abi3audit

Read the Trail of Bits blog post about how we find bugs with abi3audit!

abi3audit scans Python extensions for abi3 violations and inconsistencies.

It can scan individual (unpackaged) shared objects, packaged wheels, or entire package version histories.

This project is maintained in part by Trail of Bits. This is not an official Trail of Bits product.

Index

• Motivation
• Installation
• Usage
  • Examples
• Limitations
• Licensing

Motivation

CPython (the reference implementation of Python) defines a stable API and corresponding ABI ("abi3"). In principle, any CPython extension can be built against this API/ABI and will remain forward compatible with future minor versions of CPython. In other words: if you build against the stable ABI for Python 3.5, your extension should work without modification on Python 3.9.

The stable ABI simplifies packaging of CPython extensions, since the packager only needs to build one abi3 wheel that targets the minimum supported Python version.

To signal that a Python wheel contains abi3-compatible extensions, the Python packaging ecosystem uses the abi3 wheel tag, e.g.:

pyrage-1.0.1-cp37-abi3-manylinux_2_5_x86_64.manylinux1_x86_64.whl

Unfortunately, there is no actual enforcement of abi3 compliance in Python extensions at install or runtime: a wheel (or independent shared object) that is tagged as abi3 is assumed to be abi3, but is not validated in any way.

To make matters worse, there is no formal connection between the flag (--py-limited-api) that controls wheel tagging and the build macros (Py_LIMITED_API) that actually lock a Python extension into a specific abi3 version.

As a result: it is very easy to compile a Python extension for the wrong abi3 version, or to tag a Python wheel as abi3 without actually compiling it as abi3-compatible.

This has serious security and reliability implications: non-stable parts of the CPython ABI can change between minor versions, resulting in crashes, unpredictable behavior, or potentially exploitable memory corruption when a Python extension incorrectly assumes the parameters of a function or layout of a structure.

Installation

abi3audit is available via pip:

pip install abi3audit

Usage

You can run abi3audit as a standalone program, or via python -m abi3audit:

abi3audit --help
python -m abi3audit --help

Top-level:

usage: abi3audit [-h] [--debug] [-v] [-R] [-o OUTPUT] [-s] [-S]
                 [--assume-minimum-abi3 ASSUME_MINIMUM_ABI3]
                 SPEC [SPEC ...]

Scans Python extensions for abi3 violations and inconsistencies

positional arguments:
  SPEC                  the files or other dependency specs to scan

options:
  -h, --help            show this help message and exit
  --debug               emit debug statements; this setting also overrides
                        `ABI3AUDIT_LOGLEVEL` and is equivalent to setting it
                        to `debug`
  -v, --verbose         give more output, including pretty-printed results for
                        each audit step
  -R, --report          generate a JSON report; uses --output
  -o OUTPUT, --output OUTPUT
                        the path to write the JSON report to (default: stdout)
  -s, --summary         always output a summary even if there are no
                        violations/ABI version mismatches
  -S, --strict          fail the entire audit if an individual audit step
                        fails
  --assume-minimum-abi3 ASSUME_MINIMUM_ABI3
                        assumed abi3 version (3.x, with x>=2) if it cannot be
                        detected

Examples

Audit a single shared object, wheel, or PyPI package:

# audit a local copy of an abi3 extension
abi3audit procmaps.abi3.so

# audit a local copy of an abi3 wheel
abi3audit procmaps-0.5.0-cp36-abi3-manylinux2010_x86_64.whl

# audit every abi3 wheel for the package 'procmaps' on PyPI
abi3audit procmaps

Show additional detail (pretty tables and individual violations) while auditing:

abi3audit procmaps --verbose

yields:

[17:59:46] 👎 procmaps:
           procmaps-0.5.0-cp36-abi3-manylinux2010_x86_64.whl: procmaps.abi3.so
           uses the Python 3.10 ABI, but is tagged for the Python 3.6 ABI
           ┏━━━━━━━━━━━━━━━━━━━━━━━━━┳━━━━━━━━━┓
           ┃ Symbol                  ┃ Version ┃
           ┡━━━━━━━━━━━━━━━━━━━━━━━━━╇━━━━━━━━━┩
           │ PyUnicode_AsUTF8AndSize │ 3.10    │
           └─────────────────────────┴─────────┘
[17:59:47] 💁 procmaps: 2 extensions scanned; 1 ABI version mismatches and 0
           ABI violations found

Generate a JSON report for each input:

abi3audit procmaps --report | python -m json.tool

yields:

{
  "specs": {
    "procmaps": {
      "kind": "package",
      "package": {
        "procmaps-0.5.0-cp36-abi3-manylinux2010_x86_64.whl": [
          {
            "name": "procmaps.abi3.so",
            "result": {
              "is_abi3": true,
              "is_abi3_baseline_compatible": false,
              "baseline": "3.6",
              "computed": "3.10",
              "non_abi3_symbols": [],
              "future_abi3_objects": {
                "PyUnicode_AsUTF8AndSize": "3.10"
              }
            }
          }
        ],
        "procmaps-0.6.1-cp37-abi3-manylinux_2_5_x86_64.manylinux1_x86_64.whl": [
          {
            "name": "procmaps.abi3.so",
            "result": {
              "is_abi3": true,
              "is_abi3_baseline_compatible": true,
              "baseline": "3.7",
              "computed": "3.7",
              "non_abi3_symbols": [],
              "future_abi3_objects": {}
            }
          }
        ]
      }
    }
  }
}

Limitations

abi3audit is a best-effort tool, with some of the same limitations as auditwheel. In particular:

• abi3audit cannot check for dynamic abi3 violations, such as an extension that calls dlsym(3) to invoke a non-abi3 function at runtime.

• abi3audit can confirm the presence of abi3-compatible symbols, but does not have an exhaustive list of abi3-incompatible symbols. Instead, it looks for violations by looking for symbols that start with Py_ or _Py_ that are not in the abi3 compatibility list. This is unlikely to result in false positives, but could if an extension incorrectly uses those reserved prefixes.

• When auditing a "bare" shared object (e.g. foo.abi3.so), abi3audit cannot assume anything about the minimum intended abi3 version. Instead, it defaults to the lowest known abi3 version (abi3-cp32) and warns on any version mismatches (e.g., a symbol that was only stabilized in 3.6). This can result in false positives, so users are encouraged to audit entire wheels or packages instead (since they contain the sufficient metadata).

• abi3audit considers the abi3 version when a symbol was stabilized, not introduced. In other words: abi3audit will produce a warning when an abi3-cp36 extension contains a function stabilized in 3.7, even if that function was introduced in 3.6. This is not a false positive (it is an ABI version mismatch), but it's generally not a source of bugs.

• abi3audit checks both the "local" and "external" symbols for each extension, for formats that support both. It does this to catch symbols that have been inlined, such as _Py_DECREF. However, if the extension's symbol table has been stripped, these may be missed.

Licensing

abi3audit is licensed under the MIT license.

abi3audit includes ASN.1 and Mach-O parsers generated from definitions provided by the Kaitai Struct project. These vendored parsers are licensed by the Kaitai Struct authors under the MIT license.