linux-kernel - [PATCH] fat: Replaced 11 magic to MSDOS

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Date:   Sat, 24 Nov 2018 17:01:19 -0500
From:   Carmeli Tamir <carmeli.tamir@...il.com>
To:     axboe@...nel.dk, sergey.senozhatsky@...il.com, jthumshirn@...e.de,
        bvanassche@....org, martin.petersen@...cle.com,
        linux-kernel@...r.kernel.org, carmeli.tamir@...il.com
Subject: [PATCH] fat: Replaced 11 magic to MSDOS_NAME for volume label

The FAT file system volume label file stored in the root directory should
match the volume label field in the FAT boot sector. As consequence, the
max length of these fields ought to be the same. This patch replaces the
magic '11' usef in the struct fat_boot_sector with MSDOS_NAME,
which is used in struct msdos_dir_entry.

Please check the following references:
1. Microsoft FAT specification 2005 
(http://read.pudn.com/downloads77/ebook/294884/FAT32%20Spec%20%28SDA%20Contribution%29.pdf).
Search for 'volume label'.
2. Microsoft Extensible Firmware Initiative, FAT32 File System Specification
(https://staff.washington.edu/dittrich/misc/fatgen103.pdf). 
Search for 'volume label'.
3. User space code that creates FAT filesystem 
sometimes uses MSDOS_NAME for the label, sometimes not.
Search for 'if (memcmp(label, NO_NAME, MSDOS_NAME))'. 
I consider to make the same patch there as well.
https://github.com/dosfstools/dosfstools/blob/master/src/mkfs.fat.c

Signed-off-by: Carmeli Tamir <carmeli.tamir@...il.com>
---
 include/uapi/linux/msdos_fs.h | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/include/uapi/linux/msdos_fs.h b/include/uapi/linux/msdos_fs.h
index fde7537..1216e6c 100644
--- a/include/uapi/linux/msdos_fs.h
+++ b/include/uapi/linux/msdos_fs.h
@@ -135,7 +135,7 @@ struct fat_boot_sector {
 						   for mount state. */
 			__u8	signature;  /* extended boot signature */
 			__u8	vol_id[4];	/* volume ID */
-			__u8	vol_label[11];	/* volume label */
+			__u8	vol_label[MSDOS_NAME];	/* volume label */
 			__u8	fs_type[8];		/* file system type */
 			/* other fields are not added here */
 		} fat16;
@@ -158,7 +158,7 @@ struct fat_boot_sector {
 						   for mount state. */
 			__u8	signature;  /* extended boot signature */
 			__u8	vol_id[4];	/* volume ID */
-			__u8	vol_label[11];	/* volume label */
+			__u8	vol_label[MSDOS_NAME];	/* volume label */
 			__u8	fs_type[8];		/* file system type */
 			/* other fields are not added here */
 		} fat32;
-- 
2.7.4