https://github.com/hoodmane/clang-python-fpcast-plugin

A clang plugin to fix up common function pointer casts in Python C code
https://github.com/hoodmane/clang-python-fpcast-plugin

Last synced: 9 days ago
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A clang plugin to fix up common function pointer casts in Python C code

• Host: GitHub
• URL: https://github.com/hoodmane/clang-python-fpcast-plugin
• Owner: hoodmane
• License: mpl-2.0
• Created: 2021-11-21T00:18:15.000Z (about 3 years ago)
• Default Branch: main
• Last Pushed: 2021-11-21T08:09:53.000Z (about 3 years ago)
• Last Synced: 2024-11-15T17:09:36.692Z (2 months ago)
• Language: C++
• Size: 22.5 KB
• Stars: 0
• Watchers: 2
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • License: LICENSE

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README

        
# Clang Python Wasm Plugin

A clang plugin to fix up common function pointer casts in Python C code. Thanks

to [clang-tutor](https://github.com/banach-space/clang-tutor) for helping me to

understand. Both the code itself and especially the CMakeLists files are heavily

copied from them.

## Work in progress

Still need to figure out how to use this in Pyodide build process

(see [the fpcast removal PR](https://github.com/pyodide/pyodide/pull/1677)).

## Why?

There are two types of function pointer casting that are common in Python code

that we want to detect and patch.

```C

PyObject*

some_method(PyObject* self){

    // method body

}

// ... later

PyMethodDef {

    .ml_name = "myfunc2",

    .ml_flags = METH_NOARGS,

    .ml_meth = (PyCFunction)myfunc

}

```

The problem: `some_method` is a `METH_NOARGS` method. These should have

signature `PyObject* some_method(PyObject* self, PyObject* always_null)`. The

actual call occurs here:

https://github.com/python/cpython/blob/main/Objects/methodobject.c#L489 and

looks like `PyObject *result = meth(PyCFunction_GET_SELF(func), NULL);` In x86,

it's okay to cast `some_method` to a function of two arguments and call it with

a second argument. The second argument will be ignored and everything works

fine. In wasm, this causes an "indirect call signature mismatch" trap. We need to patch

`some_method` to take a second argument.

The second common problem is with getters and setters:

```C

PyObject*

arrayflags_updateifcopy_get(PyObject *self){

    return NULL;

}

int

arrayflags_updateifcopy_set(PyObject *self, PyObject *val){

    return 0;

}

PyGetSetDef blah = {

    "updateifcopy",                      // char* name;

    (getter)arrayflags_updateifcopy_get, // getter get;

    (setter)arrayflags_updateifcopy_set, // setter set;

    NULL,  // char* doc

    NULL   // void* closure

};

```

The type signature for `getter` is

`PyObject* func(PyObject *self, void *closure)`.

The type signature for `setter` is

`PyObject* func(PyObject *self, PyObject *value, void *closure)`.

The last field of `PyGetSetDef` is a "closure" which is always passed as a last

argument to the getter and setter. In many examples, this closure argument goes

unused, and so people leave it out entirely. Again, we get function signature

mismatch traps at runtime.

## Installing clang 13 / llvm 13

We first need to install clang 13. I think it's desirable to avoid building it,

though that is of course possible. There are instructions here:

https://apt.llvm.org/

I think the specific invocation that worked for me was:

```sh

wget https://apt.llvm.org/llvm.sh

chmod +x llvm.sh

sudo ./llvm.sh 13

apt-get install libclang-13-dev

```

## Building

I build this as follows:

```sh

mkdir build

cmake -DClang_INSTALL_DIR=/usr/lib/llvm-13/ ..

make

```

## Running

Once you've built the plugin, from the root directory you can run it as:

```sh

clang -cc1 -load build/lib/libMyPlugin.so -plugin LAC test/test.c

```

If you get an error like

`error: unable to load plugin 'lib/libMyPlugin.so': 'lib/libMyPlugin.so: undefined symbol`,

you are probably using the wrong version of clang.

Try `clang --version` and make sure it's version 13.