kernel 2.4.0-0.34 (2.4.0-test9) is where this fails.

Persists in 2.4.0-test10.

Works in test11 on an ia32 box.

Oddly, the filesystem was made on an ia32, so it's not a problem with mkdosfs.

I've never seen this fail on IA-32, regardless of kernel.

Sizeof(fat_boot_sector,fat_boot_fsinfo,msdos_dir_entry,msdos_dir_slot) are all 
identical between IA-32 and IA-64.  vfat_slot_info is different, but only 
because loff_t changes size, but that isn't stored on-disk (I don't think).

#define MODULE
#define __KERNEL__
#define MODVERSIONS
#include <linux/module.h>

#include <linux/modversions.h>


#include <linux/config.h>
#include <linux/fs.h>
#include <linux/kernel.h>
#include <linux/string.h>

#include <linux/msdos_fs.h>

int
init_module(void)
{
  printk("sizeof(struct fat_boot_sector) = %d\n", sizeof(struct fat_boot_sector)
);
  printk("sizeof(struct fat_boot_fsinfo) = %d\n", sizeof(struct fat_boot_fsinfo)
);
  printk("sizeof(struct msdos_dir_entry) = %d\n", sizeof(struct msdos_dir_entry)
);
  printk("sizeof(struct msdos_dir_slot) = %d\n", sizeof(struct msdos_dir_slot));
  printk("sizeof(struct vfat_slot_info) = %d\n", sizeof(struct vfat_slot_info));

  return 0;
}

void
cleanup_module(void)
{
}

Fixed in kernel24-2.4.0-0.41 (and 2.2.18pre23, FWIW).