Locate system linker section automatically

mobile/src/main/java/be/mygod/librootkotlinx/AppProcess.kt

@@ -5,6 +5,7 @@ import android.os.Debug
 import android.os.Process
 import androidx.annotation.RequiresApi
 import java.io.File
+import java.io.IOException
 
 object AppProcess {
     /**
@@ -32,7 +33,49 @@ object AppProcess {
         return "/system/etc/ld.config.txt"
     }
 
+    /**
+     * Based on: https://android.googlesource.com/platform/bionic/+/30f2f05/linker/linker_config.cpp#182
+     */
+    @RequiresApi(29)
+    fun findLinkerSection(lines: Sequence<String>, binaryRealPath: String): String {
+        for (untrimmed in lines) {
+            val line = untrimmed.substringBefore('#').trim()
+            if (line.isEmpty()) continue
+            if (line[0] == '[' && line.last() == ']') break
+            if (line.contains("+=")) continue
+            val chunks = line.split('=', limit = 2)
+            if (chunks.size < 2) {
+                Logger.me.w("warning: couldn't parse invalid format: $line (ignoring this line)")
+                continue
+            }
+            var (name, value) = chunks.map { it.trim() }
+            if (!name.startsWith("dir.")) {
+                Logger.me.w("warning: unexpected property name \"$name\", " +
+                        "expected format dir.<section_name> (ignoring this line)")
+                continue
+            }
+            if (value.endsWith('/')) value = value.dropLast(1)
+            if (value.isEmpty()) {
+                Logger.me.w("warning: property value is empty (ignoring this line)")
+                continue
+            }
+            try {
+                value = File(value).canonicalPath
+            } catch (e: IOException) {
+                Logger.me.i("warning: path \"$value\" couldn't be resolved: ${e.message}")
+            }
+            if (binaryRealPath.startsWith(value) && binaryRealPath[value.length] == '/') return name.substring(4)
+        }
+        throw IllegalArgumentException("No valid linker section found")
+    }
+
     val myExe get() = "/proc/${Process.myPid()}/exe"
+    val myExeCanonical get() = try {
+        File("/proc/self/exe").canonicalPath
+    } catch (e: IOException) {
+        Logger.me.i("warning: couldn't resolve self exe: ${e.message}")
+        "/system/bin/app_process"
+    }
 
     /**
      * To workaround Samsung's stupid kernel patch that prevents exec, we need to relocate exe outside of /data.
@@ -51,10 +94,17 @@ object AppProcess {
             // unfortunately native ld.config.txt only recognizes /data,/system,/system_ext as system directories;
             // to link correctly, we need to add our path to the linker config too
             val ldConfig = "$apexPath/etc/ld.config.txt"
+            val masterLdConfig = genericLdConfigFilePath
+            val section = try {
+                File(masterLdConfig).useLines { findLinkerSection(it, myExeCanonical) }
+            } catch (e: Exception) {
+                Logger.me.w("Failed to locate system section", e)
+                "system"
+            }
             script.appendln("[ -f $ldConfig ] || " +
                     "mkdir -p $apexPath/etc && " +
-                    "echo dir.system = $apexPath >$ldConfig && " +
+                    "echo dir.$section = $apexPath >$ldConfig && " +
-                    "cat $genericLdConfigFilePath >>$ldConfig || exit 1")
+                    "cat $masterLdConfig >>$ldConfig || exit 1")
             "$apexPath/bin" to "$apexPath/bin/app_process"
         }
         script.appendln("[ -f $relocated ] || " +
@@ -64,7 +114,11 @@ object AppProcess {
         return script to relocated
     }
 
-    fun launchString(packageCodePath: String, clazz: String, appProcess: String, niceName: String?): String {
+    /**
+     * Compute the shell script line that exec into the corresponding [clazz].
+     * Extra params can be simply appended to the string.
+     */
+    fun launchString(packageCodePath: String, clazz: String, appProcess: String, niceName: String? = null): String {
         val debugParams = if (Debug.isDebuggerConnected()) when (Build.VERSION.SDK_INT) {
             in 29..Int.MAX_VALUE -> "-XjdwpProvider:adbconnection"
             28 -> "-XjdwpProvider:adbconnection -XjdwpOptions:suspend=n,server=y -Xcompiler-option --debuggable"