[Python-Dev] Python 2.7, long double vs allocator alignment, GCC 8 on x86-64 (original) (raw)
Florian Weimer fw at deneb.enyo.de
Wed Jan 31 02:41:43 EST 2018
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- Gregory P. Smith:
The proper fix for this in the code would likely break ABI compatibility (ie: not possible in python 2.7 or any other stable release).
Clang's UBSAN (undefined behavior sanitizer) has been flagging this one for a long time. In Python 3 a double is used instead of long double since 2012 as I did some digging at the time: https://github.com/python/cpython/commit/e348c8d154cf6342c79d627ebfe89dfe9de23817
A slightly more ABI-safe version of that change looks like this:
diff --git a/Include/objimpl.h b/Include/objimpl.h index 55e83eced6..aa906144dc 100644 --- a/Include/objimpl.h +++ b/Include/objimpl.h @@ -248,6 +248,18 @@ PyAPI_FUNC(PyVarObject *) _PyObject_GC_Resize(PyVarObject , Py_ssize_t); / for source compatibility with 2.2 */ #define _PyObject_GC_Del PyObject_GC_Del
+/* Former over-aligned definition of PyGC_Head, used to compute the
- size of the padding for the new version below. */ +union _gc_head; +union _gc_head_old {
- struct {
union _gc_head *gc_next;union _gc_head *gc_prev;Py_ssize_t gc_refs;- } gc;
- long double dummy;
+}; + /* GC information is stored BEFORE the object structure. */ typedef union _gc_head { struct { @@ -255,7 +267,8 @@ typedef union _gc_head { union _gc_head *gc_prev; Py_ssize_t gc_refs; } gc;
- long double dummy; /* force worst-case alignment */
- double dummy; /* force worst-case alignment */
- char dummy_padding[sizeof(union _gc_head_old)];
} PyGC_Head;
extern PyGC_Head *_PyGC_generation0;
This preserves the offset used by _Py_AS_GC in case it has been built into existing binaries. It may be more appropriate to do it this way for Python 2.7. I think it's also more conservative than the allocator changes.
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