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Date: Thu, 29 Jul 2010 15:05:02 +0200 (CEST)
From: Bo Brantén <bosse@....umu.se>
To: linux-ext4@...r.kernel.org
Subject: trying to understand ext4 extents on disk format
Hello,
I am implementing support for ext4 extents in the Windows driver "Ext2Fsd"
originally written by Matt Wu (http://www.ext2fsd.com/) the driver
implements full read/write support for ext2/ext3 and is considered stable
by many users.
To understand the ext4 extents format on disk I have read a few papers by
Theodore Ts'o and studied a program called Ext2Read
(http://ext2read.blogspot.com/) but I am a bit unsure how to find all
extents when there are more than one block of them so the index has more
than one entry, if I understand it correctly the root is a single entry
that points to a block of indexes and the indexes points to different
blocks of extent entrys and they are all in increasing logical block
number order.
The full function is included below but here is the lines I am unsure
about, I want to find the block of indexes or extents to continue
searching in:
for (i = 0; i < eh->eh_entries; i++, ei++) {
if(Index >= ei->ei_block) {
/* read next block with index or entrys */
/* call this funtion recursivley on the next block */
}
}
It would be helpfull if anyone that understands extents better that me
commented this.
Bo Branten
The full funtion:
/* translate logical block number to physical block numer, Index is lbn,
Block is pbn, Number is number of contiguous pb, Alloc is extending the
file (not implemented for ext4 extents yet) The other arguments is Windows
speficic so I don't comment them here.
+NTSTATUS
+Ext2ExtentSearch(
+ IN PEXT2_IRP_CONTEXT IrpContext,
+ IN PEXT2_VCB Vcb,
+ IN PEXT2_MCB Mcb,
+ IN ULONG Index,
+ IN BOOLEAN Alloc,
+ IN PVOID ExtentHeader,
+ IN BOOLEAN UnpinIndexBlock,
+ IN PBCB BcbToUnpin,
+ OUT PULONG Block,
+ OUT PULONG Number
+ )
+{
+ EXT4_EXTENT_HEADER *eh;
+ EXT4_EXTENT_IDX *ei;
+ EXT4_EXTENT *ex;
+ LARGE_INTEGER offset;
+ PBCB bcb;
+ PVOID ib;
+ int i;
+ NTSTATUS status;
+
+ *Block = 0;
+ *Number = 0;
+
+ eh = (EXT4_EXTENT_HEADER *) ExtentHeader;
+
+ if (eh != NULL && eh->eh_magic == EXT4_EXT_MAGIC) {
+ DEBUG(DL_INF, ("eh_magic=%x eh_entries=%u eh_max=%u eh_depth=%u eh_generation=%u\n", (ULONG)eh->eh_magic, (ULONG)eh->eh_entries, (ULONG)eh->eh_max, (ULONG)eh->eh_depth, eh->eh_generation ));
+ if (eh->eh_depth == 0) {
+ ex = EXT_FIRST_EXTENT(eh);
+ for (i = 0; i < eh->eh_entries; i++, ex++) {
+ DEBUG(DL_INF, ("ee_block=%u ee_len=%u ee_start_hi=%u ee_start_lo=%u ext_to_block(extent)=%u\n", ex->ee_block, (ULONG)ex->ee_len, (ULONG)ex->ee_start_hi, ex->ee_start_lo, ext_to_block(ex) ));
+ if (Index >= ex->ee_block && Index < ex->ee_block + ex->ee_len) {
+ *Block = (ULONG) (ext_to_block(ex) + (Index - ex->ee_block));
+ *Number = ex->ee_block + ex->ee_len - Index;
+ DEBUG(DL_INF, ("Index=%u Block=%u Number=%u\n", Index, *Block, *Number ));
+ if (UnpinIndexBlock) {
+ CcUnpinData(BcbToUnpin);
+ }
+ return STATUS_SUCCESS;
+ }
+ }
+ DEBUG(DL_ERR, ("Ext2ExtentSearch: search for extent failed on file or directory %wZ\n", &Mcb->FullName ));
+ DEBUG(DL_ERR, ("eh_magic=%x eh_entries=%u eh_max=%u eh_depth=%u eh_generation=%u\n", (ULONG)eh->eh_magic, (ULONG)eh->eh_entries, (ULONG)eh->eh_max, (ULONG)eh->eh_depth, eh->eh_generation ));
+ } else {
+ ei = EXT_FIRST_INDEX(eh);
+ DEBUG(DL_INF, ("Index=%u ei_block=%u eh_entries=%u\n", Index, ei->ei_block, eh->eh_entries ));
+ for (i = 0; i < eh->eh_entries; i++, ei++) {
+ if(Index >= ei->ei_block) {
+ DEBUG(DL_INF, ("idx_to_block(index)=%u\n", idx_to_block(ei) ));
+ offset.QuadPart = idx_to_block(ei) * Vcb->BlockSize;
+ status = STATUS_SUCCESS;
+ __try {
+ if ( !CcPinRead (
+ Vcb->Volume,
+ &offset,
+ Vcb->BlockSize,
+ Ext2CanIWait(),
+ &bcb,
+ &ib )) {
+ status = STATUS_CANT_WAIT;
+ }
+ } __except (EXCEPTION_EXECUTE_HANDLER) {
+ status = GetExceptionCode();
+ }
+ if (!NT_SUCCESS(status)) {
+ DEBUG(DL_ERR, ("Ext2ExtentSearch: CcPinRead failed with status %x\n", status ));
+ return status;
+ }
+ return Ext2ExtentSearch(
+ IrpContext,
+ Vcb,
+ Mcb,
+ Index,
+ Alloc,
+ ib,
+ TRUE,
+ bcb,
+ Block,
+ Number
+ );
+ }
+ }
+ DEBUG(DL_ERR, ("Ext2ExtentSearch: search for extent failed on file or directory %wZ\n", &Mcb->FullName ));
+ if (Alloc) {
+ DEBUG(DL_ERR, ("possible because extending a file or directory that is stored using ext4 exents is not supported yet\n" ));
+ }
+ DEBUG(DL_ERR, ("eh_magic=%x eh_entries=%u eh_max=%u eh_depth=%u eh_generation=%u\n", (ULONG)eh->eh_magic, (ULONG)eh->eh_entries, (ULONG)eh->eh_max, (ULONG)eh->eh_depth, eh->eh_generation ));
+ }
+ }
+
+ if (UnpinIndexBlock) {
+ CcUnpinData(BcbToUnpin);
+ }
+
+ return STATUS_DISK_CORRUPT_ERROR;
}
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