Tài liệu Windows Internals covering windows server 2008 and windows vista- P18 pdf

840 Ntfs.sys is accessed through the directory symbolic link. File symbolic links work much the same way—you can think of them as shortcuts, except they are actually implemented on the file system instead of being .lnk files managed by Windows Explorer. Just like hard links, symbolic links can be created with the mklink utility (without the /H option) or through the CreateSymbolicLink API. Because certain legacy applications might not behave securely in the presence of symbolic links, especially across different machines, the creation of symbolic links requires the SeCreateSymbolicLink privilege, which is typically granted only to administrators. The file system also has a behavior option called SymLinkEvaluation that can be configured with the following command: 1. fsutil behavior set SymLinkEvaluation By default, the Windows default symbolic link evaluation policy allows only local-to-local and local-to-remote symbolic links but not the opposite, as shown here: 1. C:\>fsutil behavior query SymLinkEvaluation 2. Local to local symbolic links are enabled 3. Local to remote symbolic links are enabled. 4. Remote to local symbolic links are disabled. 5. Remote to Remote symbolic links are disabled. Symbolic links are based on an NTFS mechanism called reparse points. (Reparse points are discussed further in the section “Reparse Points” later in this chapter.) A reparse point is a file or directory that has a block of data called reparse data associated with it. Reparse data is user-defined data about the file or directory, such as its state or location, that can be read from the reparse point by the application that created the data, a file system filter driver, or the I/O manager. When NTFS encounters a reparse point during a file or directory lookup, it returns a reparse status code, which signals file system filter drivers that are attached to the volume and the I/O manager to examine the reparse data. Each reparse point type has a unique reparse tag. The reparse tag allows the component responsible for interpreting the reparse point’s reparse data to recognize the reparse point without having to check the reparse data. A reparse tag owner, either a file system filter driver or the I/O manager, can choose one of the following options when it recognizes reparse data: ■ The reparse tag owner can manipulate the pathname specified in the file I/O operation that crosses the reparse point and let the I/O operation reissue with the altered pathname. Junctions (described shortly) take this approach to redirect a directory lookup, for example. ■ The reparse tag owner can remove the reparse point from the file, alter the file in some way, and then reissue the file I/O operation. There are no Windows functions for creating reparse points. Instead, processes must use the FSCTL_SET_REPARSE_POINT file system control code with the Windows DeviceIoControl function. A process can query a reparse point’s contents with the FSCTL_GET_REPARSE _POINT file system control code. The FILE_ATTRIBUTE_REPARSE_POINT flag is set in a reparse point’s file attributes, so applications can check for reparse points by using the Windows GetFileAttributes function. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 841 Another type of reparse point that NTFS supports is the junction. Junctions are a legacy NTFS concept and work almost identically to directory symbolic links, except they can only be local to a volume. There is no advantage to using a junction instead of a directory symbolic link, except that junctions are compatible with older versions of Windows, while directory symbolic links are not. eXPeriMeNT: Creating a Symbolic link This experiment shows you the main difference between a symbolic link and a hard link, even when dealing with files on the same volume. Create a symbolic link called soft.txt as shown here, pointing to the test.txt file created in the previous experiment: 1. C:\>mklink soft.txt test.txt 2. symbolic link created for soft.txt <<===>> test.txt If you list the directory’s contents, you’ll notice that the symbolic link doesn’t have a file size and is identified by the type. Furthermore, you’ll note that the creation time is that of the symbolic link, not of the target file. The symbolic link can also have security permissions that are different from the permissions on the target file. 1. C:\>dir *.txt 2. Volume in drive C is OS 3. Volume Serial Number is 38D4-EA71 4. Directory of C:\ 5. 10/18/2008 11:55 PM 8 hard.txt 6. 10/19/2008 12:28 AM soft.txt [test.txt] 7. 10/18/2008 11:55 PM 8 test.txt 8. 3 File(s) 16 bytes 9. 0 Dir(s) 10,636,480,512 bytes free Finally, if you delete the original test.txt file, you can verify that both the hard link and symbolic link still exist but that the symbolic link does not point to a valid file anymore, while the hard link references the file data. Compression and Sparse Files NTFS supports compression of file data. Because NTFS performs compression and decompression procedures transparently, applications don’t have to be modified to take advantage of this feature. Directories can also be compressed, which means that any files subsequently created in the directory are compressed. Applications compress and decompress files by passing DeviceIoControl the FSCTL_SET_COMPRESSION file system control code. They query the compression state of a file or directory with the FSCTL_GET_COMPRESSION file system control code. A file or directory that is compressed has the FILE_ATTRIBUTE_COMPRESSED flag set in its attributes, so applications can also determine a file or directory’s compression state with GetFileAttributes. A second type of compression is known as sparse files. If a file is marked as sparse, NTFS doesn’t allocate space on a volume for portions of the file that an application designates as empty. NTFS returns 0-filled buffers when an application reads from empty areas of a sparse file. This Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 842 type of compression can be useful for client/server applications that implement circular-buffer logging, in which the server records information to a file and clients asynchronously read the information. Because the information that the server writes isn’t needed after a client has read it, there’s no need to store the information in the file. By making such a file sparse, the client can specify the portions of the file it reads as empty, freeing up space on the volume. The server can continue to append new information to the file without fear that the file will grow to consume all available space on the volume. As for compressed files, NTFS manages sparse files transparently. Applications specify a file’s sparseness state by passing the FSCTL_SET_SPARSE file system control code to DeviceIoControl. To set a range of a file to empty, applications use the FSCTL_SET_ZERO_ DATA code, and they can ask NTFS for a description of what parts of a file are sparse by using the control code FSCTL_QUERY_ALLOCATED_RANGES. One application of sparse files is the NTFS change journal, described next. C hange Logging Many types of applications need to monitor volumes for file and directory changes. For example, an automatic backup program might perform an initial full backup and then incremental backups based on file changes. An obvious way for an application to monitor a volume for changes is for it to scan the volume, recording the state of files and directories, and on a subsequent scan detect differences. This process can adversely affect system performance, however, especially on computers with thousands or tens of thousands of files. An alternate approach is for an application to register a directory notification by using the FindFirstChangeNotification or ReadDirectoryChangesW Windows function. As an input parameter, the application specifies the name of a directory it wants to monitor, and the function returns whenever the contents of the directory change. Although this approach is more efficient than volume scanning, it requires the application to be running at all times. Using these functions can also require an application to scan directories because FindFirstChangeNotification doesn’t indicate what changed—just that something in the directory has changed. An application can pass a buffer to ReadDirectoryChangesW that the FSD fills in with change records. If the buffer overflows, however, the application must be prepared to fall back on scanning the directory. NTFS provides a third approach that overcomes the drawbacks of the first two: an application can configure the NTFS change journal facility by using the DeviceIoControl function’s FSCTL_CREATE_USN_JOURNAL file system control code to have NTFS record information about file and directory changes to an internal file called the change journal. A change journal is usually large enough to virtually guarantee that applications get a chance to process changes without missing any. Applications use the FSCTL_QUERY_USN_JOURNAL file system control code to read records from a change journal, and they can specify that the DeviceIoControl function not complete until new records are available. Per-User Volume Quotas Systems administrators often need to track or limit user disk space usage on shared storage volumes, so NTFS includes quota-management support. NTFS quota-management support allows Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 843 for per-user specification of quota enforcement, which is useful for usage tracking and tracking when a user reaches warning and limit thresholds. NTFS can be configured to log an event indicating the occurrence to the System event log if a user surpasses his warning limit. Similarly, if a user attempts to use more volume storage then her quota limit permits, NTFS can log an event to the System event log and fail the application file I/O that would have caused the quota violation with a “disk full” error code. NTFS tracks a user’s volume usage by relying on the fact that it tags files and directories with the security ID (SID) of the user who created them. (See Chapter 6 for a definition of SIDs.) The logical sizes of files and directories a user owns count against the user’s administratordefined quota limit. Thus, a user can’t circumvent his or her quota limit by creating an empty sparse file that is larger than the quota would allow and then fill the file with nonzero data.Similarly, whereas a 50-KB file might compress to 10 KB, the full 50 KB is used for quota accounting. By default, volumes don’t have quota tracking enabled. You need to use the Quota tab of a volume’s Properties dialog box, shown in Figure 11-18, to enable quotas, to specify default warning and limit thresholds, and to configure the NTFS behavior that occurs when a user hits the warning or limit threshold. The Quota Entries tool, which you can launch from this dialog box, enables an administrator to specify different limits and behavior for each user. Applications that want to interact with NTFS quota management use COM quota interfaces, including IDiskQuotaControl, IDiskQuotaUser, and IDiskQuotaEvents. Link Tracking Shell shortcuts allow users to place files in their shell namespace (on their desktop, for example) that link to files located in the file system namespace. The Windows Start menu uses shell shortcuts extensively. Similarly, object linking and embedding (OLE) links allow documents from one application to be transparently embedded in the documents of other applications. The products of the Microsoft Office suite, including PowerPoint, Excel, and Word, use OLE linking. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 844 Although shell and OLE links provide an easy way to connect files with one another and with the shell namespace, they can be difficult to manage if a user moves the source of a shell or OLE link (a link source is the file or directory to which a link points). NTFS in Windows includes support for a service application called distributed link-tracking, which maintains the integrity of shell and OLE links when link targets move. Using the NTFS link-tracking support, if a link target located on an NTFS volume moves to any other NTFS volume within the originating volume’s domain, the link-tracking service can transparently follow the movement and update the link to reflect the change. NTFS link-tracking support is based on an optional file attribute known as an object ID. An application can assign an object ID to a file by using the FSCTL_CREATE_OR_GET_ OBJECT_ID (which assigns an ID if one isn’t already assigned) and FSCTL_SET_OBJECT_ID file system control codes. Object IDs are queried with the FSCTL_CREATE_OR_GET_ OBJECT_ID and FSCTL_GET_OBJECT_ID file system control codes. The FSCTL_DELETE_ OBJECT_ID file system control code lets applications delete object IDs from files. Encryption Corporate users often store sensitive information on their computers. Although data stored on company servers is usually safely protected with proper network security settings and physical access control, data stored on laptops can be exposed when a laptop is lost or stolen. NTFS file permissions don’t offer protection because NTFS volumes can be fully accessed without regard to security by using NTFS file-reading software that doesn’t require Windows to be running. Furthermore, NTFS file permissions are rendered useless when an alternate Windows installation is used to access files from an administrator account. Recall from Chapter 6 that the administrator account has the take-ownership and backup privileges, both of which allow it to access any secured object by overriding the object’s security settings. NTFS includes a facility called Encrypting File System (EFS), which users can use to encrypt sensitive data. The operation of EFS, as that of file compression, is completely transparent to applications, which means that file data is automatically decrypted when an application running in the account of a user authorized to view the data reads it and is automatically encrypted when an authorized application changes the data. Note NTFS doesn’t permit the encryption of files located in the system volume’s root directory or in the \Windows directory because many files in these locations are required during the boot process and EFS isn’t active during the boot process. BitLocker, described in Chapter 6, is a technology much better suited for environments in which this is a requirement because it supports full-volume encryption. EFS relies on cryptographic services supplied by Windows in user mode, so it consists of both a kernel-mode component that tightly integrates with NTFS as well as user-mode DLLs that communicate with the Local Security Authority Subsystem (Lsass) and cryptographic DLLs. Files that are encrypted can be accessed only by using the private key of an account’s EFS private/public key pair, and private keys are locked using an account’s password. Thus, EFSencrypted files on lost or stolen laptops can’t be accessed using any means (other than a Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 845 brute-force cryptographic attack) without the password of an account that is authorized to view the data. Applications can use the EncryptFile and DecryptFile Windows API functions to encrypt and decrypt files, and FileEncryptionStatus to retrieve a file or directory’s EFS-related attributes, such as whether the file or directory is encrypted. A file or directory that is encrypted has the FILE_ATTRIBUTE_ENCRYPTED flag set in its attributes, so applications can also determine a file or directory’s encryption state with GetFileAttributes. POSIX Support As explained in Chapter 2, one of the mandates for Windows was to fully support the POSIX 1003.1 standard. In the file system area, the POSIX standard requires support for casesensitive file and directory names, traversal permissions (where security for each directory of a path is used when determining whether a user has access to a file or directory), a “filechange-time” time stamp (which is different from the MS-DOS “time-last-modified” stamp), and hard links. NTFS implements each of these features. Defragmentation Even though NTFS makes efforts to keep files contiguous, a volume’s files can still become fragmented over time, especially when there is limited free space. A file is fragmented if its data occupies discontiguous clusters. For example, Figure 11-19 shows a fragmented file consisting of five fragments. However, like most file systems (including versions of FAT on Windows), NTFS makes no special efforts to keep files contiguous, other than to reserve a region of disk space known as the master file table (MFT) zone for the MFT. (NTFS lets other files allocate from the MFT zone when volume free space runs low.) Keeping an area free for the MFT can help it stay contiguous, but it, too, can become fragmented. (See the section “Master File Table” later in this chapter for more information on MFTs.) To facilitate the development of third-party disk defragmentation tools, Windows includes a defragmentation API that such tools can use to move file data so that files occupy contiguous clusters. The API consists of file system controls that let applications obtain a map of a volume’s free and in-use clusters (FSCTL_GET_VOLUME_BITMAP), obtain a map of a file’s cluster usage (FSCTL_GET_RETRIEVAL_POINTERS), and move a file (FSCTL_MOVE_FILE). Windows includes a built-in defragmentation tool that is accessible by using the Disk Defragmenter utility (\%SystemRoot%\System32\Dfrgui.exe), shown in Figure 11-20, as well as a command-line interface, \%SystemRoot%\System32\Defrag.exe, that you can run interactively or Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 846 schedule but that does not produce detailed reports or offer control—such as excluding files or directories—over the defragmentation process. The only limitation imposed by the defragmentation implementation in NTFS is that paging files and NTFS log files cannot be defragmented. Dynamic Partitioning The NTFS driver allows users to dynamically resize any partition, including the system partition, either shrinking or expanding it (if enough space is available). Expanding a partition is easy if enough space exists on the disk and is performed through the FSCTL_EXPAND_VOLUME file system control code. Shrinking a partition is a more complicated process, because it requires moving any file system data that is currently in the area to be thrown away to the region that will still remain after the shrinking process (a mechanism similar to defragmentation). Shrinking is implemented by two components: the shrinking engine and the file system driver. The shrinking engine is implemented in user mode. It communicates with NTFS to determine the maximum number of reclaimable bytes—that is, how much data can be moved from the region that will be resized into the region that will remain. The shrinking engine uses the standard defragmentation mechanism shown earlier, which doesn’t support relocating page file fragments that are in use or any other files that have been marked as unmovable with the FSCTL_MARK_HANDLE file system control code (like the hibernation file). The master file table backup and other NTFS metadata files (described later), including the change journal, also cannot be moved, which limits the minimum size of the shrunk volume and causes wasted space. The file system driver shrinking code is responsible for ensuring that the volume remains in a consistent state throughout the shrinking process. To do so, it exposes an interface that uses three requests that describe the current operation, which are sent through the FSCTL_SHRINK_ VOLUME control code: ■ The ShrinkPrepare request, which must be issued before any other operation. This request takes the desired size of the new volume in sectors and is used so that the file system can block further allocations outside the new volume boundary. The prepare request doesn’t verify whether the volume can actually be shrunk by the specified amount, but it does make sure that the amount is numerically valid and that there aren’t any other shrinking operations ongoing. Note that after a Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 847 prepare operation, the file handle to the volume becomes associated with the shrink request. If the file handle is closed, the operation is assumed to be aborted. ■ The ShrinkCommit request, which the shrinking engine issues after a ShrinkPrepare request. In this state, the file system attempts the removal of the requested number of clusters in the most recent prepare request. (If multiple prepare requests have been sent with different sizes, the last one is the determining one.) The ShrinkCommit request assumes that the shrinking engine has completed and will fail if any allocated blocks remain in the area to be shrunk. ■ The ShrinkAbort request, which can be issued by the shrinking engine or caused by events such as the closure of the file handle to the volume. This request undoes the commit operation by returning the partition to its original size and allows new allocations outside the shrunk region to occur again. However, defragmentation changes made by the shrinking engine remain. If a system is rebooted during a shrinking operation, NTFS restores the file system to a consistent state via its metadata recovery mechanism, explained later in the chapter. Because the actual shrink operation isn’t executed until all other operations have been completed, the volume retains its original size and only defragmentation operations that had already been flushed out to disk persist. Finally, shrinking a volume has several effects on the volume shadow copy mechanism (for more information on VSS, see Chapter 8). Recall that the copy-on-write mechanism allows VSS to simply retain parts of the file that were actually modified while still linking to the original file data. For deleted files, this file data will not be associated with visible files but appear as free space instead—free space that will likely be located in the area that is about to be shrunk. The shrinking engine therefore communicates with VSS to engage it in the shrinking process. In summary, the VSS mechanism’s job is to copy deleted file data into its differencing area and to increase the differencing area as required to accommodate additional data. This detail is important because it poses another constraint on the size to which even volumes with ample free space can shrink. 11.6 NTFS File System Driver As described in Chapter 7, in the framework of the Windows I/O system, NTFS and other file systems are loadable device drivers that run in kernel mode. They are invoked indirectly by applications that use Windows or other I/O APIs (such as POSIX). As Figure 11-21 shows, the Windows environment subsystems call Windows system services, which in turn locate the appropriate loaded drivers and call them. (For a description of system service dispatching, see the section “System Service Dispatching” in Chapter 3.) Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 848 The layered drivers pass I/O requests to one another by calling the Windows executive’s I/O manager. Relying on the I/O manager as an intermediary allows each driver to maintain independence so that it can be loaded or unloaded without affecting other drivers. In addition, the NTFS driver interacts with the three other Windows executive components, shown in the left side of Figure 11-22, that are closely related to file systems. The log file service (LFS) is the part of NTFS that provides services for maintaining a log of disk writes. The log file that LFS writes is used to recover an NTFS-formatted volume in the case of a system failure. (See the section “Log File Service (LFS)” later in the chapter.) The cache manager is the component of the Windows executive that provides systemwide caching services for NTFS and other file system drivers, including network file system drivers (servers and redirectors). All file systems implemented for Windows access cached files by mapping them into system address space and then accessing the virtual memory. The cache manager provides a specialized file system interface to the Windows memory manager for this purpose. When a program tries to access a part of a file that isn’t loaded into the cache (a cache Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. 849 miss), the memory manager calls NTFS to access the disk driver and obtain the file contents from disk. The cache manager optimizes disk I/O by using its lazy writer threads to call the memory manager to flush cache contents to disk as a background activity (asynchronous disk writing). (For a complete description of the cache manager, see Chapter 10.) NTFS participates in the Windows object model by implementing files as objects. This implementation allows files to be shared and protected by the object manager, the component of Windows that manages all executive-level objects. (The object manager is described in the section “Object Manager” in Chapter 3.) An application creates and accesses files just as it does other Windows objects: by means of object handles. By the time an I/O request reaches NTFS, the Windows object manager and security system have already verified that the calling process has the authority to access the file object in the way it is attempting to. The security system has compared the caller’s access token to the entries in the access control list for the file object. (See Chapter 6 for more information about access control lists.) The I/O manager has also transformed the file handle into a pointer to a file object. NTFS uses the information in the file object to access the file on disk. Figure 11-23 shows the data structures that link a file handle to the file system’s on-disk structure. NTFS follows several pointers to get from the file object to the location of the file on disk. As Figure 11-23 shows, a file object, which represents a single call to the open-file system service, points to a stream control block (SCB) for the file attribute that the caller is trying to read or write. In Figure 11-23, a process has opened both the unnamed data attribute and a named stream (alternate data attribute) for the file. The SCBs represent individual file attributes and contain information about how to find specific attributes within a file. All the SCBs for a file point to a common data structure called a file control block (FCB). The FCB contains a pointer (actually, a file reference, explained in the section “File Reference Numbers” later in this chapter) to the file’s record in the disk-based master file table (MFT), which is described in detail in the following section. Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark. [...]... formatted for use by the file system An NTFS volume, however, stores all file system data, such as bitmaps and directories, and even the system bootstrap, as ordinary files Note The on-disk format of NTFS volumes on Windows Vista and Windows Server 2008 is version 3.1, the same as on Windows XP and Windows Server 2003 The version number of a volume is stored in its $Volume metadata file Clusters The cluster... operations on their files and directories and also a file system control (FSCTL) interface for managing its resource managers Note Support for TxF was added to the NTFS driver starting with Windows Vista and Windows Server 2008 without actually changing the format of the NTFS data structures, which is why the NTFS format version number, 3.1, is the same as for Windows XP and Windows Server 2003 TxF achieves... sparse and nonsparse data The Change Journal File The change journal file, \$Extend\$UsnJrnl, is a sparse file in which NTFS stores records of changes to files and directories Applications like the Windows File Replication Service (FRS) and the Windows Search service make use of the journal to respond to file and directory changes as they occur 866 Please purchase PDF Split-Merge on www.verypdf.com... that aren’t visible to Windows and MS-DOS applications, including names with trailing periods and trailing spaces Ordinarily, creating a file using the large POSIX namespace isn’t a problem because you would do that only if you intended the POSIX subsystem or POSIX client systems to use that file The relationship between 32-bit Windows (Windows) applications and MS-DOS and 16-bit Windows applications... NTFS security identifier from the $SDH entry and storing it in the file or directory’s $STANDARD_INFORMATION attribute The NTFS $STANDARD_INFORMATION attribute, which all files and directories have, stores basic information about a file, including its attributes, time stamp information, and security identifier 873 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark If NTFS doesn’t... Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark 4 If the generated name duplicates an existing name in the directory, increment the ~n string If n is greater than 4, and a checksum was not concatenated already, truncate the string before the period to two characters and generate and concatenate a fourcharacter hex checksum string Table 11-6 shows the long Windows file names... stores file object IDs, the quota file stores quota limit and behavior information on volumes that have quotas enabled, the change journal file records file and directory changes, and the reparse point file stores information about which files and directories on the volume include reparse point data 853 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark The default resource manager... larger volume, use of a larger cluster factor can reduce fragmentation and speed allocation, at a small cost in terms of wasted disk space Both the format command available from the command prompt and the Format menu option under the All Tasks option on the Action menu in the Disk 850 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark Management MMC snap-in choose a default... much closer one, however The Windows area in Figure 11-28 represents file names that the Windows subsystem can create on an NTFS volume but that MS-DOS and 16-bit Windows applications can’t see This group includes file names longer than the 8.3 format of MS-DOS names, those containing Unicode (international) characters, those with 857 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark... that each VCN from 0 through 11 has a corresponding LCN associated with it The first entry starts at VCN 0 and covers 4 clusters, the second entry starts at VCN 4 and covers 4 clusters, and so on This entry format is typical for a noncompressed file 863 Please purchase PDF Split-Merge on www.verypdf.com to remove this watermark When a user selects a file on an NTFS volume for compression, one NTFS compression . bitmaps and directories, and even the system bootstrap, as ordinary files. Note The on-disk format of NTFS volumes on Windows Vista and Windows Server 2008. relationship between 32-bit Windows (Windows) applications and MS-DOS and 16-bit Windows applications is a much closer one, however. The Windows area in Figure