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ProGuard manual | Retrace

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ReTrace

Introduction

ReTrace is a companion tool for ProGuard that 'de-obfuscates' stack traces.

When an obfuscated program throws an exception, the resulting stack trace typically isn't very informative. Class names and method names have been replaced by short meaningless strings. Source file names and line numbers are missing altogether. While this may be intentional, it can also be inconvenient when debugging problems.

ReTrace can read an obfuscated stack trace and restore it to what it would look like without obfuscation. The restoration is based on the mapping file that ProGuard can write out while obfuscating. The mapping file links the original class names and class member names to their obfuscated names.

The standard Android build process automatically writes out mapping files to their traditional locations in your Android project, e.g. build/outputs/release/mapping/mapping.txt.

DexGuard provides advanced protection for Android applications. It is fully compatible with ProGuard. Upgrade to DexGuard

Usage

You can find the ReTrace jar in the lib directory of the ProGuard distribution. To run ReTrace, just type:

java -jar retrace.jar[options...] mapping_file[stacktrace_file]

Alternatively, the bin directory contains some short Linux and Windows scripts containing this command. These are the arguments:

mapping_file
Specifies the name of the mapping file, produced by ProGuard with the option "-printmapping mapping_file", while obfuscating the application that produced the stack trace.
stacktrace_file
Optionally specifies the name of the file containing the stack trace. If no file is specified, a stack trace is read from the standard input. The stack trace must be encoded with UTF-8 encoding. Blank lines and unrecognized lines are ignored.

The following options are supported:

-verbose
Specifies to print out more informative stack traces that include not only method names, but also method return types and arguments.
-regex regular_expression

Specifies the regular expression that is used to parse the lines in the stack trace. Specifying a different regular expression allows to de-obfuscate more general types of input than just stack traces. The default is suitable for stack traces produced by most JVMs:

(?:.*?\bat\s+%c\.%m\s*\(%s(?::%l)?\)\s*(?:~\[.*\])?)|(?:(?:.*?[:"]\s+)?%c(?::.*)?)

The regular expression is a Java regular expression (cfr. the documentation of java.util.regex.Pattern), with a few additional wildcards:

WildcardDescriptionExample
%cmatches a class namecom.example.MyClass
%Cmatches a class name with slashescom/example/MyClass
%tmatches a field type or method return typecom.example.MyClass[]
%fmatches a field namemyField
%mmatches a method namemyMethod
%amatches a list of method argumentsboolean,int
%smatches a source file nameMyClass.java
%lmatches a line number inside a method123

Elements that match these wildcards are de-obfuscated, when possible. Note that regular expressions must not contain any capturing groups. Use non-capturing groups instead: (?:...)

The default expression for instance matches the following lines:

Exception in thread "main" com.example.MyException: Some message at com.example.MyClass.myMethod(MyClass.java:123)

The restored stack trace is printed to the standard output. The completeness of the restored stack trace depends on the presence of line number tables in the obfuscated class files:

  • If all line numbers have been preserved while obfuscating the application, ReTrace will be able to restore the stack trace completely.
  • If the line numbers have been removed, mapping obfuscated method names back to their original names has become ambiguous. Retrace will list all possible original method names for each line in the stack trace. The user can then try to deduce the actual stack trace manually, based on the logic of the program.

Preserving line number tables is explained in detail in this example in the ProGuard User Manual.

Source file names are currently restored based on the names of the outer-most classes. If you prefer to keep the obfuscated name, you can replace %s in the default regular expression by .*

Unobfuscated elements and obfuscated elements for which no mapping is available will be left unchanged.

Mapping

A mapping file contains the original names and the obfuscated names of classes, fields, and methods. ProGuard can write out such a file while obfuscating an application or a library, with the option-printmapping. ReTrace requires the mapping file to restore obfuscated stack traces to more readable versions. It is a readable file with UTF-8 encoding, so you can also look up names in an ordinary text viewer. The format is pretty self-explanatory, but we describe its details here.

Specifications

A mapping file contains a sequence of records of the following form:

classline
    fieldline *
    methodline *

A classline, with a trailing colon, specifies a class and its obfuscated name:

originalclassname -> obfuscatedclassname:

A fieldline, with 4 leading spaces, specifies a field and its obfuscated name:

    originalfieldtype originalfieldname -> obfuscatedfieldname

A methodline, with 4 leading spaces, specifies a method and its obfuscated name:

    [startline:endline:]originalreturntype [originalclassname.]originalmethodname(originalargumenttype,...)[:originalstartline[:originalendline]] -> obfuscatedmethodname

An asterisk "*" means the line may occur any number of times. Square brackets "[]" mean that their contents are optional. Ellipsis dots "..." mean that any number of the preceding items may be specified. The colon ":", the separator ".", and the arrow "->" are literal tokens.

Example

The following snippet gives an impression of the structure of a mapping file:

com example.application.ArgumentWordReader -> com.example.a.a:
    java.lang.String[] arguments -> a
    int index -> a
    36:57:void <init>(java.lang.String[],java.io.File) -> <init>
    64:64:java.lang.String nextLine() -> a
    72:72:java.lang.String lineLocationDescription() -> b
com.example.application.Main -> com.example.application.Main:
    com.example.application.Configuration configuration -> a
    50:66:void <init>(com.example.application.Configuration) -> <init>
    74:228:void execute() -> a
    2039:2056:void com.example.application.GPL.check():39:56 -> a
    2039:2056:void execute():76 -> a
    2236:2252:void printConfiguration():236:252 -> a
    2236:2252:void execute():80 -> a
    3040:3042:java.io.PrintWriter com.example.application.util.PrintWriterUtil.createPrintWriterOut(java.io.File):40:42 -> a
    3040:3042:void printConfiguration():243 -> a
    3040:3042:void execute():80 -> a
    3260:3268:void readInput():260:268 -> a
    3260:3268:void execute():97 -> a

You can see the names of classes and their fields and methods:

  • The fields and methods are listed in ProGuard configuration format (javap format), with descriptors that have return types and argument types but no argument names. In the above example:

    void <init>(java.lang.String[],java.io.File)
    

    refers to a constructor with a String array argument and a Fileargument.

  • A method may have a leading line number range, if it is known from the original source code (see Producing useful obfuscated stack traces in the Examples section). Unlike method names, line numbers are unique within a class, so ReTrace can resolve lines in a stack trace without ambiguities. For example:

    74:228:void execute()
    

    refers to a method execute, defined on lines 74 to 228.

  • The obfuscated method name follows the arrow. For example:

    74:228:void execute() -> a
    

    shows that method execute has been renamed to a. Multiple fields and methods can get the same obfuscated names, as long as their descriptors are different.

Inlined methods

The mapping file accounts for the added complexity of inlined methods (as of ProGuard/ReTrace version 5.2). The optimization step may inline methods into other methods — recursively even. A single line in an obfuscated stack trace can then correspond to multiple lines in the original stack trace: the line that throws the exception followed by one or more nested method calls. In such cases, the mapping file repeats the leading line number range on subsequent lines. For example:

3040:3042:java.io.PrintWriter com.example.application.util.PrintWriterUtil.createPrintWriterOut(java.io.File):40:42 -> a
3040:3042:void printConfiguration():243 -> a
3040:3042:void execute():80 -> a
  • The subsequent lines correspond to the subsequent lines of the original stack trace. For example:

    3040:3042:java.io.PrintWriter com.example.application.util.PrintWriterUtil.createPrintWriterOut(java.io.File):40:42 -> a
    3040:3042:void printConfiguration():243 -> a
    3040:3042:void execute():80 -> a
    

    refers to method createPrintWriterOut called from and inlined inprintConfiguration, in turn called from and inlined in method execute.

  • An original method name may have a preceding class name, if the method originates from a different class. For example:

    3040:3042:java.io.PrintWriter com.example.application.util.PrintWriterUtil.createPrintWriterOut(java.io.File):40:42 -> a
    

    shows that method createPrintWriterOut was originally defined in classPrintWriterUtil.

  • A single trailing line number corresponds to an inlined method call. For example:

    3040:3042:java.io.PrintWriter com.example.application.util.PrintWriterUtil.createPrintWriterOut(java.io.File):40:42 -> a
    3040:3042:void printConfiguration():243 -> a
    3040:3042:void execute():80 -> a
    

    specifies that method execute called printConfiguration on line 80, and printconfiguration called createPrintWriterOut on line 243.

  • A traling line number range corresponds to the final inlined method body. For example:

    3040:3042:java.io.PrintWriter com.example.application.util.PrintWriterUtil.createPrintWriterOut(java.io.File):40:42 -> a
    

    shows that method createPrintWriterOut covered lines 40 to 42.

  • The leading line number range is synthetic, to avoid ambiguities with other code in the same class. ProGuard makes up the range, but tries to make it similar-looking to the original code (by adding offsets that are multiples of 1000), for convenience. For example:

    3040:3042:java.io.PrintWriter com.example.application.util.PrintWriterUtil.createPrintWriterOut(java.io.File):40:42 -> a
    

    created synthetic range 3040:3042 in the bytecode of class Main to be unique but still resemble source code range 40:42 in classPrintWriterUtil.

Tools that don't account for these repeated line number ranges, like older versions of ReTrace, may still degrade gracefully by outputting the subsequent lines without interpreting them.

Examples

Restoring a stack trace with line numbers

Assume for instance ProGuard itself has been obfuscated using the following extra options:

-printmapping mapping.txt

-renamesourcefileattribute MyApplication
-keepattributes SourceFile,LineNumberTable

Now assume the processed application throws an exception:

java.io.IOException: Can't read [dummy.jar] (No such file or directory)
    at proguard.y.a(MyApplication:188)
    at proguard.y.a(MyApplication:158)
    at proguard.y.a(MyApplication:136)
    at proguard.y.a(MyApplication:66)
    at proguard.ProGuard.c(MyApplication:218)
    at proguard.ProGuard.a(MyApplication:82)
    at proguard.ProGuard.main(MyApplication:538)
Caused by: java.io.IOException: No such file or directory
    at proguard.d.q.a(MyApplication:50)
    at proguard.y.a(MyApplication:184)
    ... 6 more

If we have saved the stack trace in a file stacktrace.txt, we can use the following command to recover the stack trace:

retrace mapping.txt stacktrace.txt

The output will correspond to the original stack trace:

java.io.IOException: Can't read [dummy.jar] (No such file or directory)
    at proguard.InputReader.readInput(InputReader.java:188)
    at proguard.InputReader.readInput(InputReader.java:158)
    at proguard.InputReader.readInput(InputReader.java:136)
    at proguard.InputReader.execute(InputReader.java:66)
    at proguard.ProGuard.readInput(ProGuard.java:218)
    at proguard.ProGuard.execute(ProGuard.java:82)
    at proguard.ProGuard.main(ProGuard.java:538)
Caused by: java.io.IOException: No such file or directory
    at proguard.io.DirectoryPump.pumpDataEntries(DirectoryPump.java:50)
    at proguard.InputReader.readInput(InputReader.java:184)
    ... 6 more

Restoring a stack trace with line numbers (verbose)

In the previous example, we could also use the verbose flag:

retrace -verbose mapping.txt stacktrace.txt

The output will then look as follows:

java.io.IOException: Can't read [dummy.jar] (No such file or directory)
    at proguard.InputReader.void readInput(java.lang.String,proguard.ClassPathEntry,proguard.io.DataEntryReader)(InputReader.java:188)
    at proguard.InputReader.void readInput(java.lang.String,proguard.ClassPath,int,int,proguard.io.DataEntryReader)(InputReader.java:158)
    at proguard.InputReader.void readInput(java.lang.String,proguard.ClassPath,proguard.io.DataEntryReader)(InputReader.java:136)
    at proguard.InputReader.void execute(proguard.classfile.ClassPool,proguard.classfile.ClassPool)(InputReader.java:66)
    at proguard.ProGuard.void readInput()(ProGuard.java:218)
    at proguard.ProGuard.void execute()(ProGuard.java:82)
    at proguard.ProGuard.void main(java.lang.String[])(ProGuard.java:538)
Caused by: java.io.IOException: No such file or directory
    at proguard.io.DirectoryPump.void pumpDataEntries(proguard.io.DataEntryReader)(DirectoryPump.java:50)
    at proguard.InputReader.void readInput(java.lang.String,proguard.ClassPathEntry,proguard.io.DataEntryReader)(InputReader.java:184)
    ... 6 more

Restoring a stack trace without line numbers

Assume for instance ProGuard itself has been obfuscated using the following extra options, this time without preserving the line number tables:

-printmapping mapping.txt

A stack trace stacktrace.txt will then lack line number information, showing "Unknown source" instead:

java.io.IOException: Can't read [dummy.jar] (No such file or directory)
    at proguard.y.a(Unknown Source)
    at proguard.y.a(Unknown Source)
    at proguard.y.a(Unknown Source)
    at proguard.y.a(Unknown Source)
    at proguard.ProGuard.c(Unknown Source)
    at proguard.ProGuard.a(Unknown Source)
    at proguard.ProGuard.main(Unknown Source)
Caused by: java.io.IOException: No such file or directory
    at proguard.d.q.a(Unknown Source)
    ... 7 more

We can still use the same command to recover the stack trace:

java -jar retrace.jar mapping.txt stacktrace.txt

The output will now list all alternative original method names for each ambiguous obfuscated method name:

java.io.IOException: Can't read [dummy.jar] (No such file or directory)
    at proguard.InputReader.execute(InputReader.java)
                            readInput(InputReader.java)
    at proguard.InputReader.execute(InputReader.java)
                            readInput(InputReader.java)
    at proguard.InputReader.execute(InputReader.java)
                            readInput(InputReader.java)
    at proguard.InputReader.execute(InputReader.java)
                            readInput(InputReader.java)
    at proguard.ProGuard.readInput(ProGuard.java)
    at proguard.ProGuard.execute(ProGuard.java)
                         optimize(ProGuard.java)
                         createPrintStream(ProGuard.java)
                         closePrintStream(ProGuard.java)
                         fileName(ProGuard.java)
    at proguard.ProGuard.main(ProGuard.java)
Caused by: java.io.IOException: No such file or directory
    at proguard.io.DirectoryPump.pumpDataEntries(DirectoryPump.java)
                                 readFiles(DirectoryPump.java)

For instance, ReTrace can't tell if the method a corresponds toexecute or to readInput, so it lists both. You need to figure it out based on your knowledge of the application. Having line numbers and unambiguous names clearly is a lot easier, so you should consider preserving the line numbers when you obfuscate your application.