bug#42530: 28.0.50; Integer overflows in alloc.c on macOS

[Philipp]

-fsanitize=undefined finds the following integer overflows in alloc.c:

alloc.c:4641:33: runtime error: addition of unsigned offset to 0x000102496c05 
overflowed to 0x000102496c00
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior alloc.c:4641:33 in 
alloc.c:4852:9: runtime error: pointer index expression with base 
0xffffffffffffffff overflowed to 0x00010344053f
SUMMARY: UndefinedBehaviorSanitizer: undefined-behavior alloc.c:4852:9 in 

I briefly checked the code, but couldn't find anything obviously wrong.
Note that UBSan also checks for unsigned integer overflows, which are
technically not undefined, but might still be fishy.  If these overflows
are intended, we should probably use INT_ADD_WRAPV to make that clear
and suppress the sanitizer.


In GNU Emacs 28.0.50 (build 66, x86_64-apple-darwin19.5.0, NS appkit-1894.50 
Version 10.15.5 (Build 19F101))
 of 2020-07-25
Repository revision: 3b44829823f43d3736b8ec9db2258eeff7f6c16a
Repository branch: master
Windowing system distributor 'Apple', version 10.3.1894
System Description:  Mac OS X 10.15.5

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --with-modules --without-xml2 --without-pop --with-mailutils
 --enable-gcc-warnings=warn-only --enable-checking=all
 --enable-check-lisp-object-type 'CFLAGS=-g3 -O1 -fsanitize=address
 -fsanitize=undefined -fno-omit-frame-pointer''

Configured features:
JPEG TIFF GIF PNG NOTIFY KQUEUE ACL GNUTLS ZLIB TOOLKIT_SCROLL_BARS NS
MODULES THREADS JSON PDUMPER LCMS2

Important settings:
  value of $LANG: de_DE.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc dired dired-loaddefs rfc822
mml easymenu mml-sec epa epg epg-config gnus-util rmail rmail-loaddefs
text-property-search time-date mm-decode mm-bodies mm-encode mail-parse
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ietf-drums mm-util mail-prsvr mail-utils phst skeleton derived edmacro
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password-cache json map url-vars mailcap subr-x rx gnutls puny seq
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ansi-color ring cl-loaddefs cl-lib tooltip eldoc electric uniquify
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custom widget hashtable-print-readable backquote threads kqueue cocoa ns
lcms2 multi-tty make-network-process emacs)

Memory information:
((conses 16 69705 5415)
 (symbols 48 8650 1)
 (strings 32 23527 1769)
 (string-bytes 1 768093)
 (vectors 16 14130)
 (vector-slots 8 172256 4253)
 (floats 8 25 30)
 (intervals 56 210 0)
 (buffers 992 10))