ext4: reject too-large filesystems on 32-bit kernels

ext4 will happily mount a > 16T filesystem on a 32-bit box, but
this is not safe; writes to the block device will wrap past 16T
and the page cache can't index past 16T (232 index * 4k pages).

Adding another test to the existing "too many sectors" test
should do the trick.

Add a comment, a relevant return value, and fix the reference
to the CONFIG_LBD(AF) option as well.
Signed-off-by: NEric Sandeen <sandeen@redhat.com>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>

fs/ext4/super.c

@@ -2550,12 +2550,19 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 		goto failed_mount;
 	}
 
-	if (ext4_blocks_count(es) >
-		    (sector_t)(~0ULL) >> (sb->s_blocksize_bits - 9)) {
+	/*
+	 * Test whether we have more sectors than will fit in sector_t,
+	 * and whether the max offset is addressable by the page cache.
+	 */
+	if ((ext4_blocks_count(es) >
+	     (sector_t)(~0ULL) >> (sb->s_blocksize_bits - 9)) ||
+	    (ext4_blocks_count(es) >
+	     (pgoff_t)(~0ULL) >> (PAGE_CACHE_SHIFT - sb->s_blocksize_bits))) {
 		ext4_msg(sb, KERN_ERR, "filesystem"
-			" too large to mount safely");
+			 " too large to mount safely on this system");
 		if (sizeof(sector_t) < 8)
 			ext4_msg(sb, KERN_WARNING, "CONFIG_LBDAF not enabled");
+		ret = -EFBIG;
 		goto failed_mount;
 	}