lua: add flex array field to TString type

Linux 6.10+ with CONFIG_FORTIFY_SOURCE notices memcpy() accessing past
the end of TString, because it has no indication that there there may be
an additional allocation there.

There's no appropriate upstream change for this (ancient) version of
Lua, so this is the narrowest change I could come up with to add a flex
array field to the end of TString to satisfy the check. It's loosely
based on changes from lua/lua@ca41b43f and lua/lua@9514abc2.

Sponsored-by: https://despairlabs.com/sponsor/
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Closes openzfs#16541
Closes openzfs#16583

module/lua/lobject.h

@@ -404,19 +404,22 @@ typedef TValue *StkId;  /* index to stack elements */
 /*
 ** Header for string value; string bytes follow the end of this structure
 */
-typedef union TString {
-  L_Umaxalign dummy;  /* ensures maximum alignment for strings */
-  struct {
-    CommonHeader;
-    lu_byte extra;  /* reserved words for short strings; "has hash" for longs */
-    unsigned int hash;
-    size_t len;  /* number of characters in string */
-  } tsv;
+typedef struct TString {
+  union {
+    L_Umaxalign dummy;  /* ensures maximum alignment for strings */
+    struct {
+      CommonHeader;
+      lu_byte extra;  /* reserved words for short strings; "has hash" for longs */
+      unsigned int hash;
+      size_t len;  /* number of characters in string */
+    } tsv;
+  };
+  char contents[];
 } TString;
 
 
 /* get the actual string (array of bytes) from a TString */
-#define getstr(ts)	cast(const char *, (ts) + 1)
+#define getstr(ts)	((ts)->contents)
 
 /* get the actual string (array of bytes) from a Lua value */
 #define svalue(o)       getstr(rawtsvalue(o))

module/lua/lstate.h

@@ -185,7 +185,7 @@ struct lua_State {
 */
 union GCObject {
   GCheader gch;  /* common header */
-  union TString ts;
+  struct TString ts;
   union Udata u;
   union Closure cl;
   struct Table h;

module/lua/lstring.c

@@ -103,7 +103,7 @@ static TString *createstrobj (lua_State *L, const char *str, size_t l,
   ts->tsv.len = l;
   ts->tsv.hash = h;
   ts->tsv.extra = 0;
-  sbuf = (char *)(TString *)(ts + 1);
+  sbuf = ts->contents;
   memcpy(sbuf, str, l*sizeof(char));
   sbuf[l] = '\0';  /* ending 0 */
   return ts;

module/lua/lstring.h

@@ -12,7 +12,7 @@
 #include "lstate.h"
 
 
-#define sizestring(s)	(sizeof(union TString)+((s)->len+1)*sizeof(char))
+#define sizestring(s)	(sizeof(struct TString)+((s)->len+1)*sizeof(char))
 
 #define sizeudata(u)	(sizeof(union Udata)+(u)->len)