From a173b50b22b2f884853fcde45fec9bffb0fc5aae Mon Sep 17 00:00:00 2001 From: Finn Thain Date: Tue, 3 Dec 2024 11:32:55 +1100 Subject: [PATCH] Specify minimum PyGC_Head and PyObject alignment to fix build failure As documented in InternalDocs/garbage_collector.md, the garbage collector stores flags in the least significant two bits of the _gc_prev pointer in struct PyGC_Head. Consequently, this pointer is only capable of storing a location that's aligned to a 4-byte boundary. This alignment requirement is documented but it's not actually encoded. The code only works when python happens to run on a platform that has a sufficiently large minimum alignment for the structs in question. The same problem arises with PyObject pointers because the least significant bits get used for PyStackRef tags. Since we know that 2 bits are needed, we also know the minimum alignment that's needed. Let's make that explicit, so the compiler can then make those bits available. This patch fixes a segfault in _bootstrap_python. In 3.14.0 beta 2 this fixes the "Assertion `!PyStackRef_IsTaggedInt(ref)' failed" when built with --config-pydebug. Also, making the requirements explicit improves clarity. This bug was previously investigated by Adrian Glaubitz here: https://lists.debian.org/debian-68k/2024/11/msg00020.html https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=1087600 Although Adrian's patch isn't really correct (because natural alignment is not needed), he deserves full credit for finding the root cause. Origin: Backported from upstream, https://github.com/fthain/cpython/commit/a173b50b22b2f884853fcde45fec9bffb0fc5aae Bug-Upstream: https://github.com/python/cpython/issues/127545 Bug-Debian: https://bugs.debian.org/1105110 --- Include/internal/pycore_gc.h | 5 +++-- Include/object.h | 9 +++++++-- .../Build/2024-12-04-10-00-35.gh-issue-127545.t0THjE.rst | 1 + 3 files changed, 11 insertions(+), 4 deletions(-) create mode 100644 Misc/NEWS.d/next/Build/2024-12-04-10-00-35.gh-issue-127545.t0THjE.rst --- a/Include/internal/pycore_gc.h +++ b/Include/internal/pycore_gc.h @@ -18,8 +18,9 @@ // Pointer to previous object in the list. // Lowest two bits are used for flags documented later. + // Those bits are made available by the struct's minimum alignment. uintptr_t _gc_prev; -} PyGC_Head; +} PyGC_Head Py_ALIGNED(1 << _PyObject_ALIGNMENT_SHIFT); #define _PyGC_Head_UNUSED PyGC_Head @@ -146,7 +147,7 @@ /* Bit 1 is set when the object is in generation which is GCed currently. */ #define _PyGC_PREV_MASK_COLLECTING (2) /* The (N-2) most significant bits contain the real address. */ -#define _PyGC_PREV_SHIFT (2) +#define _PyGC_PREV_SHIFT _PyObject_ALIGNMENT_SHIFT #define _PyGC_PREV_MASK (((uintptr_t) -1) << _PyGC_PREV_SHIFT) /* set for debugging information */ --- a/Include/object.h +++ b/Include/object.h @@ -154,6 +154,11 @@ #define PyObject_VAR_HEAD PyVarObject ob_base; #define Py_INVALID_SIZE (Py_ssize_t)-1 +/* PyObjects are given a minimum alignment so that the least significant bits + * of an object pointer become available for other purposes. + */ +#define _PyObject_ALIGNMENT_SHIFT 2 + /* Nothing is actually declared to be a PyObject, but every pointer to * a Python object can be cast to a PyObject*. This is inheritance built * by hand. Similarly every pointer to a variable-size Python object can, @@ -183,7 +188,7 @@ #endif PyTypeObject *ob_type; -}; +} Py_ALIGNED(1 << _PyObject_ALIGNMENT_SHIFT); #else // Objects that are not owned by any thread use a thread id (tid) of zero. // This includes both immortal objects and objects whose reference count @@ -215,7 +220,7 @@ uint32_t ob_ref_local; // local reference count Py_ssize_t ob_ref_shared; // shared (atomic) reference count PyTypeObject *ob_type; -}; +} Py_ALIGNED(1 << _PyObject_ALIGNMENT_SHIFT); #endif /* Cast argument to PyObject* type. */ --- /dev/null +++ b/Misc/NEWS.d/next/Build/2024-12-04-10-00-35.gh-issue-127545.t0THjE.rst @@ -0,0 +1 @@ +Fix crash when building on Linux/m68k.