x86/kprobes: Fix crash when probing CS CALL instructionsgrafted

When using eBPF to probe CS CALL instructions within a function,
a crash can be triggered.

The eBPF tool probes offset 257 of the __hrtimer_run_queues()
function:

<__hrtimer_run_queues+249>:  nopl   0x0(%rax,%rax,1)
<__hrtimer_run_queues+254>:  mov    %r14,%rdi
<__hrtimer_run_queues+257>:  cs call <__x86_indirect_thunk_r12>
<__hrtimer_run_queues+263>:  mov    %eax,%r12d
<__hrtimer_run_queues+266>:  xchg   %ax,%ax
<__hrtimer_run_queues+268>:  mov    %r13,%rdi

Which triggers this crash:

  BUG: unable to handle page fault for address: 00000000000f41c9
  #PF: supervisor write access in kernel mode
  #PF: error_code(0x0002) - not-present page
  PGD 0 P4D 0
  Oops: 0002 [#1] SMP NOPTI
  CPU: 1 PID: 0 Comm: swapper/1 Kdump: loaded Tainted: P
  RIP: 0010:__hrtimer_run_queues+0x106/0x230

Note that __hrtimer_run_queues+0x106 is __hrtimer_run_queues+262, which is
at the 6th byte of the above CS CALL instruction. Since the CS CALL
instruction occupies 6 bytes, the exception occurred in the middle of that
call instruction.

The root cause is that when using eBPF tools to probe in the middle of a
function, a kprobe with INT3 is used as the underlying implementation.

During single-step emulation of the original CALL instruction,
int3_emulate_call() assumes that the probed CALL instruction is 5 bytes
long. However, the actual CS-prefixed CALL instruction occupies 6 bytes,
so it constructs an incorrect exception return address. When the CPU
returns from the kprobe handler, the next instruction to be executed is at
the address of the last byte of that CS CALL instruction. Coincidentally,
starting from that address, the CPU fetches and decodes a completely
different instruction, which ultimately triggers a kernel crash.

Fix the issue by using the actual instruction length obtained from
the instruction decoder when constructing the exception return
address, rather than relying on the hardcoded CALL_INSN_SIZE macro.

[ mingo: Refined the changelog ]

Fixes: 6256e668b7af ("x86/kprobes: Use int3 instead of debug trap for single-step")
Suggested-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Signed-off-by: Jinke Han <jinkehan@didiglobal.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Masami Hiramatsu (Google) <mhiramat@kernel.org>
Acked-by: Yafang Shao <laoar.shao@gmail.com>
Acked-by: Borislav Petkov <bp@alien8.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: https://patch.msgid.link/20260908073742.GA10517@didi-ThinkCentre-M920t-N000

1 files changed, 776 insertions, 0 deletions

diff --git a/drivers/mtd/inftlmount.c b/drivers/mtd/inftlmount.c
new file mode 100644
index 000000000..87e246a6f
--- /dev/null
+++ b/drivers/mtd/inftlmount.c
@@ -0,0 +1,776 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * inftlmount.c -- INFTL mount code with extensive checks.
+ *
+ * Author: Greg Ungerer (gerg@snapgear.com)
+ * Copyright © 2002-2003, Greg Ungerer (gerg@snapgear.com)
+ *
+ * Based heavily on the nftlmount.c code which is:
+ * Author: Fabrice Bellard (fabrice.bellard@netgem.com)
+ * Copyright © 2000 Netgem S.A.
+ */
+
+#include <linux/kernel.h>
+#include <linux/module.h>
+#include <asm/errno.h>
+#include <asm/io.h>
+#include <linux/uaccess.h>
+#include <linux/delay.h>
+#include <linux/slab.h>
+#include <linux/mtd/mtd.h>
+#include <linux/mtd/nftl.h>
+#include <linux/mtd/inftl.h>
+
+/*
+ * find_boot_record: Find the INFTL Media Header and its Spare copy which
+ *	contains the various device information of the INFTL partition and
+ *	Bad Unit Table. Update the PUtable[] table according to the Bad
+ *	Unit Table. PUtable[] is used for management of Erase Unit in
+ *	other routines in inftlcore.c and inftlmount.c.
+ */
+static int find_boot_record(struct INFTLrecord *inftl)
+{
+	struct inftl_unittail h1;
+	//struct inftl_oob oob;
+	unsigned int i, block;
+	u8 buf[SECTORSIZE];
+	struct INFTLMediaHeader *mh = &inftl->MediaHdr;
+	struct mtd_info *mtd = inftl->mbd.mtd;
+	struct INFTLPartition *ip;
+	size_t retlen;
+
+	pr_debug("INFTL: find_boot_record(inftl=%p)\n", inftl);
+
+        /*
+	 * Assume logical EraseSize == physical erasesize for starting the
+	 * scan. We'll sort it out later if we find a MediaHeader which says
+	 * otherwise.
+	 */
+	inftl->EraseSize = inftl->mbd.mtd->erasesize;
+        inftl->nb_blocks = (u32)inftl->mbd.mtd->size / inftl->EraseSize;
+
+	inftl->MediaUnit = BLOCK_NIL;
+
+	/* Search for a valid boot record */
+	for (block = 0; block < inftl->nb_blocks; block++) {
+		int ret;
+
+		/*
+		 * Check for BNAND header first. Then whinge if it's found
+		 * but later checks fail.
+		 */
+		ret = mtd_read(mtd, block * inftl->EraseSize, SECTORSIZE,
+			       &retlen, buf);
+		/* We ignore ret in case the ECC of the MediaHeader is invalid
+		   (which is apparently acceptable) */
+		if (retlen != SECTORSIZE) {
+			static int warncount = 5;
+
+			if (warncount) {
+				pr_warn("INFTL: block read at 0x%x "
+					"of mtd%d failed: %d\n",
+					block * inftl->EraseSize,
+					inftl->mbd.mtd->index, ret);
+				if (!--warncount)
+					pr_warn("INFTL: further "
+						"failures for this block will "
+						"not be printed\n");
+			}
+			continue;
+		}
+
+		if (retlen < 6 || memcmp(buf, "BNAND", 6)) {
+			/* BNAND\0 not found. Continue */
+			continue;
+		}
+
+		/* To be safer with BIOS, also use erase mark as discriminant */
+		ret = inftl_read_oob(mtd,
+				     block * inftl->EraseSize + SECTORSIZE + 8,
+				     8, &retlen,(char *)&h1);
+		if (ret < 0) {
+			pr_warn("INFTL: ANAND header found at "
+				"0x%x in mtd%d, but OOB data read failed "
+				"(err %d)\n", block * inftl->EraseSize,
+				inftl->mbd.mtd->index, ret);
+			continue;
+		}
+
+
+		/*
+		 * This is the first we've seen.
+		 * Copy the media header structure into place.
+		 */
+		memcpy(mh, buf, sizeof(struct INFTLMediaHeader));
+
+		/* Read the spare media header at offset 4096 */
+		mtd_read(mtd, block * inftl->EraseSize + 4096, SECTORSIZE,
+			 &retlen, buf);
+		if (retlen != SECTORSIZE) {
+			pr_warn("INFTL: Unable to read spare "
+			       "Media Header\n");
+			return -1;
+		}
+		/* Check if this one is the same as the first one we found. */
+		if (memcmp(mh, buf, sizeof(struct INFTLMediaHeader))) {
+			pr_warn("INFTL: Primary and spare Media "
+			       "Headers disagree.\n");
+			return -1;
+		}
+
+		mh->NoOfBootImageBlocks = le32_to_cpu(mh->NoOfBootImageBlocks);
+		mh->NoOfBinaryPartitions = le32_to_cpu(mh->NoOfBinaryPartitions);
+		mh->NoOfBDTLPartitions = le32_to_cpu(mh->NoOfBDTLPartitions);
+		mh->BlockMultiplierBits = le32_to_cpu(mh->BlockMultiplierBits);
+		mh->FormatFlags = le32_to_cpu(mh->FormatFlags);
+		mh->PercentUsed = le32_to_cpu(mh->PercentUsed);
+
+		pr_debug("INFTL: Media Header ->\n"
+			 "    bootRecordID          = %s\n"
+			 "    NoOfBootImageBlocks   = %d\n"
+			 "    NoOfBinaryPartitions  = %d\n"
+			 "    NoOfBDTLPartitions    = %d\n"
+			 "    BlockMultiplierBits   = %d\n"
+			 "    FormatFlgs            = %d\n"
+			 "    OsakVersion           = 0x%x\n"
+			 "    PercentUsed           = %d\n",
+			 mh->bootRecordID, mh->NoOfBootImageBlocks,
+			 mh->NoOfBinaryPartitions,
+			 mh->NoOfBDTLPartitions,
+			 mh->BlockMultiplierBits, mh->FormatFlags,
+			 mh->OsakVersion, mh->PercentUsed);
+
+		if (mh->NoOfBDTLPartitions == 0) {
+			pr_warn("INFTL: Media Header sanity check "
+				"failed: NoOfBDTLPartitions (%d) == 0, "
+				"must be at least 1\n", mh->NoOfBDTLPartitions);
+			return -1;
+		}
+
+		if ((mh->NoOfBDTLPartitions + mh->NoOfBinaryPartitions) > 4) {
+			pr_warn("INFTL: Media Header sanity check "
+				"failed: Total Partitions (%d) > 4, "
+				"BDTL=%d Binary=%d\n", mh->NoOfBDTLPartitions +
+				mh->NoOfBinaryPartitions,
+				mh->NoOfBDTLPartitions,
+				mh->NoOfBinaryPartitions);
+			return -1;
+		}
+
+		if (mh->BlockMultiplierBits > 1) {
+			pr_warn("INFTL: sorry, we don't support "
+				"UnitSizeFactor 0x%02x\n",
+				mh->BlockMultiplierBits);
+			return -1;
+		} else if (mh->BlockMultiplierBits == 1) {
+			pr_warn("INFTL: support for INFTL with "
+				"UnitSizeFactor 0x%02x is experimental\n",
+				mh->BlockMultiplierBits);
+			inftl->EraseSize = inftl->mbd.mtd->erasesize <<
+				mh->BlockMultiplierBits;
+			inftl->nb_blocks = (u32)inftl->mbd.mtd->size / inftl->EraseSize;
+			block >>= mh->BlockMultiplierBits;
+		}
+
+		/* Scan the partitions */
+		for (i = 0; (i < 4); i++) {
+			ip = &mh->Partitions[i];
+			ip->virtualUnits = le32_to_cpu(ip->virtualUnits);
+			ip->firstUnit = le32_to_cpu(ip->firstUnit);
+			ip->lastUnit = le32_to_cpu(ip->lastUnit);
+			ip->flags = le32_to_cpu(ip->flags);
+			ip->spareUnits = le32_to_cpu(ip->spareUnits);
+			ip->Reserved0 = le32_to_cpu(ip->Reserved0);
+
+			pr_debug("    PARTITION[%d] ->\n"
+				 "        virtualUnits    = %d\n"
+				 "        firstUnit       = %d\n"
+				 "        lastUnit        = %d\n"
+				 "        flags           = 0x%x\n"
+				 "        spareUnits      = %d\n",
+				 i, ip->virtualUnits, ip->firstUnit,
+				 ip->lastUnit, ip->flags,
+				 ip->spareUnits);
+
+			if (ip->Reserved0 != ip->firstUnit) {
+				struct erase_info *instr = &inftl->instr;
+
+				/*
+				 * 	Most likely this is using the
+				 * 	undocumented qiuck mount feature.
+				 * 	We don't support that, we will need
+				 * 	to erase the hidden block for full
+				 * 	compatibility.
+				 */
+				instr->addr = ip->Reserved0 * inftl->EraseSize;
+				instr->len = inftl->EraseSize;
+				mtd_erase(mtd, instr);
+			}
+			if ((ip->lastUnit - ip->firstUnit + 1) < ip->virtualUnits) {
+				pr_warn("INFTL: Media Header "
+					"Partition %d sanity check failed\n"
+					"    firstUnit %d : lastUnit %d  >  "
+					"virtualUnits %d\n", i, ip->lastUnit,
+					ip->firstUnit, ip->Reserved0);
+				return -1;
+			}
+			if (ip->Reserved1 != 0) {
+				pr_warn("INFTL: Media Header "
+					"Partition %d sanity check failed: "
+					"Reserved1 %d != 0\n",
+					i, ip->Reserved1);
+				return -1;
+			}
+
+			if (ip->flags & INFTL_BDTL)
+				break;
+		}
+
+		if (i >= 4) {
+			pr_warn("INFTL: Media Header Partition "
+				"sanity check failed:\n       No partition "
+				"marked as Disk Partition\n");
+			return -1;
+		}
+
+		inftl->nb_boot_blocks = ip->firstUnit;
+		inftl->numvunits = ip->virtualUnits;
+		if (inftl->numvunits > (inftl->nb_blocks -
+		    inftl->nb_boot_blocks - 2)) {
+			pr_warn("INFTL: Media Header sanity check "
+				"failed:\n        numvunits (%d) > nb_blocks "
+				"(%d) - nb_boot_blocks(%d) - 2\n",
+				inftl->numvunits, inftl->nb_blocks,
+				inftl->nb_boot_blocks);
+			return -1;
+		}
+
+		inftl->mbd.size  = inftl->numvunits *
+			(inftl->EraseSize / SECTORSIZE);
+
+		/*
+		 * Block count is set to last used EUN (we won't need to keep
+		 * any meta-data past that point).
+		 */
+		inftl->firstEUN = ip->firstUnit;
+		inftl->lastEUN = ip->lastUnit;
+		inftl->nb_blocks = ip->lastUnit + 1;
+
+		/* Memory alloc */
+		inftl->PUtable = kmalloc_array(inftl->nb_blocks, sizeof(u16),
+					       GFP_KERNEL);
+		if (!inftl->PUtable)
+			return -ENOMEM;
+
+		inftl->VUtable = kmalloc_array(inftl->nb_blocks, sizeof(u16),
+					       GFP_KERNEL);
+		if (!inftl->VUtable) {
+			kfree(inftl->PUtable);
+			return -ENOMEM;
+		}
+
+		/* Mark the blocks before INFTL MediaHeader as reserved */
+		for (i = 0; i < inftl->nb_boot_blocks; i++)
+			inftl->PUtable[i] = BLOCK_RESERVED;
+		/* Mark all remaining blocks as potentially containing data */
+		for (; i < inftl->nb_blocks; i++)
+			inftl->PUtable[i] = BLOCK_NOTEXPLORED;
+
+		/* Mark this boot record (NFTL MediaHeader) block as reserved */
+		inftl->PUtable[block] = BLOCK_RESERVED;
+
+		/* Read Bad Erase Unit Table and modify PUtable[] accordingly */
+		for (i = 0; i < inftl->nb_blocks; i++) {
+			int physblock;
+			/* If any of the physical eraseblocks are bad, don't
+			   use the unit. */
+			for (physblock = 0; physblock < inftl->EraseSize; physblock += inftl->mbd.mtd->erasesize) {
+				if (mtd_block_isbad(inftl->mbd.mtd,
+						    i * inftl->EraseSize + physblock))
+					inftl->PUtable[i] = BLOCK_RESERVED;
+			}
+		}
+
+		inftl->MediaUnit = block;
+		return 0;
+	}
+
+	/* Not found. */
+	return -1;
+}
+
+static int memcmpb(void *a, int c, int n)
+{
+	int i;
+	for (i = 0; i < n; i++) {
+		if (c != ((unsigned char *)a)[i])
+			return 1;
+	}
+	return 0;
+}
+
+/*
+ * check_free_sector: check if a free sector is actually FREE,
+ *	i.e. All 0xff in data and oob area.
+ */
+static int check_free_sectors(struct INFTLrecord *inftl, unsigned int address,
+	int len, int check_oob)
+{
+	struct mtd_info *mtd = inftl->mbd.mtd;
+	size_t retlen;
+	int i, ret;
+	u8 *buf;
+
+	buf = kmalloc(SECTORSIZE + mtd->oobsize, GFP_KERNEL);
+	if (!buf)
+		return -ENOMEM;
+
+	ret = -1;
+	for (i = 0; i < len; i += SECTORSIZE) {
+		if (mtd_read(mtd, address, SECTORSIZE, &retlen, buf))
+			goto out;
+		if (memcmpb(buf, 0xff, SECTORSIZE) != 0)
+			goto out;
+
+		if (check_oob) {
+			if(inftl_read_oob(mtd, address, mtd->oobsize,
+					  &retlen, &buf[SECTORSIZE]) < 0)
+				goto out;
+			if (memcmpb(buf + SECTORSIZE, 0xff, mtd->oobsize) != 0)
+				goto out;
+		}
+		address += SECTORSIZE;
+	}
+
+	ret = 0;
+
+out:
+	kfree(buf);
+	return ret;
+}
+
+/*
+ * INFTL_format: format a Erase Unit by erasing ALL Erase Zones in the Erase
+ *		 Unit and Update INFTL metadata. Each erase operation is
+ *		 checked with check_free_sectors.
+ *
+ * Return: 0 when succeed, -1 on error.
+ *
+ * ToDo: 1. Is it necessary to check_free_sector after erasing ??
+ */
+int INFTL_formatblock(struct INFTLrecord *inftl, int block)
+{
+	size_t retlen;
+	struct inftl_unittail uci;
+	struct erase_info *instr = &inftl->instr;
+	struct mtd_info *mtd = inftl->mbd.mtd;
+	int physblock;
+
+	pr_debug("INFTL: INFTL_formatblock(inftl=%p,block=%d)\n", inftl, block);
+
+	memset(instr, 0, sizeof(struct erase_info));
+
+	/* FIXME: Shouldn't we be setting the 'discarded' flag to zero
+	   _first_? */
+
+	/* Use async erase interface, test return code */
+	instr->addr = block * inftl->EraseSize;
+	instr->len = inftl->mbd.mtd->erasesize;
+	/* Erase one physical eraseblock at a time, even though the NAND api
+	   allows us to group them.  This way we if we have a failure, we can
+	   mark only the failed block in the bbt. */
+	for (physblock = 0; physblock < inftl->EraseSize;
+	     physblock += instr->len, instr->addr += instr->len) {
+		int ret;
+
+		ret = mtd_erase(inftl->mbd.mtd, instr);
+		if (ret) {
+			pr_warn("INFTL: error while formatting block %d\n",
+				block);
+			goto fail;
+		}
+
+		/*
+		 * Check the "freeness" of Erase Unit before updating metadata.
+		 * FixMe: is this check really necessary? Since we have check
+		 * the return code after the erase operation.
+		 */
+		if (check_free_sectors(inftl, instr->addr, instr->len, 1) != 0)
+			goto fail;
+	}
+
+	uci.EraseMark = cpu_to_le16(ERASE_MARK);
+	uci.EraseMark1 = cpu_to_le16(ERASE_MARK);
+	uci.Reserved[0] = 0;
+	uci.Reserved[1] = 0;
+	uci.Reserved[2] = 0;
+	uci.Reserved[3] = 0;
+	instr->addr = block * inftl->EraseSize + SECTORSIZE * 2;
+	if (inftl_write_oob(mtd, instr->addr + 8, 8, &retlen, (char *)&uci) < 0)
+		goto fail;
+	return 0;
+fail:
+	/* could not format, update the bad block table (caller is responsible
+	   for setting the PUtable to BLOCK_RESERVED on failure) */
+	mtd_block_markbad(inftl->mbd.mtd, instr->addr);
+	return -1;
+}
+
+/*
+ * format_chain: Format an invalid Virtual Unit chain. It frees all the Erase
+ *	Units in a Virtual Unit Chain, i.e. all the units are disconnected.
+ *
+ *	Since the chain is invalid then we will have to erase it from its
+ *	head (normally for INFTL we go from the oldest). But if it has a
+ *	loop then there is no oldest...
+ */
+static void format_chain(struct INFTLrecord *inftl, unsigned int first_block)
+{
+	unsigned int block = first_block, block1;
+
+	pr_warn("INFTL: formatting chain at block %d\n",
+		first_block);
+
+	for (;;) {
+		block1 = inftl->PUtable[block];
+
+		pr_warn("INFTL: formatting block %d\n", block);
+		if (INFTL_formatblock(inftl, block) < 0) {
+			/*
+			 * Cannot format !!!! Mark it as Bad Unit,
+			 */
+			inftl->PUtable[block] = BLOCK_RESERVED;
+		} else {
+			inftl->PUtable[block] = BLOCK_FREE;
+		}
+
+		/* Goto next block on the chain */
+		block = block1;
+
+		if (block == BLOCK_NIL || block >= inftl->lastEUN)
+			break;
+	}
+}
+
+void INFTL_dumptables(struct INFTLrecord *s)
+{
+	int i;
+
+	pr_debug("-------------------------------------------"
+		"----------------------------------\n");
+
+	pr_debug("VUtable[%d] ->", s->nb_blocks);
+	for (i = 0; i < s->nb_blocks; i++) {
+		if ((i % 8) == 0)
+			pr_debug("\n%04x: ", i);
+		pr_debug("%04x ", s->VUtable[i]);
+	}
+
+	pr_debug("\n-------------------------------------------"
+		"----------------------------------\n");
+
+	pr_debug("PUtable[%d-%d=%d] ->", s->firstEUN, s->lastEUN, s->nb_blocks);
+	for (i = 0; i <= s->lastEUN; i++) {
+		if ((i % 8) == 0)
+			pr_debug("\n%04x: ", i);
+		pr_debug("%04x ", s->PUtable[i]);
+	}
+
+	pr_debug("\n-------------------------------------------"
+		"----------------------------------\n");
+
+	pr_debug("INFTL ->\n"
+		"  EraseSize       = %d\n"
+		"  h/s/c           = %d/%d/%d\n"
+		"  numvunits       = %d\n"
+		"  firstEUN        = %d\n"
+		"  lastEUN         = %d\n"
+		"  numfreeEUNs     = %d\n"
+		"  LastFreeEUN     = %d\n"
+		"  nb_blocks       = %d\n"
+		"  nb_boot_blocks  = %d",
+		s->EraseSize, s->heads, s->sectors, s->cylinders,
+		s->numvunits, s->firstEUN, s->lastEUN, s->numfreeEUNs,
+		s->LastFreeEUN, s->nb_blocks, s->nb_boot_blocks);
+
+	pr_debug("\n-------------------------------------------"
+		"----------------------------------\n");
+}
+
+void INFTL_dumpVUchains(struct INFTLrecord *s)
+{
+	int logical, block, i;
+
+	pr_debug("-------------------------------------------"
+		"----------------------------------\n");
+
+	pr_debug("INFTL Virtual Unit Chains:\n");
+	for (logical = 0; logical < s->nb_blocks; logical++) {
+		block = s->VUtable[logical];
+		if (block >= s->nb_blocks)
+			continue;
+		pr_debug("  LOGICAL %d --> %d ", logical, block);
+		for (i = 0; i < s->nb_blocks; i++) {
+			if (s->PUtable[block] == BLOCK_NIL)
+				break;
+			block = s->PUtable[block];
+			pr_debug("%d ", block);
+		}
+		pr_debug("\n");
+	}
+
+	pr_debug("-------------------------------------------"
+		"----------------------------------\n");
+}
+
+int INFTL_mount(struct INFTLrecord *s)
+{
+	struct mtd_info *mtd = s->mbd.mtd;
+	unsigned int block, first_block, prev_block, last_block;
+	unsigned int first_logical_block, logical_block, erase_mark;
+	int chain_length, do_format_chain;
+	struct inftl_unithead1 h0;
+	struct inftl_unittail h1;
+	size_t retlen;
+	int i;
+	u8 *ANACtable, ANAC;
+
+	pr_debug("INFTL: INFTL_mount(inftl=%p)\n", s);
+
+	/* Search for INFTL MediaHeader and Spare INFTL Media Header */
+	if (find_boot_record(s) < 0) {
+		pr_warn("INFTL: could not find valid boot record?\n");
+		return -ENXIO;
+	}
+
+	/* Init the logical to physical table */
+	for (i = 0; i < s->nb_blocks; i++)
+		s->VUtable[i] = BLOCK_NIL;
+
+	logical_block = block = BLOCK_NIL;
+
+	/* Temporary buffer to store ANAC numbers. */
+	ANACtable = kcalloc(s->nb_blocks, sizeof(u8), GFP_KERNEL);
+	if (!ANACtable)
+		return -ENOMEM;
+
+	/*
+	 * First pass is to explore each physical unit, and construct the
+	 * virtual chains that exist (newest physical unit goes into VUtable).
+	 * Any block that is in any way invalid will be left in the
+	 * NOTEXPLORED state. Then at the end we will try to format it and
+	 * mark it as free.
+	 */
+	pr_debug("INFTL: pass 1, explore each unit\n");
+	for (first_block = s->firstEUN; first_block <= s->lastEUN; first_block++) {
+		if (s->PUtable[first_block] != BLOCK_NOTEXPLORED)
+			continue;
+
+		do_format_chain = 0;
+		first_logical_block = BLOCK_NIL;
+		last_block = BLOCK_NIL;
+		block = first_block;
+
+		for (chain_length = 0; ; chain_length++) {
+
+			if ((chain_length == 0) &&
+			    (s->PUtable[block] != BLOCK_NOTEXPLORED)) {
+				/* Nothing to do here, onto next block */
+				break;
+			}
+
+			if (inftl_read_oob(mtd, block * s->EraseSize + 8,
+					   8, &retlen, (char *)&h0) < 0 ||
+			    inftl_read_oob(mtd, block * s->EraseSize +
+					   2 * SECTORSIZE + 8, 8, &retlen,
+					   (char *)&h1) < 0) {
+				/* Should never happen? */
+				do_format_chain++;
+				break;
+			}
+
+			logical_block = le16_to_cpu(h0.virtualUnitNo);
+			prev_block = le16_to_cpu(h0.prevUnitNo);
+			erase_mark = le16_to_cpu((h1.EraseMark | h1.EraseMark1));
+			ANACtable[block] = h0.ANAC;
+
+			/* Previous block is relative to start of Partition */
+			if (prev_block < s->nb_blocks)
+				prev_block += s->firstEUN;
+
+			/* Already explored partial chain? */
+			if (s->PUtable[block] != BLOCK_NOTEXPLORED) {
+				/* Check if chain for this logical */
+				if (logical_block == first_logical_block) {
+					if (last_block != BLOCK_NIL)
+						s->PUtable[last_block] = block;
+				}
+				break;
+			}
+
+			/* Check for invalid block */
+			if (erase_mark != ERASE_MARK) {
+				pr_warn("INFTL: corrupt block %d "
+					"in chain %d, chain length %d, erase "
+					"mark 0x%x?\n", block, first_block,
+					chain_length, erase_mark);
+				/*
+				 * Assume end of chain, probably incomplete
+				 * fold/erase...
+				 */
+				if (chain_length == 0)
+					do_format_chain++;
+				break;
+			}
+
+			/* Check for it being free already then... */
+			if ((logical_block == BLOCK_FREE) ||
+			    (logical_block == BLOCK_NIL)) {
+				s->PUtable[block] = BLOCK_FREE;
+				break;
+			}
+
+			/* Sanity checks on block numbers */
+			if ((logical_block >= s->nb_blocks) ||
+			    ((prev_block >= s->nb_blocks) &&
+			     (prev_block != BLOCK_NIL))) {
+				if (chain_length > 0) {
+					pr_warn("INFTL: corrupt "
+						"block %d in chain %d?\n",
+						block, first_block);
+					do_format_chain++;
+				}
+				break;
+			}
+
+			if (first_logical_block == BLOCK_NIL) {
+				first_logical_block = logical_block;
+			} else {
+				if (first_logical_block != logical_block) {
+					/* Normal for folded chain... */
+					break;
+				}
+			}
+
+			/*
+			 * Current block is valid, so if we followed a virtual
+			 * chain to get here then we can set the previous
+			 * block pointer in our PUtable now. Then move onto
+			 * the previous block in the chain.
+			 */
+			s->PUtable[block] = BLOCK_NIL;
+			if (last_block != BLOCK_NIL)
+				s->PUtable[last_block] = block;
+			last_block = block;
+			block = prev_block;
+
+			/* Check for end of chain */
+			if (block == BLOCK_NIL)
+				break;
+
+			/* Validate next block before following it... */
+			if (block > s->lastEUN) {
+				pr_warn("INFTL: invalid previous "
+					"block %d in chain %d?\n", block,
+					first_block);
+				do_format_chain++;
+				break;
+			}
+		}
+
+		if (do_format_chain) {
+			format_chain(s, first_block);
+			continue;
+		}
+
+		/*
+		 * Looks like a valid chain then. It may not really be the
+		 * newest block in the chain, but it is the newest we have
+		 * found so far. We might update it in later iterations of
+		 * this loop if we find something newer.
+		 */
+		s->VUtable[first_logical_block] = first_block;
+		logical_block = BLOCK_NIL;
+	}
+
+	INFTL_dumptables(s);
+
+	/*
+	 * Second pass, check for infinite loops in chains. These are
+	 * possible because we don't update the previous pointers when
+	 * we fold chains. No big deal, just fix them up in PUtable.
+	 */
+	pr_debug("INFTL: pass 2, validate virtual chains\n");
+	for (logical_block = 0; logical_block < s->numvunits; logical_block++) {
+		block = s->VUtable[logical_block];
+		last_block = BLOCK_NIL;
+
+		/* Check for free/reserved/nil */
+		if (block >= BLOCK_RESERVED)
+			continue;
+
+		ANAC = ANACtable[block];
+		for (i = 0; i < s->numvunits; i++) {
+			if (s->PUtable[block] == BLOCK_NIL)
+				break;
+			if (s->PUtable[block] > s->lastEUN) {
+				pr_warn("INFTL: invalid prev %d, "
+					"in virtual chain %d\n",
+					s->PUtable[block], logical_block);
+				s->PUtable[block] = BLOCK_NIL;
+
+			}
+			if (ANACtable[block] != ANAC) {
+				/*
+				 * Chain must point back to itself. This is ok,
+				 * but we will need adjust the tables with this
+				 * newest block and oldest block.
+				 */
+				s->VUtable[logical_block] = block;
+				s->PUtable[last_block] = BLOCK_NIL;
+				break;
+			}
+
+			ANAC--;
+			last_block = block;
+			block = s->PUtable[block];
+		}
+
+		if (i >= s->nb_blocks) {
+			/*
+			 * Uhoo, infinite chain with valid ANACS!
+			 * Format whole chain...
+			 */
+			format_chain(s, first_block);
+		}
+	}
+
+	INFTL_dumptables(s);
+	INFTL_dumpVUchains(s);
+
+	/*
+	 * Third pass, format unreferenced blocks and init free block count.
+	 */
+	s->numfreeEUNs = 0;
+	s->LastFreeEUN = BLOCK_NIL;
+
+	pr_debug("INFTL: pass 3, format unused blocks\n");
+	for (block = s->firstEUN; block <= s->lastEUN; block++) {
+		if (s->PUtable[block] == BLOCK_NOTEXPLORED) {
+			pr_warn("INFTL: unreferenced block %d, formatting it\n",
+				block);
+			if (INFTL_formatblock(s, block) < 0)
+				s->PUtable[block] = BLOCK_RESERVED;
+			else
+				s->PUtable[block] = BLOCK_FREE;
+		}
+		if (s->PUtable[block] == BLOCK_FREE) {
+			s->numfreeEUNs++;
+			if (s->LastFreeEUN == BLOCK_NIL)
+				s->LastFreeEUN = block;
+		}
+	}
+
+	kfree(ANACtable);
+	return 0;
+}