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

[Walter Harms]

> Michal Srb <msrb at suse.com> hat am 15. März 2018 um 09:50 geschrieben:
> 
> 
> 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[] */
>  };


looks good to me.

re,
 wh

>  
>  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
> 
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