https://github.com/python/cpython/commit/0b91f8a668201fc58fa732b8acc496caedf... commit: 0b91f8a668201fc58fa732b8acc496caedfdbae0 branch: 2.7 author: Florian Weimer <fw@deneb.enyo.de> committer: Benjamin Peterson <benjamin@python.org> date: 2018-04-29T12:18:33-07:00 summary: Indicate that _PyGC_Head is only 8-byte aligned. (closes bpo-33374) By spec, the "long double" in _PyGC_Head requires the union to always be 16-byte aligned. However, obmalloc only yields 8-byte alignment. Compilers including GCC 8 are starting to use alignment information to do store-merging. So, the "long double" needs to be changed to a simple "double" as was long ago done in Python 3 by e348c8d154cf6342c79d627ebfe89dfe9de23817. For 2.7, we need to add some dummy padding to make sure _PyGC_Head stays the same size. files: A Misc/NEWS.d/next/Core and Builtins/2018-04-29-12-07-00.bpo-33374.-xegL6.rst M Include/objimpl.h diff --git a/Include/objimpl.h b/Include/objimpl.h index 5f2868332955..cbf6bc3f8763 100644 --- a/Include/objimpl.h +++ b/Include/objimpl.h @@ -248,6 +248,20 @@ 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_old *gc_next; + union _gc_head_old *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 +269,8 @@ 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 at least 8-byte alignment. */ + char dummy_padding[sizeof(union _gc_head_old)]; } PyGC_Head; extern PyGC_Head *_PyGC_generation0; diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-04-29-12-07-00.bpo-33374.-xegL6.rst b/Misc/NEWS.d/next/Core and Builtins/2018-04-29-12-07-00.bpo-33374.-xegL6.rst new file mode 100644 index 000000000000..9ec1a605c8f2 --- /dev/null +++ b/Misc/NEWS.d/next/Core and Builtins/2018-04-29-12-07-00.bpo-33374.-xegL6.rst @@ -0,0 +1,3 @@ +Tweak the definition of PyGC_Head, so compilers do not believe it is always +16-byte aligned on x86. This prevents crashes with more aggressive +optimizations present in GCC 8.