Proot: bind mounts for plugin sandbox, safer exec

app/src/main/java/io/github/abrarul/modularai/proot/Proot.kt

@@ -22,7 +22,7 @@ class Proot(private val ctx: Context) {
     val rootfs = File(ctx.filesDir, "rootfs/alpine")
 
     val binaryAvailable get() = bin.exists() && loader.exists()
-    val installed get() = File(rootfs, "bin/sh").exists() || File(rootfs, "bin/busybox").exists()
+    val installed get() = File(rootfs, "bin/busybox").exists() || File(rootfs, "bin/sh").exists()
 
     suspend fun installAlpine(onLog: (String) -> Unit) = withContext(Dispatchers.IO) {
         val url = "https://dl-cdn.alpinelinux.org/alpine/v3.20/releases/aarch64/alpine-minirootfs-3.20.3-aarch64.tar.gz"
@@ -32,23 +32,28 @@ class Proot(private val ctx: Context) {
         URL(url).openStream().use { Tar.extract(it, rootfs) { n++ } }
         File(rootfs, "etc/resolv.conf").apply { parentFile?.mkdirs(); writeText("nameserver 8.8.8.8\nnameserver 1.1.1.1\n") }
         onLog("Extracted $n entries into ${rootfs.path}")
+        if (!binaryAvailable) onLog("Note: libproot.so is not bundled in this build, so commands cannot run yet (see jniLibs/README.md).")
     }
 
     fun remove() = rootfs.deleteRecursively()
 
-    /** Runs [cmd] with /bin/sh inside the rootfs and streams combined output line by line. */
-    fun exec(cmd: String, onLine: (String) -> Unit): Int {
+    /**
+     * Runs [cmd] with /bin/sh inside the rootfs and streams combined output line by line.
+     * [binds] are "hostPath:guestPath" pairs. Nothing else of the app's private data is visible inside.
+     */
+    fun exec(cmd: String, onLine: (String) -> Unit, binds: List<String> = emptyList()): Int {
         check(binaryAvailable) { "proot binary not bundled in this build (use 'termux ...' instead)" }
         check(installed) { "Rootfs not installed. Run: proot install" }
         val tmp = File(ctx.filesDir, "tmp").apply { mkdirs() }
-        val pb = ProcessBuilder(
-            bin.path, "-0", "-r", rootfs.path,
-            "-b", "/dev", "-b", "/proc", "-b", "/sys",
+        val args = mutableListOf(bin.path, "-0", "-r", rootfs.path, "-b", "/dev", "-b", "/proc", "-b", "/sys")
+        binds.forEach { args += listOf("-b", it) }
+        args += listOf(
             "-w", "/root",
             "/usr/bin/env", "-i", "HOME=/root", "TERM=xterm-256color",
             "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin",
             "/bin/sh", "-c", cmd
-        ).redirectErrorStream(true)
+        )
+        val pb = ProcessBuilder(args).redirectErrorStream(true)
         pb.environment()["PROOT_LOADER"] = loader.path
         pb.environment()["PROOT_TMP_DIR"] = tmp.path
         val p = pb.start()
@@ -97,7 +102,7 @@ object Tar {
             longName = null; paxPath = null; paxLink = null
 
             val out = File(dest, name.trimStart('/')).let { File(it.parentFile?.canonicalFile ?: dest, it.name) }
-            if (!out.path.startsWith(root)) { skip(inp, size + pad); continue } // path traversal guard
+            if (out.path != root && !out.path.startsWith(root + File.separator)) { skip(inp, size + pad); continue } // traversal guard
             onEntry()
 
             when (type) {
@@ -109,6 +114,7 @@ object Tar {
                 }
                 '0', '\u0000' -> {
                     out.parentFile?.mkdirs()
+                    out.delete()
                     out.outputStream().use { o ->
                         var left = size
                         val buf = ByteArray(1 shl 16)