[PATCH 1/2] filetype: Improve FAT detection

[Franck Jullien]

We may have some disk with MBR as a first sector. In this case, the
current FAT check returns an error. However, the FAT sector exist and
the MBR can tell us where it is.

This patch make the FAT fs try to find the FAT boot sector on the first
sector of the first partition in case it is not on sector 0.

Signed-off-by: Franck Jullien <franck.jullien at gmail.com>
---
 common/filetype.c  |   55 +++++++++++++++++++++++++++++++++++++++++----------
 include/filetype.h |    1 +
 2 files changed, 45 insertions(+), 11 deletions(-)

diff --git a/common/filetype.c b/common/filetype.c
index e736d43..d07505b 100644
--- a/common/filetype.c
+++ b/common/filetype.c
@@ -26,6 +26,8 @@
 #include <fcntl.h>
 #include <fs.h>
 #include <malloc.h>
+#include <fat.h>
+#include <errno.h>
 
 static const char *filetype_str[] = {
 	[filetype_unknown] = "unknown",
@@ -52,26 +54,45 @@ const char *file_type_to_string(enum filetype f)
 	return NULL;
 }
 
-static int is_fat(u8 *buf)
+int fd;
+
+#define MBR_StartSector		8	/* MBR: Offset of Starting Sector in Partition Table Entry */
+#define BS_55AA			510	/* Boot sector signature (2) */
+#define MBR_Table		446	/* MBR: Partition table offset (2) */
+#define BS_FilSysType32		82	/* File system type (1) */
+#define BS_FilSysType		54	/* File system type (1) */
+
+int is_fat_boot_sector(unsigned char *sector, unsigned long *bootsec)
 {
-	if (get_unaligned_le16(&buf[510]) != 0xAA55)
+	if(bootsec)
+		*bootsec = 0;
+
+	/* Check record signature (always placed at offset 510 even if the sector size is>512) */
+	if (get_unaligned_le16(&sector[BS_55AA]) != 0xAA55)
+		return -ENODEV;
+
+	/* Check "FAT" string */
+	if ((get_unaligned_le32(&sector[BS_FilSysType]) & 0xFFFFFF) == 0x544146)
 		return 0;
 
-	/* FAT */
-	if ((get_unaligned_le32(&buf[54]) & 0xFFFFFF) == 0x544146)
-		return 1;
+	if ((get_unaligned_le32(&sector[BS_FilSysType32]) & 0xFFFFFF) == 0x544146)
+		return 0;
 
-	/* FAT32 */
-	if ((get_unaligned_le32(&buf[82]) & 0xFFFFFF) == 0x544146)
-		return 1;
+	if(bootsec)
+		/* This must be an MBR, so return the starting sector of the
+		 * first partition so we could check if there is a FAT boot
+		 * sector there */
+		*bootsec = get_unaligned_le16(&sector[MBR_Table + MBR_StartSector]);
 
-	return 0;
+	return -ENODEV;
 }
 
 enum filetype file_detect_type(void *_buf)
 {
 	u32 *buf = _buf;
 	u8 *buf8 = _buf;
+	unsigned long bootsec;
+	int ret;
 
 	if (strncmp(buf8, "#!/bin/sh", 9) == 0)
 		return filetype_sh;
@@ -99,7 +120,19 @@ enum filetype file_detect_type(void *_buf)
 		return filetype_aimage;
 	if (strncmp(buf8 + 0x10, "barebox", 7) == 0)
 		return filetype_mips_barebox;
-	if (is_fat(buf8))
+
+	ret = is_fat_boot_sector(buf8, &bootsec);
+	if (ret && bootsec != 0) {
+		/* The first sector is an MBR, now check for FAT in
+		 * the first partition */
+		lseek(fd, (bootsec) * 512, SEEK_SET);
+		ret = read(fd, buf, 512);
+		if (ret < 0)
+			return filetype_unknown;
+		ret = is_fat_boot_sector(buf8, NULL);
+		if (!ret)
+			return filetype_fat;
+	} else
 		return filetype_fat;
 
 	return filetype_unknown;
@@ -107,7 +140,7 @@ enum filetype file_detect_type(void *_buf)
 
 enum filetype file_name_detect_type(const char *filename)
 {
-	int fd, ret;
+	int ret;
 	void *buf;
 	enum filetype type = filetype_unknown;
 
diff --git a/include/filetype.h b/include/filetype.h
index 179ec0f..c60f654 100644
--- a/include/filetype.h
+++ b/include/filetype.h
@@ -24,5 +24,6 @@ enum filetype {
 const char *file_type_to_string(enum filetype f);
 enum filetype file_detect_type(void *_buf);
 enum filetype file_name_detect_type(const char *filename);
+int is_fat_boot_sector(unsigned char *sector, unsigned long *bootsec);
 
 #endif /* __FILE_TYPE_H */
-- 
1.7.1