/java/com/android/tools/metalava/OptionsTest.kt
index 82b06f6..cf31e96 100644
--- a/src/test/java/com/android/tools/metalava/OptionsTest.kt
+++ b/src/test/java/com/android/tools/metalava/OptionsTest.kt
@@ -19,7 +19,10 @@ package com.android.tools.metalava
 import com.android.tools.metalava.doclava1.Issues
 import com.android.tools.metalava.model.defaultConfiguration
 import org.junit.Assert.assertEquals
+import org.junit.Assert.assertFalse
+import org.junit.Assert.assertTrue
 import org.junit.Test
+import java.io.File
 import java.io.PrintWriter
 import java.io.StringWriter
 
@@ -397,6 +400,23 @@ Extracting API Levels:
                                              Points to the current API jar, if any
 
 
+Sandboxing:
+--no-implicit-root                           
+                                             Disable implicit root directory detection. Otherwise, metalava adds in
+                                             source roots implied by the source files
+--strict-input-files <file>                  
+                                             Do not read files that are not explicitly specified in the command line.
+                                             All violations are written to the given file. Reads on directories are
+                                             always allowed, but metalava still trackes reads on directories that are
+                                             not specified in the command line, and write them to the file.
+--strict-input-files:stack <file>            
+                                             Same as --strict-input-files but also print stacktraces.
+--strict-input-files-exempt <files or dirs>  
+                                             Used with --strict-input-files. Explicitly allow access to files and/or
+                                             directories (separated by `:). Can also be @ followed by a path to a text
+                                             file containing paths to the full set of files and/or directories.
+
+
 Environment Variables:
 METALAVA_DUMP_ARGV                           
                                              Set to true to have metalava emit all the arguments it was invoked with.
@@ -523,4 +543,30 @@ $FLAGS
             expectedFail = "Unknown issue id: --hide ThisIssueDoesNotExist"
         )
     }
+
+    @Test
+    fun `Test for --strict-input-files-exempt`() {
+        val top = temporaryFolder.newFolder()
+
+        val dir = File(top, "childdir").apply { mkdirs() }
+        val grandchild1 = File(dir, "grandchiild1").apply { createNewFile() }
+        val grandchild2 = File(dir, "grandchiild2").apply { createNewFile() }
+        val file1 = File(top, "file1").apply { createNewFile() }
+        val file2 = File(top, "file2").apply { createNewFile() }
+
+        try {
+            check(
+                extraArguments = arrayOf("--strict-input-files-exempt",
+                    file1.path + File.pathSeparatorChar + dir.path)
+            )
+
+            assertTrue(FileReadSandbox.isAccessAllowed(file1))
+            assertTrue(FileReadSandbox.isAccessAllowed(grandchild1))
+            assertTrue(FileReadSandbox.isAccessAllowed(grandchild2))
+
+            assertFalse(FileReadSandbox.isAccessAllowed(file2)) // Access *not* allowed
+        } finally {
+            FileReadSandbox.reset()
+        }
+    }
 }