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From a608eb640f151f08ffd810e53eb29f58ef68f493 Mon Sep 17 00:00:00 2001
From: Hood Chatham <roberthoodchatham@gmail.com>
Date: Fri, 3 Dec 2021 20:55:40 -0800
Subject: [PATCH] call trampolines to handle fpcast troubles
The wasm call_indirect instruction takes a function signature as an immediate
argument (an immediate argument is one which is determined statically at compile
time). The web assembly runtime checks at runtime that the function pointer we
are attempting to call has a signature which matches the asserted function
signature, if they don't match, the runtime traps with "indirect call signature
mismatch". The codegen provided by default by the emscripten toolchain produces
an ABI that also has this constraint.
By contrast, typical native ABIs handle function pointer casts very gracefully:
extra arguments are ignored, missing arguments are filled in as 0, even wrong
return values are completely fine.
The Python ecosystem is full of code which contain function pointer casts.
Emscripten offers a second "EMULATE_FPCASTS" ABI that fixes many of these
issues, but it shims in the function pointer cast support in a binaryen pass
that happens on the fully linked wasm module. At this point, lots of information
has been destroyed and as a result the only possible solutions are extremely
costly in run time, stack space, and code size.
We wish to avoid these costs. Patching the packages is prohibitively time
consuming and boring, especially given our limited developer effort. I have
explored making automated detection tools, and these work. However, getting
these tools included into the CI of Python ecosystem packages would be
prohibitively time consuming.
There is a best of both worlds solution. It is possible to define an ABI which
allows for specific types of function pointer casts with neglible costs. I hope
to do this in future work. However, this will require custom llvm passes.
Because llvm is implemented in C++ which has very poor ABI compatibility, this
means our whole toolchain needs to be built against the same version of llvm,
from llvm all the way down to binaryen and the wasm binary toolkit.
In the meantime, most bad function pointer calls happen in a small number of
locations. Calling a function pointer from Javascript has no restrictions on
arguments. Extra arguments can be provided, arguments can be left off, etc with
no issue (this mimics Javascript's typical function call semantics). So at the
locations where the bad calls occur, we patch in a trampoline which calls out to
Javascript to make the call for us. Like magic, the problem is gone. Research
shows that the performance cost of this is surprisingly low too.
---
Objects/call.c | 5 ++++-
Objects/descrobject.c | 28 +++++++++++++++++++---------
Objects/methodobject.c | 12 +++++++++---
3 files changed, 32 insertions(+), 13 deletions(-)
diff --git a/Objects/call.c b/Objects/call.c
index 87dc0dbbdb..f8c7811521 100644
--- a/Objects/call.c
+++ b/Objects/call.c
@@ -143,6 +143,9 @@ PyObject_VectorcallDict(PyObject *callable, PyObject *const *args,
}
+// Defined in methodobject patch
+PyObject* method_call_trampoline(PyCFunction func, PyObject *self, PyObject *arg1, PyObject *arg2);
+
PyObject *
_PyObject_MakeTpCall(PyThreadState *tstate, PyObject *callable,
PyObject *const *args, Py_ssize_t nargs,
@@ -188,7 +191,7 @@ _PyObject_MakeTpCall(PyThreadState *tstate, PyObject *callable,
PyObject *result = NULL;
if (_Py_EnterRecursiveCall(tstate, " while calling a Python object") == 0)
{
- result = call(callable, argstuple, kwdict);
+ result = method_call_trampoline((PyCFunction)call, callable, argstuple, kwdict);
_Py_LeaveRecursiveCall(tstate);
}
diff --git a/Objects/descrobject.c b/Objects/descrobject.c
index fce9cdd309..326844f569 100644
--- a/Objects/descrobject.c
+++ b/Objects/descrobject.c
@@ -174,6 +174,11 @@ member_get(PyMemberDescrObject *descr, PyObject *obj, PyObject *type)
return PyMember_GetOne((char *)obj, descr->d_member);
}
+#include <emscripten.h>
+EM_JS(PyObject*, getter_call_trampoline, (getter get, PyObject *obj, void *closure), {
+ return wasmTable.get(get)(obj, closure);
+});
+
static PyObject *
getset_get(PyGetSetDescrObject *descr, PyObject *obj, PyObject *type)
{
@@ -182,7 +187,7 @@ getset_get(PyGetSetDescrObject *descr, PyObject *obj, PyObject *type)
if (descr_check((PyDescrObject *)descr, obj, &res))
return res;
if (descr->d_getset->get != NULL)
- return descr->d_getset->get(obj, descr->d_getset->closure);
+ return getter_call_trampoline(descr->d_getset->get, obj, descr->d_getset->closure);
PyErr_Format(PyExc_AttributeError,
"attribute '%V' of '%.100s' objects is not readable",
descr_name((PyDescrObject *)descr), "?",
@@ -228,6 +233,10 @@ member_set(PyMemberDescrObject *descr, PyObject *obj, PyObject *value)
return PyMember_SetOne((char *)obj, descr->d_member, value);
}
+EM_JS(PyObject*, setter_call_trampoline, (setter set, PyObject *obj, PyObject *value, void *closure), {
+ return wasmTable.get(set)(obj, value, closure);
+});
+
static int
getset_set(PyGetSetDescrObject *descr, PyObject *obj, PyObject *value)
{
@@ -236,8 +245,7 @@ getset_set(PyGetSetDescrObject *descr, PyObject *obj, PyObject *value)
if (descr_setcheck((PyDescrObject *)descr, obj, value, &res))
return res;
if (descr->d_getset->set != NULL)
- return descr->d_getset->set(obj, value,
- descr->d_getset->closure);
+ return setter_call_trampoline(descr->d_getset->set, obj, value, descr->d_getset->closure);
PyErr_Format(PyExc_AttributeError,
"attribute '%V' of '%.100s' objects is not writable",
descr_name((PyDescrObject *)descr), "?",
@@ -291,6 +299,9 @@ method_enter_call(PyThreadState *tstate, PyObject *func)
return (funcptr)((PyMethodDescrObject *)func)->d_method->ml_meth;
}
+// Defined in methodobject patch
+PyObject* method_call_trampoline(PyCFunction func, PyObject *self, PyObject *arg1, PyObject *arg2);
+
/* Now the actual vectorcall functions */
static PyObject *
method_vectorcall_VARARGS(
@@ -310,7 +321,7 @@ method_vectorcall_VARARGS(
Py_DECREF(argstuple);
return NULL;
}
- PyObject *result = meth(args[0], argstuple);
+ PyObject *result = method_call_trampoline(meth, args[0], argstuple, NULL);
Py_DECREF(argstuple);
_Py_LeaveRecursiveCall(tstate);
return result;
@@ -338,12 +349,11 @@ method_vectorcall_VARARGS_KEYWORDS(
goto exit;
}
}
- PyCFunctionWithKeywords meth = (PyCFunctionWithKeywords)
- method_enter_call(tstate, func);
+ PyCFunction meth = (PyCFunction)method_enter_call(tstate, func);
if (meth == NULL) {
goto exit;
}
- result = meth(args[0], argstuple, kwdict);
+ result = method_call_trampoline(meth, args[0], argstuple, kwdict);
_Py_LeaveRecursiveCall(tstate);
exit:
Py_DECREF(argstuple);
@@ -431,7 +441,7 @@ method_vectorcall_NOARGS(
if (meth == NULL) {
return NULL;
}
- PyObject *result = meth(args[0], NULL);
+ PyObject *result = method_call_trampoline(meth, args[0], NULL, NULL);
_Py_LeaveRecursiveCall(tstate);
return result;
}
@@ -459,7 +469,7 @@ method_vectorcall_O(
if (meth == NULL) {
return NULL;
}
- PyObject *result = meth(args[0], args[1]);
+ PyObject *result = method_call_trampoline(meth, args[0], args[1], NULL);
_Py_LeaveRecursiveCall(tstate);
return result;
}
diff --git a/Objects/methodobject.c b/Objects/methodobject.c
index 7b430416c5..b57d7a37b2 100644
--- a/Objects/methodobject.c
+++ b/Objects/methodobject.c
@@ -460,6 +460,12 @@ cfunction_vectorcall_FASTCALL_KEYWORDS_METHOD(
return result;
}
+#include <emscripten.h>
+
+EM_JS(PyObject*, method_call_trampoline, (PyCFunction func, PyObject *self, PyObject *arg1, PyObject *arg2), {
+ return wasmTable.get(func)(self, arg1, arg2);
+});
+
static PyObject *
cfunction_vectorcall_NOARGS(
PyObject *func, PyObject *const *args, size_t nargsf, PyObject *kwnames)
@@ -482,7 +488,7 @@ cfunction_vectorcall_NOARGS(
if (meth == NULL) {
return NULL;
}
- PyObject *result = meth(PyCFunction_GET_SELF(func), NULL);
+ PyObject *result = method_call_trampoline(meth, PyCFunction_GET_SELF(func), NULL, NULL);
_Py_LeaveRecursiveCall(tstate);
return result;
}
@@ -536,7 +542,7 @@ cfunction_call(PyObject *func, PyObject *args, PyObject *kwargs)
PyObject *result;
if (flags & METH_KEYWORDS) {
- result = (*(PyCFunctionWithKeywords)(void(*)(void))meth)(self, args, kwargs);
+ result = method_call_trampoline(meth, self, args, kwargs);
}
else {
if (kwargs != NULL && PyDict_GET_SIZE(kwargs) != 0) {
@@ -545,7 +551,7 @@ cfunction_call(PyObject *func, PyObject *args, PyObject *kwargs)
((PyCFunctionObject*)func)->m_ml->ml_name);
return NULL;
}
- result = meth(self, args);
+ result = method_call_trampoline(meth, self, args, NULL);
}
return _Py_CheckFunctionResult(tstate, func, result, NULL);
}
--
2.25.1
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