GoReSym

GoReSym

GoReSym is a Go symbol parser that extracts program metadata (such as CPU architecture, OS, endianness, compiler version, etc), function metadata (start & end addresses, names, sources), filename and line number metadata, and embedded structures and types. This cross platform program is based directly on the open source Go compiler and runtime code.

The upstream Go runtime code is extended to handle:

• stripped binaries
• malformed unpacked binaries, such as from UPX
• binaries that split single data ranges across multiple sections
• the location of the moduledata structure

Usage

Refer to https://www.mandiant.com/resources/blog/golang-internals-symbol-recovery for reverse engineering details and example usage.

You can download pre-built linux and windows GoReSym binaries from the Releases tab.

To build from source with a recent Go compiler, invoke the Go compiler:

go build

Once built invoke GoReSym like this:

GoReSym.exe -t -d -p /path/to/input.exe

In this example, we ask GoReSym to recover type names (-t), user package names, standard Go package names (-d), and input file paths (-p) embedded within the file /path/to/input.exe. The output looks like this:

{
    "Version": "1.14.15",
    "BuildId": "Zb9QmokKTiOUgHKmaIwz/wd2rtE3W9PN-um1Ocdzh/qTdqcTY_jVajHy_-TtYv/Z_kJu9M77OjfijEiHMcF",
    "Arch": "amd64",
    "TabMeta": {
        "VA": 5174784,
        "Version": "1.2",
        "Endianess": "LittleEndian",
        "CpuQuantum": 1,
        "CpuQuantumStr": "x86/x64",
        "PointerSize": 8
    },
    "ModuleMeta": {
        "VA": 5678816,
        "Types": 4845568,
        "ETypes": 5171904,
        "Typelinks": {
            "Data": 5171904,
            "Len": 695,
            "Capacity": 695
        },
        "ITablinks": {
            "Data": 5174688,
            "Len": 11,
            "Capacity": 11
        },
        "LegacyTypes": {
            "Data": 0,
            "Len": 0,
            "Capacity": 0
        }
    },
    "Types": [ ... ],
    "Files": [ ... ],
    "UserFunctions": [ ... ],
    "StdFunctions": [ ... ]
}

Here are all the available flags:

• -d ("default", optional) flag will print standard Go packages in addition to user packages.
• -p ("paths", optional) flag will print any file paths embedded in the pclntab.
• -t ("types", optional) flag will print Go type names.
• -m <virtual address> ("manual", optional) flag will dump the RTYPE structure recursively at the given virtual address
• -v <version string> ("version", optional) flag will override automated version detection and use the provided version. This is needed for some stripped binaries. Type parsing will fail if the version is not accurate.
• -human (optional) flag will print a flat text listing instead of JSON. Especially useful when printing structure and interface types.

To import this information into IDA Pro you can run the script found in https://github.com/mandiant/GoReSym/blob/master/IDAPython/goresym_rename.py. It will read a json file produced by GoReSym and set symbols/labels in IDA.

Version Support

As the Go compiler and runtime have changed, so have the embedded metadata structures. GoReSym supports the following combinations of Go releases & metadata:

• all combinations of ARM64 𝒙 Intel x86/x64 𝒙 MACH-O/ELF/PE 𝒙 big/little endian
• pclntab parsing: >= Go 1.2
• moduledata location: >= Go 1.2
• moduledata type parsing: >= Go 1.5

The moduledata table used to extract types doesn't exist prior to Go 1.5, so this library will never support extracting types from very old Go versions.

This library current handles legacy pclntab (pre Go 1.2), 1.2, 1.16, 1.18, 1.19, and 1.20.

Contributions

Much of the source code from GoReSym is copied from the upstream Go compiler source directory /internal. To make this work, we've had to massage the source a bit. If you want to contribute to GoReSym, read on so we can explain this import process.

Due to the way Go packages work, we needed to remove the /internal path from the source file tree. This resulted in a lot of copying of internal Go files, where the directory tree is mostly intact but with small changes to many files' imports: references to /internal paths were replaced with github.com/mandiant/GoReSym/.

We also modified many internal structures to export fields and methods. These are not exported by Go upstream because users should not rely upon them. However, the purpose of this tool is to extract internal information, so we're taking on the task of maintaining these structures. It's not a great situation, but it's not easily avoidable. If you update this repository, you must take care to keep these modifications intact. It's probably better to manually merge in commits from upstream rather than copying upstream files wholesale.

I am open to suggestions on how to better structure this project to avoid these issues while still compiling with the typical go build. There is a previous discussion involving Go maintainers here.

Ignoring some trivial changes, most new logic exists in /objfile. For example, the file objfile/internals defines the reversed internal Go structures that GoReSym parses.

References

• pclntab Specification: golang.org/s/go12symtab
• pclntab Magics: pclntab.go#L169
• objfile Bug(s):
  • golang/go#42954
  • golang/go#47981
  • golang/go#47852
• buildID Legacy bug: golang/go#50809

Changes

• GoReSym will now also attempt to find the pclntab based on a signature of the runtime_modulesinit initialization method and attempt to repair the pclntab magic (in cases where the pclntab magic has been modified).
• Extended pcln() functions in objfile/<fileformat> to support byte scanning the pclntab magic
• Added routines such as DataAfterSection to support signature scan in file format parsers in /debug/<fileformat>
• Added check to debug/gosym/symtab.go's walksymtab to bail early when the optional symtab section is empty
• Exported many members and internal structs (changes are too many to enumerate)
• Removed goobj liner support in objfile/objfile.go's PCLineTable()
• Added extra sanity checks around loadPeTable (and other format variants) to avoid panic when symbols are present but maliciously modified to be invalid (ref: golang/go#47981)
• Modified the signatures of some internal functions to provide lower level access to information such as section addresses and offsets
• Implemented read_memory routines for supported file formats to read file data by virtual address
• Introduced moduledata scan routines to help locate moduledata in support of scanning for types and interfaces (via typelinks)
• Added size guards to readStringTable for invalid symbol tables. Parsing failures are ignored as well.

License

MIT