ubifs: correct UBIFS_DFS_DIR_LEN macro definition and improve code clarity

The UBIFS_DFS_DIR_LEN macro, which defines the maximum length of the UBIFS
debugfs directory name, has an incorrect formula and misleading comments.
The current formula is (3 + 1 + 2*2 + 1), which assumes that both UBI device
number and volume ID are limited to 2 characters. However, UBI device number
ranges from 0 to 31 (2 characters), and volume ID ranges from 0 to 127 (up
to 3 characters).

Although the current code works due to the cancellation of mathematical
errors (9 + 1 = 10, which matches the correct UBIFS_DFS_DIR_LEN value), it
can lead to confusion and potential issues in the future.

This patch aims to improve the code clarity and maintainability by making
the following changes:

1. Corrects the UBIFS_DFS_DIR_LEN macro definition to (3 + 1 + 2 + 3 + 1),
   accommodating the maximum lengths of both UBI device number and volume ID,
   plus the separators and null terminator.
2. Updates the snprintf calls to use UBIFS_DFS_DIR_LEN instead of
   UBIFS_DFS_DIR_LEN + 1, removing the unnecessary +1.
3. Modifies the error checks to compare against UBIFS_DFS_DIR_LEN using >=
   instead of >, aligning with the corrected macro definition.
4. Removes the redundant +1 in the dfs_dir_name array definitions in ubi.h
   and debug.h.

While these changes do not affect the runtime behavior, they make the code
more readable, maintainable, and less prone to future errors.

v2->v3:

 - Removes the duplicated UBIFS_DFS_DIR_LEN and UBIFS_DFS_DIR_NAME macro
   definitions in ubifs.h, as they are already defined in debug.h.
Signed-off-by: ZhaoLong Wang <wangzhaolong1@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/debug.c

@@ -598,9 +598,9 @@ int ubi_debugfs_init_dev(struct ubi_device *ubi)
 	if (!IS_ENABLED(CONFIG_DEBUG_FS))
 		return 0;
 
-	n = snprintf(d->dfs_dir_name, UBI_DFS_DIR_LEN + 1, UBI_DFS_DIR_NAME,
+	n = snprintf(d->dfs_dir_name, UBI_DFS_DIR_LEN, UBI_DFS_DIR_NAME,
 		     ubi->ubi_num);
-	if (n > UBI_DFS_DIR_LEN) {
+	if (n >= UBI_DFS_DIR_LEN) {
 		/* The array size is too small */
 		return -EINVAL;
 	}

drivers/mtd/ubi/ubi.h

@@ -420,7 +420,7 @@ struct ubi_debug_info {
 	unsigned int power_cut_min;
 	unsigned int power_cut_max;
 	unsigned int emulate_failures;
-	char dfs_dir_name[UBI_DFS_DIR_LEN + 1];
+	char dfs_dir_name[UBI_DFS_DIR_LEN];
 	struct dentry *dfs_dir;
 	struct dentry *dfs_chk_gen;
 	struct dentry *dfs_chk_io;

fs/ubifs/debug.c

@@ -2827,9 +2827,9 @@ void dbg_debugfs_init_fs(struct ubifs_info *c)
 	const char *fname;
 	struct ubifs_debug_info *d = c->dbg;
 
-	n = snprintf(d->dfs_dir_name, UBIFS_DFS_DIR_LEN + 1, UBIFS_DFS_DIR_NAME,
+	n = snprintf(d->dfs_dir_name, UBIFS_DFS_DIR_LEN, UBIFS_DFS_DIR_NAME,
 		     c->vi.ubi_num, c->vi.vol_id);
-	if (n > UBIFS_DFS_DIR_LEN) {
+	if (n >= UBIFS_DFS_DIR_LEN) {
 		/* The array size is too small */
 		return;
 	}

fs/ubifs/debug.h

@@ -19,10 +19,11 @@ typedef int (*dbg_znode_callback)(struct ubifs_info *c,
 
 /*
  * The UBIFS debugfs directory name pattern and maximum name length (3 for "ubi"
- * + 1 for "_" and plus 2x2 for 2 UBI numbers and 1 for the trailing zero byte.
+ * + 1 for "_" and 2 for UBI device numbers and 3 for volume number and 1 for
+ * the trailing zero byte.
  */
 #define UBIFS_DFS_DIR_NAME "ubi%d_%d"
-#define UBIFS_DFS_DIR_LEN  (3 + 1 + 2*2 + 1)
+#define UBIFS_DFS_DIR_LEN  (3 + 1 + 2 + 3 + 1)
 
 /**
  * ubifs_debug_info - per-FS debugging information.
@@ -103,7 +104,7 @@ struct ubifs_debug_info {
 	unsigned int chk_fs:1;
 	unsigned int tst_rcvry:1;
 
-	char dfs_dir_name[UBIFS_DFS_DIR_LEN + 1];
+	char dfs_dir_name[UBIFS_DFS_DIR_LEN];
 	struct dentry *dfs_dir;
 	struct dentry *dfs_dump_lprops;
 	struct dentry *dfs_dump_budg;

fs/ubifs/sysfs.c

@@ -91,17 +91,17 @@ static struct kset ubifs_kset = {
 int ubifs_sysfs_register(struct ubifs_info *c)
 {
 	int ret, n;
-	char dfs_dir_name[UBIFS_DFS_DIR_LEN+1];
+	char dfs_dir_name[UBIFS_DFS_DIR_LEN];
 
 	c->stats = kzalloc(sizeof(struct ubifs_stats_info), GFP_KERNEL);
 	if (!c->stats) {
 		ret = -ENOMEM;
 		goto out_last;
 	}
-	n = snprintf(dfs_dir_name, UBIFS_DFS_DIR_LEN + 1, UBIFS_DFS_DIR_NAME,
+	n = snprintf(dfs_dir_name, UBIFS_DFS_DIR_LEN, UBIFS_DFS_DIR_NAME,
 		     c->vi.ubi_num, c->vi.vol_id);
 
-	if (n > UBIFS_DFS_DIR_LEN) {
+	if (n >= UBIFS_DFS_DIR_LEN) {
 		/* The array size is too small */
 		ret = -EINVAL;
 		goto out_free;

fs/ubifs/ubifs.h

@@ -157,13 +157,6 @@
 #define UBIFS_HMAC_ARR_SZ 0
 #endif
 
-/*
- * The UBIFS sysfs directory name pattern and maximum name length (3 for "ubi"
- * + 1 for "_" and plus 2x2 for 2 UBI numbers and 1 for the trailing zero byte.
- */
-#define UBIFS_DFS_DIR_NAME "ubi%d_%d"
-#define UBIFS_DFS_DIR_LEN  (3 + 1 + 2*2 + 1)
-
 /*
  * Lockdep classes for UBIFS inode @ui_mutex.
  */