lua-5.1 restorestack memory handling bug

[Marko Lindqvist <cazfi74@...>]

Freeciv ( http://www.freeciv.org/ ) source tree includes copy of
lua-5.1. We got a bug report about build failure, and this might be a
real bug (at least on some platforms and compiler options)

I managed to reproduce myself building with clang, and I think this
error message describes problem best:

dependencies/lua-5.1/src/ldebug.c:620:21: error: cast
from 'char ' to 'TValue ' (aka 'struct lua_TValue *') increases required
alignment from 1 to 8 [-Werror,-Wcast-align]
StkId errfunc = restorestack(L, L->errfunc);
^~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from
../../../../../src.patched/dependencies/lua-5.1/src/ldebug.c:21:
../../../../../src.patched/dependencies/lua-5.1/src/ldo.h:25:28: note:
instantiated from:
#define restorestack(L,n) ((TValue )((char )L->stack + (n)))
^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


So, "L->stack + (n)" is calculated as char (1 byte space reserved),
but then cast to pointer (8 bytes). In theory that pointer now has 7*8
of its bits in undefined state.


Original bug report: http://gna.org/bugs/?18481


 - ML