external/python3: Silence UBSan errors with --with-pydebug

...that happen when building ExternalProject_python3 itself after
12142490cd43f8568ab29e0ddfa75b334d6d39d5 "Enable Python Py_DEBUG setting when
built with --enable-dbgutil on Linux":

For one, silence

> Modules/posixmodule.c:14395:9: runtime error: left shift of 34 by 26 places cannot be represented in type 'int'
>  #0 in all_ins at workdir/UnpackedTarball/python3/./Modules/posixmodule.c:14395:9

where at least my kernel-headers-5.9.9-200.fc33.x86_64
/usr/include/linux/memfd.h has

> #define MFD_HUGE_16GB	HUGETLB_FLAG_ENCODE_16GB

and /usr/include/asm-generic/hugetlb_encode.h has

> #define HUGETLB_FLAG_ENCODE_16GB	(34 << HUGETLB_FLAG_ENCODE_SHIFT)

For another (and as predicted in 29d47d22c43e6adc1850b7db5880028dcd07d1b3 "Fix
passing --disable-optimized into external/python3": "in a Linux UBsan build,
making ExternalProject_python3 would have started to cause some 'applying zero
offset to null pointer' failures, but which would have been easy to fix"),
silence

> Objects/listobject.c:551:24: runtime error: applying zero offset to null pointer
>  #0 in list_concat at workdir/UnpackedTarball/python3/Objects/listobject.c:551:24

Change-Id: I0523cd35e393000c8e67629a0522b2db1d8c16f0
Reviewed-on: https://gerrit.libreoffice.org/c/core/+/106984
Tested-by: Jenkins
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/external/python3/ubsan.patch.0 b/external/python3/ubsan.patch.0
index 938acc1f1725..9f458b1e0f95 100644
--- a/external/python3/ubsan.patch.0
+++ b/external/python3/ubsan.patch.0
@@ -18,3 +18,26 @@
  
  	/*	Set the carry bit if the function uses any sse registers.
  		This is clc or stc, together with the first byte of the jmp.  */
+--- Modules/posixmodule.c
++++ Modules/posixmodule.c
+@@ -23,6 +23,9 @@
+ };
+ 
+ static int
++#if defined __clang__
++__attribute__((no_sanitize("shift-base"))) // MFD_HUGE_16GB in /usr/include/linux/memfd.h
++#endif
+ all_ins(PyObject *m)
+ {
+ #ifdef F_OK
+--- Objects/listobject.c
++++ Objects/listobject.c
+@@ -548,7 +548,7 @@
+         dest[i] = v;
+     }
+     src = b->ob_item;
+-    dest = np->ob_item + Py_SIZE(a);
++    dest = Py_SIZE(a) == 0 ? np->ob_item : np->ob_item + Py_SIZE(a);
+     for (i = 0; i < Py_SIZE(b); i++) {
+         PyObject *v = src[i];
+         Py_INCREF(v);