[Bug libc/3002] New: iconvconfig misalignes hash table on ARM

["mikpe at it dot uu dot se" <sourceware-bugzilla at sourceware dot org>]

Iconvconfig explicitly aligns the hash table part of its output file
(gconv-modules.cache) to a two-byte boundary. That's not OK for ARM
because the hash table is an array of structures, and those structures
have 4-byte alignment on ARM. When compiling for gcc's default CPU
type (the ARMv3 arm6 processor), gcc uses combinations of 4-byte loads
and shifts to read 2-byte fields from the hash table; this occurs in
__gconv_lookup_cache(). Due the broken code to align the hash table,
there is a very high probability (50%, depends on the size of the
preceeding string table) that each of these loads will cause an alignment
exception and require kernel fixup.

The fix is to use __alignof__ (struct hash_entry) not sizeof (gidx_t)
when aligning the hash table. This has no effect on other architectures
since they make __alignof__ (struct hash_entry) == __alignof__ (gidx_t).

--- glibc-2.3.6/iconv/iconvconfig.c.~1~ 2005-04-05 20:37:52.000000000 -0400
+++ glibc-2.3.6/iconv/iconvconfig.c     2006-07-28 09:59:22.000000000 -0400
@@ -1028,7 +1028,7 @@ write_output (void)
   size_t n;
   int idx;
   struct iovec iov[6];
-  static const gidx_t null_word;
+  static const char null_word[__alignof__ (struct hash_entry)];
   size_t total;
   char finalname[prefix_len + sizeof GCONV_MODULES_CACHE];
   char tmpfname[(output_file == NULL ? sizeof finalname : output_file_len + 1)
@@ -1193,11 +1193,11 @@ write_output (void)
   total += iov[1].iov_len;
 
   idx = 2;
-  if ((string_table_size & (sizeof (gidx_t) - 1)) != 0)
+  if ((string_table_size & (__alignof__ (struct hash_entry) - 1)) != 0)
     {
       iov[2].iov_base = (void *) &null_word;
-      iov[2].iov_len = (sizeof (gidx_t)
-                       - (string_table_size & (sizeof (gidx_t) - 1)));
+      iov[2].iov_len = (__alignof__ (struct hash_entry)
+                       - (string_table_size & (__alignof__ (struct hash_entry)
- 1)));
       total += iov[2].iov_len;
       ++idx;
     }

-- 
           Summary: iconvconfig misalignes hash table on ARM
           Product: glibc
           Version: 2.3.6
            Status: NEW
          Severity: normal
          Priority: P2
         Component: libc
        AssignedTo: drepper at redhat dot com
        ReportedBy: mikpe at it dot uu dot se
                CC: glibc-bugs at sources dot redhat dot com
 GCC build triplet: armv5b-unknown-linux
  GCC host triplet: armv5b-unknown-linux
GCC target triplet: armv5b-unknown-linux


http://sourceware.org/bugzilla/show_bug.cgi?id=3002

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