Freeldr提供了对fat12、fat32、fatx、ntfs等文件系统的只读功能。这部分代码主要集中在boot\freeldr\freeldr\fs\fs.c文件中。
VOID BootMain (LPSTR CmdLine) ...... MachInit (CmdLine);FsInit ();...... RunLoader ();}
BootMain会对硬件(MachInit)和文件系统(FsInit)进行检测和初始化。所有准备工作进行完毕后就会调用RunLoader进行系统的加载工作。
VOID FsInit (VOID) ULONG i; RtlZeroMemory (FileData, sizeof (FileData));for (i = 0 ; i < MAX_FDS; i++)FileData[i].DeviceId = (ULONG)-1 ; InitializeListHead (&DeviceListHead);}
FsInit初始化FileData数组。和一个和磁盘分区相关的链表DeviceListHead。
typedef struct tagDEVVTBL { ARC_CLOSE Close; ARC_GET_FILE_INFORMATION GetFileInformation; ARC_OPEN Open; ARC_READ Read; ARC_SEEK Seek; LPCWSTR ServiceName; } DEVVTBL; typedef struct tagFILEDATA { ULONG DeviceId; ULONG ReferenceCount; const DEVVTBL* FuncTable; const DEVVTBL* FileFuncTable; VOID* Specific; } FILEDATA;
每一个成功打开的文件会返回一个文件句柄，这个句柄实际上就是FileData数组的索引。所以每个打开的文件都有一个对应的FileData。这个结构就类似windows中的FILE_OBJECT
typedef struct tagDEVICE { LIST_ENTRY ListEntry; const DEVVTBL* FuncTable; CHAR* Prefix; ULONG DeviceId; ULONG ReferenceCount; } DEVICE;
用户电脑中的每一个硬盘和硬盘中的每一分区都对应了一个DEVICE结构。
VOID FsRegisterDevice (CHAR* Prefix, const DEVVTBL* FuncTable) DEVICE* pNewEntry; ULONG dwLength; dwLength = strlen (Prefix) + 1 ; pNewEntry = MmHeapAlloc (sizeof (DEVICE) + dwLength); if (!pNewEntry)return ;pNewEntry->FuncTable = FuncTable; pNewEntry->ReferenceCount = 0 ; pNewEntry->Prefix = (CHAR*)(pNewEntry + 1 ); memcpy (pNewEntry->Prefix, Prefix, dwLength);InsertHeadList (&DeviceListHead, &pNewEntry->ListEntry);}
这么函数非常简单。Prefix就是Arc路径，FuncTable是操作这个分区（磁盘）对应的函数数组。FsRegisterDevice生成了一个DEVICE结构，把ArcName和FuncTable复制进去。之后连入了DeviceListHead链表。
static VOIDDetectBiosDisks (PCONFIGURATION_COMPONENT_DATA BusKey) BOOLEAN BootDriveReported = FALSE; ULONG i; ULONG DiskCount = GetDiskCount (BusKey); CHAR BootPath[512 ]; ...... for (i = 0 ; i < DiskCount; i++){ ULONG Size; CHAR Identifier[20 ]; ...... if (BootDrive == 0x80 + i)BootDriveReported = TRUE; GetHarddiskIdentifier (Identifier, 0x80 + i);} }
首先使用GetDiskCount从Freeldr注册表的System键中读取硬盘总数。System键的初始化在DetectSystem(freeldr\freeldr\arch\i386\hardware.c)中，一会儿再看。
static VOIDGetHarddiskIdentifier (PCHAR Identifier, ULONG DriveNumber) PMASTER_BOOT_RECORD Mbr; ULONG *Buffer; ULONG i; ULONG Checksum; ULONG Signature; CHAR ArcName[256 ]; PARTITION_TABLE_ENTRY PartitionTableEntry; if (!MachDiskReadLogicalSectors (DriveNumber, 0ULL , 1 , (PVOID)DISKREADBUFFER)){ DPRINTM (DPRINT_HWDETECT, "Reading MBR failed\n" );return ;} Buffer = (ULONG*)DISKREADBUFFER; Mbr = (PMASTER_BOOT_RECORD)DISKREADBUFFER; Signature = Mbr->Signature; DPRINTM (DPRINT_HWDETECT, "Signature: %x\n" , Signature);Checksum = 0 ; for (i = 0 ; i < 128 ; i++){ Checksum += Buffer[i]; } Checksum = ~Checksum + 1 ; DPRINTM (DPRINT_HWDETECT, "Checksum: %x\n" , Checksum);reactos_arc_disk_info[reactos_disk_count].Signature = Signature; reactos_arc_disk_info[reactos_disk_count].CheckSum = Checksum; sprintf (ArcName, "multi(0)disk(0)rdisk(%lu)" , reactos_disk_count);strcpy (reactos_arc_strings[reactos_disk_count], ArcName);reactos_arc_disk_info[reactos_disk_count].ArcName = reactos_arc_strings[reactos_disk_count]; reactos_disk_count++; sprintf (ArcName, "multi(0)disk(0)rdisk(%lu)partition(0)" , DriveNumber - 0x80 );FsRegisterDevice (ArcName, &DiskVtbl);i = 1 ; DiskReportError (FALSE);while (DiskGetPartitionEntry (DriveNumber, i, &PartitionTableEntry)){ if (PartitionTableEntry.SystemIndicator != PARTITION_ENTRY_UNUSED){ sprintf (ArcName, "multi(0)disk(0)rdisk(%lu)partition(%lu)" , DriveNumber - 0x80 , i);FsRegisterDevice (ArcName, &DiskVtbl);} i++; } DiskReportError (TRUE);Identifier[0 ] = Hex[(Checksum >> 28 ) & 0x0F ]; Identifier[1 ] = Hex[(Checksum >> 24 ) & 0x0F ]; Identifier[2 ] = Hex[(Checksum >> 20 ) & 0x0F ]; Identifier[3 ] = Hex[(Checksum >> 16 ) & 0x0F ]; Identifier[4 ] = Hex[(Checksum >> 12 ) & 0x0F ]; Identifier[5 ] = Hex[(Checksum >> 8 ) & 0x0F ]; Identifier[6 ] = Hex[(Checksum >> 4 ) & 0x0F ]; Identifier[7 ] = Hex[Checksum & 0x0F ]; Identifier[8 ] = '-' ; Identifier[9 ] = Hex[(Signature >> 28 ) & 0x0F ]; Identifier[10 ] = Hex[(Signature >> 24 ) & 0x0F ]; Identifier[11 ] = Hex[(Signature >> 20 ) & 0x0F ]; Identifier[12 ] = Hex[(Signature >> 16 ) & 0x0F ]; Identifier[13 ] = Hex[(Signature >> 12 ) & 0x0F ]; Identifier[14 ] = Hex[(Signature >> 8 ) & 0x0F ]; Identifier[15 ] = Hex[(Signature >> 4 ) & 0x0F ]; Identifier[16 ] = Hex[Signature & 0x0F ]; Identifier[17 ] = '-' ; Identifier[18 ] = 'A' ; Identifier[19 ] = 0 ; }
函数首先使用MachDiskReadLogicalSectors读取指定硬盘的MBR。对于PC机而言MachDiskReadLogicalSectors使用int 13h中断实现对硬盘的读操作。里面包括了16、32位代码的互转，和本节内容无关，以后再做说明。http://en.wikipedia.org/wiki/Master_boot_record )
typedef struct _MASTER_BOOT_RECORD { UCHAR MasterBootRecordCodeAndData[0x1b8 ]; ULONG Signature; USHORT Reserved; PARTITION_TABLE_ENTRY PartitionTable[4 ]; USHORT MasterBootRecordMagic; } MASTER_BOOT_RECORD, *PMASTER_BOOT_RECORD;
GetHarddiskIdentifier在获取了Signature、计算了Checksum后 。
i = 1 ; while (DiskGetPartitionEntry (DriveNumber, i, &PartitionTableEntry)){ if (PartitionTableEntry.SystemIndicator != PARTITION_ENTRY_UNUSED){ sprintf (ArcName, "multi(0)disk(0)rdisk(%lu)partition(%lu)" , DriveNumber - 0x80 , i);FsRegisterDevice (ArcName, &DiskVtbl);} i++; }
DiskGetParititionEntry将会解析DriveNumber对应磁盘的分区表，填充第i个分区的信息到PartitionTableEntry结构。如果分区存在则使用FsRegisterDevice注册分区。
static const DEVVTBL DiskVtbl = {DiskClose, DiskGetFileInformation, DiskOpen, DiskRead, DiskSeek, };
这里面包含了对磁盘扇区读写的全部函数。我们之后再介绍。
LONG ArcOpen (CHAR* Path, OPENMODE OpenMode, ULONG* FileId) ...... *FileId = MAX_FDS; FileName = strrchr (Path, ')' ); if (!FileName)return EINVAL;FileName++; dwCount = 0 ; for (p = Path; p != FileName; p++)if (*p == '(' && *(p + 1 ) == ')' )dwCount++; dwLength = FileName - Path + dwCount; if (dwCount != 0 ){ DeviceName = MmHeapAlloc (FileName - Path + dwCount); if (!DeviceName)return ENOMEM;for (p = Path, q = DeviceName; p != FileName; p++){ *q++ = *p; if (*p == '(' && *(p + 1 ) == ')' )*q++ = '0' ; } } else DeviceName = Path; ......
这个函数有三个参数Path是文件名的Arc路径，如multi(0)disk(0)rdisk(0)partition(1)Freeldr.sys就表示C盘中的Freeldr.sys文件。
...... pEntry = DeviceListHead.Flink; while (pEntry != &DeviceListHead){ pDevice = CONTAINING_RECORD (pEntry, DEVICE, ListEntry); if (strncmp (pDevice->Prefix, DeviceName, dwLength) == 0 ){ if (pDevice->ReferenceCount == 0 ){ for (DeviceId = 0 ; DeviceId < MAX_FDS; DeviceId++)if (!FileData[DeviceId].FuncTable)break ;if (DeviceId == MAX_FDS)return EMFILE;FileData[DeviceId].FuncTable = pDevice->FuncTable; ret = pDevice->FuncTable->Open (pDevice->Prefix, DeviceOpenMode, &DeviceId); if (ret != ESUCCESS){ FileData[DeviceId].FuncTable = NULL ; return ret;} else if (!*FileName){ *FileId = DeviceId; pDevice->ReferenceCount++; return ESUCCESS;} FileData[DeviceId].FileFuncTable = FatMount (DeviceId); if (!FileData[DeviceId].FileFuncTable)FileData[DeviceId].FileFuncTable = NtfsMount (DeviceId); if (!FileData[DeviceId].FileFuncTable)FileData[DeviceId].FileFuncTable = Ext2Mount (DeviceId); if (!FileData[DeviceId].FileFuncTable){ pDevice->FuncTable->Close (DeviceId); FileData[DeviceId].FuncTable = NULL ; return ENODEV;} pDevice->DeviceId = DeviceId; } else { DeviceId = pDevice->DeviceId; } pDevice->ReferenceCount++; break ;} pEntry = pEntry->Flink; } if (pEntry == &DeviceListHead)return ENODEV;
一个循环，遍历DEVICE链表，找到DEVICE->Prefix (磁盘、分区的Arc路径，上一节说过)和刚刚分解出来的DeviceName相等的节点。如果没有则函数直接失败。
for (DeviceId = 0 ; DeviceId < MAX_FDS; DeviceId++)if (!FileData[DeviceId].FuncTable)break ;if (DeviceId == MAX_FDS)return EMFILE;
首先在FileData数组中找到空闲项，数组的索引即将成为设备句柄。
FileData[DeviceId].FuncTable = pDevice->FuncTable; ret = pDevice->FuncTable->Open (pDevice->Prefix, DeviceOpenMode, &DeviceId); if (ret != ESUCCESS){ FileData[DeviceId].FuncTable = NULL ; return ret;} else if (!*FileName){ *FileId = DeviceId; pDevice->ReferenceCount++; return ESUCCESS;}
之后把DEVICE中存储的设别操作函数数组FuncTable赋值给对应FileData中的FuncTable。之后对该句柄的读写操作将直接传递给FileData.FuncTable中的函数。
ArcOpen -> DiskOpen (freeldr\freeldr\arch\i386\hardware.c) static LONG DiskOpen (CHAR* Path, OPENMODE OpenMode, ULONG* FileId) ...... if (!DissectArcPath (Path, FileName, &DriveNumber, &DrivePartition))return EINVAL;if (DrivePartition == 0xff ){ SectorSize = 2048 ; } else { SectorSize = 512 ; } if (DrivePartition != 0xff && DrivePartition != 0 ){ if (!DiskGetPartitionEntry (DriveNumber, DrivePartition, &PartitionTableEntry))return EINVAL;SectorOffset = PartitionTableEntry.SectorCountBeforePartition; SectorCount = PartitionTableEntry.PartitionSectorCount; } Context = MmHeapAlloc (sizeof (DISKCONTEXT)); if (!Context)return ENOMEM;Context->DriveNumber = DriveNumber; Context->SectorSize = SectorSize; Context->SectorOffset = SectorOffset; Context->SectorCount = SectorCount; Context->SectorNumber = 0 ; FsSetDeviceSpecific (*FileId, Context);return ESUCCESS;}
这个函数非常简单，使用DissectArcPath根据设备的Arc路径分解出文件名FileName、BIOS驱动器号DriveNumber、和分区号DrivePartition（第0个分区的编号是1，0代表整个硬盘）
VOID FsSetDeviceSpecific (ULONG FileId, VOID* Specific) if (FileId >= MAX_FDS || !FileData[FileId].FuncTable)return ;FileData[FileId].Specific = Specific; }
实际上DiskOpen的作用就是获得该设备（分区）的基本信息——BIOS驱动器号、扇区大小、开始扇区号、扇区数量和当前读写指针（SectorNumber）。生成DISKCONTENT结构使用FsSetDeviceSpecific和FileID绑定。
FileData[DeviceId].FileFuncTable = FatMount (DeviceId); if (!FileData[DeviceId].FileFuncTable)FileData[DeviceId].FileFuncTable = NtfsMount (DeviceId); if (!FileData[DeviceId].FileFuncTable)FileData[DeviceId].FileFuncTable = Ext2Mount (DeviceId); if (!FileData[DeviceId].FileFuncTable){ pDevice->FuncTable->Close (DeviceId); FileData[DeviceId].FuncTable = NULL ; return ENODEV;} pDevice->DeviceId = DeviceId;
现在FileData[DeviceId]已经代表刚刚打开的设备了，开始挂载分区。啥叫挂载分区，就是让文件系统提供个接口，能让我们操作分区里面的文件。而这个接口就是个DEVVTBL指针，和直接操作硬盘的接口一样，只不过这次这个可以操作文件了。如果分区识别成功，XxxMount函数将会返回另外一个DEVVTBL指针数组，这个指针赋值给设备对象的FileFuncTable成员。使用这个指针数组就可以在文件级别操作了。比如打开freeldr.sys文件就可以调用FileData[DeviceId].FileFuncTable->open函数。FileFuncTable和FuncTable是不同的哦！ ：）
for (i = 0 ; i < MAX_FDS; i++)if (!FileData[i].FuncTable)break ;if (i == MAX_FDS)return EMFILE;
跳过文件名开始的 “" 字符
if (*FileName == '\\' )FileName++;
我们前面说的，为FileData.FuncTable赋值。FileData.DeviceId是文件所在分区的句柄。FuncTable内部函数将通过这个句柄调用读写分区内容，为用户提供文件的读写接口。
FileData[i].FuncTable = FileData[DeviceId].FileFuncTable; FileData[i].DeviceId = DeviceId; *FileId = i; ret = FileData[i].FuncTable->Open (FileName, OpenMode, FileId); if (ret != ESUCCESS){ FileData[i].FuncTable = NULL ; *FileId = MAX_FDS; }
至此打开文件的操作结束。这里略去了XxxMount和文件的Open函数。以后再说。 ：）
