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commit 0b91f8a668201fc58fa732b8acc496caedfdbae0
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Author: Florian Weimer <fw@deneb.enyo.de>
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Date: Sun Apr 29 12:18:33 2018 -0700
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Indicate that _PyGC_Head is only 8-byte aligned. (closes bpo-33374)
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By spec, the "long double" in _PyGC_Head requires the union to always be 16-byte
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aligned. However, obmalloc only yields 8-byte alignment. Compilers including GCC
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8 are starting to use alignment information to do store-merging. So, the "long
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double" needs to be changed to a simple "double" as was long ago done in Python
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3 by e348c8d154cf6342c79d627ebfe89dfe9de23817. For 2.7, we need to add some
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dummy padding to make sure _PyGC_Head stays the same size.
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diff --git a/Include/objimpl.h b/Include/objimpl.h
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index 5f28683329..cbf6bc3f87 100644
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--- Python-2.7.13/Include/objimpl.h
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+++ Python-2.7.13/Include/objimpl.h
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@@ -248,6 +248,20 @@ PyAPI_FUNC(PyVarObject *) _PyObject_GC_Resize(PyVarObject *, Py_ssize_t);
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/* for source compatibility with 2.2 */
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#define _PyObject_GC_Del PyObject_GC_Del
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+/*
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+ * Former over-aligned definition of PyGC_Head, used to compute the size of the
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+ * padding for the new version below.
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+ */
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+union _gc_head;
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+union _gc_head_old {
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+ struct {
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+ union _gc_head_old *gc_next;
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+ union _gc_head_old *gc_prev;
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+ Py_ssize_t gc_refs;
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+ } gc;
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+ long double dummy;
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+};
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+
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/* GC information is stored BEFORE the object structure. */
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typedef union _gc_head {
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struct {
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@@ -255,7 +269,8 @@ typedef union _gc_head {
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union _gc_head *gc_prev;
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Py_ssize_t gc_refs;
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} gc;
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- long double dummy; /* force worst-case alignment */
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+ double dummy; /* Force at least 8-byte alignment. */
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+ char dummy_padding[sizeof(union _gc_head_old)];
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} PyGC_Head;
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extern PyGC_Head *_PyGC_generation0;
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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
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new file mode 100644
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index 0000000000..9ec1a605c8
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--- /dev/null
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+++ Python-2.7.13/Misc/NEWS.d/next/Core and Builtins/2018-04-29-12-07-00.bpo-33374.-xegL6.rst
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@@ -0,0 +1,3 @@
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+Tweak the definition of PyGC_Head, so compilers do not believe it is always
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+16-byte aligned on x86. This prevents crashes with more aggressive
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+optimizations present in GCC 8.
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