linux-kernel - [PATCH] fat: Relax checks for sector size and media type

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Date:   Sun,  2 Sep 2018 15:19:32 +0200
From:   Pali Rohár <pali.rohar@...il.com>
To:     OGAWA Hirofumi <hirofumi@...l.parknet.co.jp>
Cc:     linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org
Subject: [PATCH] fat: Relax checks for sector size and media type

Windows fastfat.sys driver accepts also media types 0x00 and 0x01 and
sector sizes 128 and 256 bytes. Linux mkfs.fat can format disk also to
larger FAT sector sizes then 4096 bytes, therefore relax also upper limit
restriction.

Signed-off-by: Pali Rohár <pali.rohar@...il.com>

---
Source code of Windows 10 Anniversary Update fastfat.sys driver is now
available on github.

Check for valid media types in fastfat.sys is there:
https://github.com/Microsoft/Windows-driver-samples/blob/96eb96dfb613e4c745db6bd1f53a92fe7e2290fc/filesys/fastfat/fsctrl.c#L2601-L2611

And check for valid sector size is there:
https://github.com/Microsoft/Windows-driver-samples/blob/96eb96dfb613e4c745db6bd1f53a92fe7e2290fc/filesys/fastfat/fsctrl.c#L2542-L2547
---
 fs/fat/inode.c           | 4 +---
 include/linux/msdos_fs.h | 2 +-
 2 files changed, 2 insertions(+), 4 deletions(-)

diff --git a/fs/fat/inode.c b/fs/fat/inode.c
index d6b81e31..f2556f71 100644
--- a/fs/fat/inode.c
+++ b/fs/fat/inode.c
@@ -1506,9 +1506,7 @@ static int fat_read_bpb(struct super_block *sb, struct fat_boot_sector *b,
 		goto out;
 	}
 
-	if (!is_power_of_2(bpb->fat_sector_size)
-	    || (bpb->fat_sector_size < 512)
-	    || (bpb->fat_sector_size > 4096)) {
+	if (!is_power_of_2(bpb->fat_sector_size)) {
 		if (!silent)
 			fat_msg(sb, KERN_ERR, "bogus logical sector size %u",
 			       (unsigned)bpb->fat_sector_size);
diff --git a/include/linux/msdos_fs.h b/include/linux/msdos_fs.h
index b7a5d4c7..e5b3f613 100644
--- a/include/linux/msdos_fs.h
+++ b/include/linux/msdos_fs.h
@@ -7,6 +7,6 @@
 /* media of boot sector */
 static inline int fat_valid_media(u8 media)
 {
-	return 0xf8 <= media || media == 0xf0;
+	return 0xf8 <= media || media == 0xf0 || media == 0x00 || media == 0x01;
 }
 #endif /* !_LINUX_MSDOS_FS_H */
-- 
2.11.0