quota: Fix maximum quota limit settings

Currently quota format that supports 64-bit usage sets maximum quota
limit as 2^64-1. However quota core code uses signed numbers to track
usage and even limits themselves are stored in long long. Checking of
maximum allowable limits worked by luck until commit 14bf61ffe6
(quota: Switch ->get_dqblk() and ->set_dqblk() to use bytes as space
units) because variable we compared with was unsigned. After that commit
the type we compared against changed to signed and thus checks for
maximum limits with the newest VFS quota format started to refuse any
non-negative value. Later the problem was inadvertedly fixed by commit
b10a08194c (quota: Store maximum space limit in bytes) because we
started to compare against unsigned type as well.

Fix possible future problems of this kind by setting maximum limits to
2^63-1 to avoid overflow issues.

Reported-by: Carlos Carvalho <carlos@fisica.ufpr.br>
Signed-off-by: Jan Kara <jack@suse.cz>

fs/quota/quota_v2.c

@@ -117,12 +117,16 @@ static int v2_read_file_info(struct super_block *sb, int type)
 	qinfo = info->dqi_priv;
 	if (version == 0) {
 		/* limits are stored as unsigned 32-bit data */
-		info->dqi_max_spc_limit = 0xffffffffULL << QUOTABLOCK_BITS;
+		info->dqi_max_spc_limit = 0xffffffffLL << QUOTABLOCK_BITS;
 		info->dqi_max_ino_limit = 0xffffffff;
 	} else {
-		/* used space is stored as unsigned 64-bit value in bytes */
-		info->dqi_max_spc_limit = 0xffffffffffffffffULL; /* 2^64-1 */
-		info->dqi_max_ino_limit = 0xffffffffffffffffULL;
+		/*
+		 * Used space is stored as unsigned 64-bit value in bytes but
+		 * quota core supports only signed 64-bit values so use that
+		 * as a limit
+		 */
+		info->dqi_max_spc_limit = 0x7fffffffffffffffLL; /* 2^63-1 */
+		info->dqi_max_ino_limit = 0x7fffffffffffffffLL;
 	}
 	info->dqi_bgrace = le32_to_cpu(dinfo.dqi_bgrace);
 	info->dqi_igrace = le32_to_cpu(dinfo.dqi_igrace);