[PATCH libX11] Use flexible array member instead of fake size.

[Michal Srb]

The _XimCacheStruct structure is followed in memory by two strings containing
fname and encoding. The memory was accessed using the last member of the
structure `char fname[1]`. That is a lie, prohibits us from using sizeof and
confuses checkers. Lets declare it properly as a flexible array, so compilers
don't complain about writing past that array. As bonus we can replace the
XOffsetOf with regular sizeof.

Fixes GCC8 error:
  In function 'strcpy',
      inlined from '_XimWriteCachedDefaultTree' at imLcIm.c:479:5,
      inlined from '_XimCreateDefaultTree' at imLcIm.c:616:2,
      inlined from '_XimLocalOpenIM' at imLcIm.c:700:5:
  /usr/include/bits/string_fortified.h:90:10: error: '__builtin_strcpy'
  forming offset 2 is out of the bounds [0, 1] [-Werror=array-bounds]
     return __builtin___strcpy_chk (__dest, __src, __bos (__dest));

Caused by this line seemingly writing past the fname[1] array:
  imLcIm.c:479:  strcpy (m->fname+strlen(name)+1, encoding);
---
 modules/im/ximcp/imLcIm.c | 8 ++++----
 1 file changed, 4 insertions(+), 4 deletions(-)

diff --git a/modules/im/ximcp/imLcIm.c b/modules/im/ximcp/imLcIm.c
index c19695df..743df77b 100644
--- a/modules/im/ximcp/imLcIm.c
+++ b/modules/im/ximcp/imLcIm.c
@@ -82,8 +82,8 @@ struct _XimCacheStruct {
     DTCharIndex     mbused;
     DTCharIndex     wcused;
     DTCharIndex     utf8used;
-    char            fname[1];
-    /* char encoding[1] */
+    char            fname[];
+    /* char encoding[] */
 };
 
 static struct  _XimCacheStruct* _XimCache_mmap = NULL;
@@ -281,7 +281,7 @@ _XimReadCachedDefaultTree(
     assert (m->id == XIM_CACHE_MAGIC);
     assert (m->version == XIM_CACHE_VERSION);
     if (size != m->size ||
-	size < XOffsetOf (struct _XimCacheStruct, fname) + namelen + encodinglen) {
+	size < sizeof (struct _XimCacheStruct) + namelen + encodinglen) {
 	fprintf (stderr, "Ignoring broken XimCache %s [%s]\n", name, encoding);
         munmap (m, size);
         return False;
@@ -442,7 +442,7 @@ _XimWriteCachedDefaultTree(
     int   fd;
     FILE *fp;
     struct _XimCacheStruct *m;
-    int   msize = (XOffsetOf(struct _XimCacheStruct, fname)
+    int   msize = (sizeof(struct _XimCacheStruct)
 		   + strlen(name) + strlen(encoding) + 2
 		   + XIM_CACHE_TREE_ALIGNMENT-1) & -XIM_CACHE_TREE_ALIGNMENT;
     DefTreeBase *b = &im->private.local.base;
-- 
2.13.6