bug#11288: avoid buffer overrun in display code

[Eli Zaretskii]

> From: Jim Meyering <jim@meyering.net>
> Date: Fri, 20 Apr 2012 13:42:05 +0200
> Cc: Jakub Jelinek <jakub@redhat.com>,
>       Richard Guenther <richard.guenther@gmail.com>
> 
> This bug leads to a seemingly unterminated loop in swap_glyph_pointers,
> when compiled with gcc-4.8.0 (from April 19 or newer).
> At first I thought it was a code-gen bug and reported it as
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53053.  But then Richard
> Guenther guessed at the cause and Jakub Jelinek confirmed that the
> seemingly-infinite-loop was in fact just part of the undefined behavior
> we may now expect from buggy code.
> 
> 2012-04-20  Jim Meyering  <meyering@redhat.com>
> 
>       * dispextern.h (glyph_row.used): Increase size by 1, to avoid buffer
>         overrun in swap_glyph_pointers, which reads and writes 
> used[LAST_AREA].
>         Reported as a gcc bug 
> http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53053
>         where Jakub Jelinek spotted the root cause.
> 
> === modified file 'src/dispextern.h'
> --- src/dispextern.h  2012-03-26 05:43:05 +0000
> +++ src/dispextern.h  2012-04-20 11:14:29 +0000
> @@ -748,7 +748,7 @@
>    struct glyph *glyphs[1 + LAST_AREA];
> 
>    /* Number of glyphs actually filled in areas.  */
> -  short used[LAST_AREA];
> +  short used[1 + LAST_AREA];
> 
>    /* Window-relative x and y-position of the top-left corner of this
>       row.  If y < 0, this means that eabs (y) pixels of the row are

Thanks, I fixed it somewhat differently, see below.

=== modified file 'src/ChangeLog'
--- src/ChangeLog       2012-04-20 06:39:29 +0000
+++ src/ChangeLog       2012-04-20 14:07:46 +0000
@@ -1,3 +1,8 @@
+2012-04-20  Eli Zaretskii  <eliz@gnu.org>
+
+       * dispnew.c (swap_glyph_pointers, copy_row_except_pointers): Don't
+       overrun array limits of glyph row's used[] array.  (Bug#11288)
+
 2012-04-20  Chong Yidong  <cyd@gnu.org>
 
        * process.c (wait_reading_process_output): If EIO occurs on a pty,

=== modified file 'src/dispnew.c'
--- src/dispnew.c       2012-03-20 08:52:11 +0000
+++ src/dispnew.c       2012-04-20 14:04:13 +0000
@@ -1085,12 +1085,16 @@ swap_glyph_pointers (struct glyph_row *a
   for (i = 0; i < LAST_AREA + 1; ++i)
     {
       struct glyph *temp = a->glyphs[i];
-      short used_tem = a->used[i];
 
       a->glyphs[i] = b->glyphs[i];
       b->glyphs[i] = temp;
-      a->used[i] = b->used[i];
-      b->used[i] = used_tem;
+      if (i < LAST_AREA)
+       {
+         short used_tem = a->used[i];
+
+         a->used[i] = b->used[i];
+         b->used[i] = used_tem;
+       }
     }
   a->hash = b->hash;
   b->hash = hash_tem;
@@ -1105,7 +1109,7 @@ static inline void
 copy_row_except_pointers (struct glyph_row *to, struct glyph_row *from)
 {
   struct glyph *pointers[1 + LAST_AREA];
-  short used[1 + LAST_AREA];
+  short used[LAST_AREA];
   unsigned hashval;
 
   /* Save glyph pointers of TO.  */