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, 710 insertions, 0 deletions

diff --git a/fs/xfs/scrub/rtrefcount.c b/fs/xfs/scrub/rtrefcount.c
new file mode 100644
index 000000000..3d916d71a
--- /dev/null
+++ b/fs/xfs/scrub/rtrefcount.c
@@ -0,0 +1,710 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (c) 2021-2024 Oracle.  All Rights Reserved.
+ * Author: Darrick J. Wong <djwong@kernel.org>
+ */
+#include "xfs_platform.h"
+#include "xfs_fs.h"
+#include "xfs_shared.h"
+#include "xfs_format.h"
+#include "xfs_log_format.h"
+#include "xfs_trans_resv.h"
+#include "xfs_mount.h"
+#include "xfs_trans.h"
+#include "xfs_btree.h"
+#include "xfs_rmap.h"
+#include "xfs_refcount.h"
+#include "xfs_inode.h"
+#include "xfs_rtbitmap.h"
+#include "xfs_rtgroup.h"
+#include "xfs_metafile.h"
+#include "xfs_rtrefcount_btree.h"
+#include "xfs_rtalloc.h"
+#include "xfs_ag.h"
+#include "scrub/scrub.h"
+#include "scrub/common.h"
+#include "scrub/btree.h"
+#include "scrub/repair.h"
+
+/* Set us up with the realtime refcount metadata locked. */
+int
+xchk_setup_rtrefcountbt(
+	struct xfs_scrub	*sc)
+{
+	int			error;
+
+	if (xchk_need_intent_drain(sc))
+		xchk_fsgates_enable(sc, XCHK_FSGATES_DRAIN);
+
+	if (xchk_could_repair(sc)) {
+		error = xrep_setup_rtrefcountbt(sc);
+		if (error)
+			return error;
+	}
+
+	error = xchk_rtgroup_init(sc, sc->sm->sm_agno, &sc->sr);
+	if (error)
+		return error;
+
+	error = xchk_setup_rt(sc);
+	if (error)
+		return error;
+
+	error = xchk_install_live_inode(sc, rtg_refcount(sc->sr.rtg));
+	if (error)
+		return error;
+
+	return xchk_rtgroup_lock(sc, &sc->sr, XCHK_RTGLOCK_ALL);
+}
+
+/* Realtime Reference count btree scrubber. */
+
+/*
+ * Confirming Reference Counts via Reverse Mappings
+ *
+ * We want to count the reverse mappings overlapping a refcount record
+ * (bno, len, refcount), allowing for the possibility that some of the
+ * overlap may come from smaller adjoining reverse mappings, while some
+ * comes from single extents which overlap the range entirely.  The
+ * outer loop is as follows:
+ *
+ * 1. For all reverse mappings overlapping the refcount extent,
+ *    a. If a given rmap completely overlaps, mark it as seen.
+ *    b. Otherwise, record the fragment (in agbno order) for later
+ *       processing.
+ *
+ * Once we've seen all the rmaps, we know that for all blocks in the
+ * refcount record we want to find $refcount owners and we've already
+ * visited $seen extents that overlap all the blocks.  Therefore, we
+ * need to find ($refcount - $seen) owners for every block in the
+ * extent; call that quantity $target_nr.  Proceed as follows:
+ *
+ * 2. Pull the first $target_nr fragments from the list; all of them
+ *    should start at or before the start of the extent.
+ *    Call this subset of fragments the working set.
+ * 3. Until there are no more unprocessed fragments,
+ *    a. Find the shortest fragments in the set and remove them.
+ *    b. Note the block number of the end of these fragments.
+ *    c. Pull the same number of fragments from the list.  All of these
+ *       fragments should start at the block number recorded in the
+ *       previous step.
+ *    d. Put those fragments in the set.
+ * 4. Check that there are $target_nr fragments remaining in the list,
+ *    and that they all end at or beyond the end of the refcount extent.
+ *
+ * If the refcount is correct, all the check conditions in the algorithm
+ * should always hold true.  If not, the refcount is incorrect.
+ */
+struct xchk_rtrefcnt_frag {
+	struct list_head	list;
+	struct xfs_rmap_irec	rm;
+};
+
+struct xchk_rtrefcnt_check {
+	struct xfs_scrub	*sc;
+	struct list_head	fragments;
+
+	/* refcount extent we're examining */
+	xfs_rgblock_t		bno;
+	xfs_extlen_t		len;
+	xfs_nlink_t		refcount;
+
+	/* number of owners seen */
+	xfs_nlink_t		seen;
+};
+
+/*
+ * Decide if the given rmap is large enough that we can redeem it
+ * towards refcount verification now, or if it's a fragment, in
+ * which case we'll hang onto it in the hopes that we'll later
+ * discover that we've collected exactly the correct number of
+ * fragments as the rtrefcountbt says we should have.
+ */
+STATIC int
+xchk_rtrefcountbt_rmap_check(
+	struct xfs_btree_cur		*cur,
+	const struct xfs_rmap_irec	*rec,
+	void				*priv)
+{
+	struct xchk_rtrefcnt_check	*refchk = priv;
+	struct xchk_rtrefcnt_frag	*frag;
+	xfs_rgblock_t			rm_last;
+	xfs_rgblock_t			rc_last;
+	int				error = 0;
+
+	if (xchk_should_terminate(refchk->sc, &error))
+		return error;
+
+	rm_last = rec->rm_startblock + rec->rm_blockcount - 1;
+	rc_last = refchk->bno + refchk->len - 1;
+
+	/* Confirm that a single-owner refc extent is a CoW stage. */
+	if (refchk->refcount == 1 && rec->rm_owner != XFS_RMAP_OWN_COW) {
+		xchk_btree_xref_set_corrupt(refchk->sc, cur, 0);
+		return 0;
+	}
+
+	if (rec->rm_startblock <= refchk->bno && rm_last >= rc_last) {
+		/*
+		 * The rmap overlaps the refcount record, so we can confirm
+		 * one refcount owner seen.
+		 */
+		refchk->seen++;
+	} else {
+		/*
+		 * This rmap covers only part of the refcount record, so
+		 * save the fragment for later processing.  If the rmapbt
+		 * is healthy each rmap_irec we see will be in agbno order
+		 * so we don't need insertion sort here.
+		 */
+		frag = kmalloc_obj(struct xchk_rtrefcnt_frag, XCHK_GFP_FLAGS);
+		if (!frag)
+			return -ENOMEM;
+		memcpy(&frag->rm, rec, sizeof(frag->rm));
+		list_add_tail(&frag->list, &refchk->fragments);
+	}
+
+	return 0;
+}
+
+/*
+ * Given a bunch of rmap fragments, iterate through them, keeping
+ * a running tally of the refcount.  If this ever deviates from
+ * what we expect (which is the rtrefcountbt's refcount minus the
+ * number of extents that totally covered the rtrefcountbt extent),
+ * we have a rtrefcountbt error.
+ */
+STATIC void
+xchk_rtrefcountbt_process_rmap_fragments(
+	struct xchk_rtrefcnt_check	*refchk)
+{
+	struct list_head		worklist;
+	struct xchk_rtrefcnt_frag	*frag;
+	struct xchk_rtrefcnt_frag	*n;
+	xfs_rgblock_t			bno;
+	xfs_rgblock_t			rbno;
+	xfs_rgblock_t			next_rbno;
+	xfs_nlink_t			nr;
+	xfs_nlink_t			target_nr;
+
+	target_nr = refchk->refcount - refchk->seen;
+	if (target_nr == 0)
+		return;
+
+	/*
+	 * There are (refchk->rc.rc_refcount - refchk->nr refcount)
+	 * references we haven't found yet.  Pull that many off the
+	 * fragment list and figure out where the smallest rmap ends
+	 * (and therefore the next rmap should start).  All the rmaps
+	 * we pull off should start at or before the beginning of the
+	 * refcount record's range.
+	 */
+	INIT_LIST_HEAD(&worklist);
+	rbno = NULLRGBLOCK;
+
+	/* Make sure the fragments actually /are/ in bno order. */
+	bno = 0;
+	list_for_each_entry(frag, &refchk->fragments, list) {
+		if (frag->rm.rm_startblock < bno)
+			goto done;
+		bno = frag->rm.rm_startblock;
+	}
+
+	/*
+	 * Find all the rmaps that start at or before the refc extent,
+	 * and put them on the worklist.
+	 */
+	nr = 0;
+	list_for_each_entry_safe(frag, n, &refchk->fragments, list) {
+		if (frag->rm.rm_startblock > refchk->bno || nr > target_nr)
+			break;
+		bno = frag->rm.rm_startblock + frag->rm.rm_blockcount;
+		if (bno < rbno)
+			rbno = bno;
+		list_move_tail(&frag->list, &worklist);
+		nr++;
+	}
+
+	/*
+	 * We should have found exactly $target_nr rmap fragments starting
+	 * at or before the refcount extent.
+	 */
+	if (nr != target_nr)
+		goto done;
+
+	while (!list_empty(&refchk->fragments)) {
+		/* Discard any fragments ending at rbno from the worklist. */
+		nr = 0;
+		next_rbno = NULLRGBLOCK;
+		list_for_each_entry_safe(frag, n, &worklist, list) {
+			bno = frag->rm.rm_startblock + frag->rm.rm_blockcount;
+			if (bno != rbno) {
+				if (bno < next_rbno)
+					next_rbno = bno;
+				continue;
+			}
+			list_del(&frag->list);
+			kfree(frag);
+			nr++;
+		}
+
+		/* Try to add nr rmaps starting at rbno to the worklist. */
+		list_for_each_entry_safe(frag, n, &refchk->fragments, list) {
+			bno = frag->rm.rm_startblock + frag->rm.rm_blockcount;
+			if (frag->rm.rm_startblock != rbno)
+				goto done;
+			list_move_tail(&frag->list, &worklist);
+			if (next_rbno > bno)
+				next_rbno = bno;
+			nr--;
+			if (nr == 0)
+				break;
+		}
+
+		/*
+		 * If we get here and nr > 0, this means that we added fewer
+		 * items to the worklist than we discarded because the fragment
+		 * list ran out of items.  Therefore, we cannot maintain the
+		 * required refcount.  Something is wrong, so we're done.
+		 */
+		if (nr)
+			goto done;
+
+		rbno = next_rbno;
+	}
+
+	/*
+	 * Make sure the last extent we processed ends at or beyond
+	 * the end of the refcount extent.
+	 */
+	if (rbno < refchk->bno + refchk->len)
+		goto done;
+
+	/* Actually record us having seen the remaining refcount. */
+	refchk->seen = refchk->refcount;
+done:
+	/* Delete fragments and work list. */
+	list_for_each_entry_safe(frag, n, &worklist, list) {
+		list_del(&frag->list);
+		kfree(frag);
+	}
+	list_for_each_entry_safe(frag, n, &refchk->fragments, list) {
+		list_del(&frag->list);
+		kfree(frag);
+	}
+}
+
+/* Use the rmap entries covering this extent to verify the refcount. */
+STATIC void
+xchk_rtrefcountbt_xref_rmap(
+	struct xfs_scrub		*sc,
+	const struct xfs_refcount_irec	*irec)
+{
+	struct xchk_rtrefcnt_check	refchk = {
+		.sc			= sc,
+		.bno			= irec->rc_startblock,
+		.len			= irec->rc_blockcount,
+		.refcount		= irec->rc_refcount,
+		.seen			= 0,
+	};
+	struct xfs_rmap_irec		low;
+	struct xfs_rmap_irec		high;
+	struct xchk_rtrefcnt_frag	*frag;
+	struct xchk_rtrefcnt_frag	*n;
+	int				error;
+
+	if (!sc->sr.rmap_cur || xchk_skip_xref(sc->sm))
+		return;
+
+	/* Cross-reference with the rmapbt to confirm the refcount. */
+	memset(&low, 0, sizeof(low));
+	low.rm_startblock = irec->rc_startblock;
+	memset(&high, 0xFF, sizeof(high));
+	high.rm_startblock = irec->rc_startblock + irec->rc_blockcount - 1;
+
+	INIT_LIST_HEAD(&refchk.fragments);
+	error = xfs_rmap_query_range(sc->sr.rmap_cur, &low, &high,
+			xchk_rtrefcountbt_rmap_check, &refchk);
+	if (!xchk_should_check_xref(sc, &error, &sc->sr.rmap_cur))
+		goto out_free;
+
+	xchk_rtrefcountbt_process_rmap_fragments(&refchk);
+	if (irec->rc_refcount != refchk.seen)
+		xchk_btree_xref_set_corrupt(sc, sc->sr.rmap_cur, 0);
+
+out_free:
+	list_for_each_entry_safe(frag, n, &refchk.fragments, list) {
+		list_del(&frag->list);
+		kfree(frag);
+	}
+}
+
+/* Cross-reference with the other btrees. */
+STATIC void
+xchk_rtrefcountbt_xref(
+	struct xfs_scrub		*sc,
+	const struct xfs_refcount_irec	*irec)
+{
+	if (sc->sm->sm_flags & XFS_SCRUB_OFLAG_CORRUPT)
+		return;
+
+	xchk_xref_is_used_rt_space(sc,
+			xfs_rgbno_to_rtb(sc->sr.rtg, irec->rc_startblock),
+			irec->rc_blockcount);
+	xchk_rtrefcountbt_xref_rmap(sc, irec);
+}
+
+struct xchk_rtrefcbt_records {
+	/* Previous refcount record. */
+	struct xfs_refcount_irec	prev_rec;
+
+	/* The next rtgroup block where we aren't expecting shared extents. */
+	xfs_rgblock_t			next_unshared_rgbno;
+
+	/* Number of CoW blocks we expect. */
+	xfs_extlen_t			cow_blocks;
+
+	/* Was the last record a shared or CoW staging extent? */
+	enum xfs_refc_domain		prev_domain;
+};
+
+static inline bool
+xchk_rtrefcount_mergeable(
+	struct xchk_rtrefcbt_records	*rrc,
+	const struct xfs_refcount_irec	*r2)
+{
+	const struct xfs_refcount_irec	*r1 = &rrc->prev_rec;
+
+	/* Ignore if prev_rec is not yet initialized. */
+	if (r1->rc_blockcount == 0)
+		return false;
+
+	if (r1->rc_startblock + r1->rc_blockcount != r2->rc_startblock)
+		return false;
+	if (r1->rc_refcount != r2->rc_refcount)
+		return false;
+	if ((unsigned long long)r1->rc_blockcount + r2->rc_blockcount >
+			XFS_REFC_LEN_MAX)
+		return false;
+
+	return true;
+}
+
+/* Flag failures for records that could be merged. */
+STATIC void
+xchk_rtrefcountbt_check_mergeable(
+	struct xchk_btree		*bs,
+	struct xchk_rtrefcbt_records	*rrc,
+	const struct xfs_refcount_irec	*irec)
+{
+	if (bs->sc->sm->sm_flags & XFS_SCRUB_OFLAG_CORRUPT)
+		return;
+
+	if (xchk_rtrefcount_mergeable(rrc, irec))
+		xchk_btree_set_corrupt(bs->sc, bs->cur, 0);
+
+	memcpy(&rrc->prev_rec, irec, sizeof(struct xfs_refcount_irec));
+}
+
+STATIC int
+xchk_rtrefcountbt_rmap_check_gap(
+	struct xfs_btree_cur		*cur,
+	const struct xfs_rmap_irec	*rec,
+	void				*priv)
+{
+	xfs_rgblock_t			*next_bno = priv;
+
+	if (*next_bno != NULLRGBLOCK && rec->rm_startblock < *next_bno)
+		return -ECANCELED;
+
+	*next_bno = rec->rm_startblock + rec->rm_blockcount;
+	return 0;
+}
+
+/*
+ * Make sure that a gap in the reference count records does not correspond to
+ * overlapping records (i.e. shared extents) in the reverse mappings.
+ */
+static inline void
+xchk_rtrefcountbt_xref_gaps(
+	struct xfs_scrub	*sc,
+	struct xchk_rtrefcbt_records *rrc,
+	xfs_rgblock_t		bno)
+{
+	struct xfs_rmap_irec	low;
+	struct xfs_rmap_irec	high;
+	xfs_rgblock_t		next_bno = NULLRGBLOCK;
+	int			error;
+
+	if (bno <= rrc->next_unshared_rgbno || !sc->sr.rmap_cur ||
+            xchk_skip_xref(sc->sm))
+		return;
+
+	memset(&low, 0, sizeof(low));
+	low.rm_startblock = rrc->next_unshared_rgbno;
+	memset(&high, 0xFF, sizeof(high));
+	high.rm_startblock = bno - 1;
+
+	error = xfs_rmap_query_range(sc->sr.rmap_cur, &low, &high,
+			xchk_rtrefcountbt_rmap_check_gap, &next_bno);
+	if (error == -ECANCELED)
+		xchk_btree_xref_set_corrupt(sc, sc->sr.rmap_cur, 0);
+	else
+		xchk_should_check_xref(sc, &error, &sc->sr.rmap_cur);
+}
+
+/* Scrub a rtrefcountbt record. */
+STATIC int
+xchk_rtrefcountbt_rec(
+	struct xchk_btree		*bs,
+	const union xfs_btree_rec	*rec)
+{
+	struct xfs_mount		*mp = bs->cur->bc_mp;
+	struct xchk_rtrefcbt_records	*rrc = bs->private;
+	struct xfs_refcount_irec	irec;
+	u32				mod;
+
+	xfs_refcount_btrec_to_irec(rec, &irec);
+	if (xfs_rtrefcount_check_irec(to_rtg(bs->cur->bc_group), &irec) !=
+			NULL) {
+		xchk_btree_set_corrupt(bs->sc, bs->cur, 0);
+		return 0;
+	}
+
+	/* We can only share full rt extents. */
+	mod = xfs_rgbno_to_rtxoff(mp, irec.rc_startblock);
+	if (mod)
+		xchk_btree_set_corrupt(bs->sc, bs->cur, 0);
+	mod = xfs_extlen_to_rtxmod(mp, irec.rc_blockcount);
+	if (mod)
+		xchk_btree_set_corrupt(bs->sc, bs->cur, 0);
+
+	if (irec.rc_domain == XFS_REFC_DOMAIN_COW)
+		rrc->cow_blocks += irec.rc_blockcount;
+
+	/* Shared records always come before CoW records. */
+	if (irec.rc_domain == XFS_REFC_DOMAIN_SHARED &&
+	    rrc->prev_domain == XFS_REFC_DOMAIN_COW)
+		xchk_btree_set_corrupt(bs->sc, bs->cur, 0);
+	rrc->prev_domain = irec.rc_domain;
+
+	xchk_rtrefcountbt_check_mergeable(bs, rrc, &irec);
+	xchk_rtrefcountbt_xref(bs->sc, &irec);
+
+	/*
+	 * If this is a record for a shared extent, check that all blocks
+	 * between the previous record and this one have at most one reverse
+	 * mapping.
+	 */
+	if (irec.rc_domain == XFS_REFC_DOMAIN_SHARED) {
+		xchk_rtrefcountbt_xref_gaps(bs->sc, rrc, irec.rc_startblock);
+		rrc->next_unshared_rgbno = irec.rc_startblock +
+					   irec.rc_blockcount;
+	}
+
+	return 0;
+}
+
+/* Count the number of blocks used by the rtrefcount btree file in this AG. */
+static int
+xchk_rtrefcount_count_agblocks(
+	struct xfs_scrub	*sc,
+	xfs_agnumber_t		agno,
+	const struct xfs_owner_info *btree_oinfo,
+	xfs_filblks_t		*blocks)
+{
+	xfs_filblks_t		agblocks = 0;
+	int			error;
+
+	error = xchk_ag_init_existing(sc, agno, &sc->sa);
+	if (error)
+		goto out_free;
+
+	/*
+	 * If we don't have an rmap cursor, we can't complete the cross
+	 * referencing, so return EFSCORRUPTED to end the loop and trigger the
+	 * XFAIL flag.
+	 */
+	if (!sc->sa.rmap_cur) {
+		error = -EFSCORRUPTED;
+		goto out_free;
+	}
+
+	error = xchk_count_rmap_ownedby_ag(sc, sc->sa.rmap_cur, btree_oinfo,
+			&agblocks);
+	if (error)
+		goto out_free;
+
+	*blocks += agblocks;
+out_free:
+	xchk_ag_free(sc, &sc->sa);
+	return error;
+}
+
+/* Make sure we have as many refc blocks as the rmap says. */
+STATIC void
+xchk_rtrefcount_xref_rmap(
+	struct xfs_scrub	*sc,
+	const struct xfs_owner_info *btree_oinfo,
+	xfs_extlen_t		cow_blocks)
+{
+	xfs_filblks_t		refcbt_blocks = 0;
+	xfs_filblks_t		blocks = 1; /* one for the iroot */
+	xfs_agnumber_t		agno;
+	int			error = 0;
+
+	if (!xfs_has_rmapbt(sc->mp) || xchk_skip_xref(sc->sm))
+		return;
+
+	/* Check that we saw as many refcbt blocks as the rmap knows about. */
+	error = xfs_btree_count_blocks(sc->sr.refc_cur, &refcbt_blocks);
+	if (!xchk_btree_process_error(sc, sc->sr.refc_cur, 0, &error))
+		return;
+
+	for (agno = 0; agno < sc->mp->m_sb.sb_agcount; agno++) {
+		error = xchk_rtrefcount_count_agblocks(sc, agno, btree_oinfo,
+				&blocks);
+		if (error)
+			break;
+	}
+	if (!xchk_fblock_xref_process_error(sc, XFS_DATA_FORK, 0, &error))
+		return;
+	if (blocks != refcbt_blocks)
+		xchk_fblock_xref_set_corrupt(sc, XFS_DATA_FORK, 0);
+
+	if (!sc->sr.rmap_cur || xchk_skip_xref(sc->sm))
+		return;
+
+	/* Check that we saw as many cow blocks as the rmap knows about. */
+	error = xchk_count_rmap_ownedby_ag(sc, sc->sr.rmap_cur,
+			&XFS_RMAP_OINFO_COW, &blocks);
+	if (!xchk_should_check_xref(sc, &error, &sc->sr.rmap_cur))
+		return;
+	if (blocks != cow_blocks)
+		xchk_btree_xref_set_corrupt(sc, sc->sr.rmap_cur, 0);
+}
+
+/* Scrub the refcount btree for some rtgroup. */
+int
+xchk_rtrefcountbt(
+	struct xfs_scrub	*sc)
+{
+	struct xfs_owner_info	btree_oinfo;
+	struct xchk_rtrefcbt_records rrc = {
+		.cow_blocks		= 0,
+		.next_unshared_rgbno	= 0,
+		.prev_domain		= XFS_REFC_DOMAIN_SHARED,
+	};
+	int			error;
+
+	error = xchk_metadata_inode_forks(sc);
+	if (error || (sc->sm->sm_flags & XFS_SCRUB_OFLAG_CORRUPT))
+		return error;
+
+	xfs_rmap_inode_bmbt_owner(&btree_oinfo, rtg_refcount(sc->sr.rtg),
+			XFS_DATA_FORK);
+	error = xchk_btree(sc, sc->sr.refc_cur, xchk_rtrefcountbt_rec,
+			&btree_oinfo, &rrc);
+	if (error || (sc->sm->sm_flags & XFS_SCRUB_OFLAG_CORRUPT))
+		return error;
+
+	/*
+	 * Check that all blocks between the last refcount > 1 record and the
+	 * end of the rtgroup have at most one reverse mapping.
+	 */
+	xchk_rtrefcountbt_xref_gaps(sc, &rrc,
+			xfs_rtx_to_rgbno(sc->sr.rtg, sc->mp->m_sb.sb_rgextents));
+	xchk_rtrefcount_xref_rmap(sc, &btree_oinfo, rrc.cow_blocks);
+
+	return 0;
+}
+
+/* xref check that a cow staging extent is marked in the rtrefcountbt. */
+void
+xchk_xref_is_rt_cow_staging(
+	struct xfs_scrub		*sc,
+	xfs_rgblock_t			bno,
+	xfs_extlen_t			len)
+{
+	struct xfs_refcount_irec	rc;
+	int				has_refcount;
+	int				error;
+
+	if (!sc->sr.refc_cur || xchk_skip_xref(sc->sm))
+		return;
+
+	/* Find the CoW staging extent. */
+	error = xfs_refcount_lookup_le(sc->sr.refc_cur, XFS_REFC_DOMAIN_COW,
+			bno, &has_refcount);
+	if (!xchk_should_check_xref(sc, &error, &sc->sr.refc_cur))
+		return;
+	if (!has_refcount) {
+		xchk_btree_xref_set_corrupt(sc, sc->sr.refc_cur, 0);
+		return;
+	}
+
+	error = xfs_refcount_get_rec(sc->sr.refc_cur, &rc, &has_refcount);
+	if (!xchk_should_check_xref(sc, &error, &sc->sr.refc_cur))
+		return;
+	if (!has_refcount) {
+		xchk_btree_xref_set_corrupt(sc, sc->sr.refc_cur, 0);
+		return;
+	}
+
+	/* CoW lookup returned a shared extent record? */
+	if (rc.rc_domain != XFS_REFC_DOMAIN_COW)
+		xchk_btree_xref_set_corrupt(sc, sc->sr.refc_cur, 0);
+
+	/* Can't start after bno */
+	if (rc.rc_startblock > bno)
+		xchk_btree_xref_set_corrupt(sc, sc->sr.refc_cur, 0);
+
+	/* Must be at least as long as what was passed in */
+	if (rc.rc_startblock + rc.rc_blockcount < bno + len)
+		xchk_btree_xref_set_corrupt(sc, sc->sr.refc_cur, 0);
+}
+
+/*
+ * xref check that the extent is not shared.  Only file data blocks
+ * can have multiple owners.
+ */
+void
+xchk_xref_is_not_rt_shared(
+	struct xfs_scrub	*sc,
+	xfs_rgblock_t		bno,
+	xfs_extlen_t		len)
+{
+	enum xbtree_recpacking	outcome;
+	int			error;
+
+	if (!sc->sr.refc_cur || xchk_skip_xref(sc->sm))
+		return;
+
+	error = xfs_refcount_has_records(sc->sr.refc_cur,
+			XFS_REFC_DOMAIN_SHARED, bno, len, &outcome);
+	if (!xchk_should_check_xref(sc, &error, &sc->sr.refc_cur))
+		return;
+	if (outcome != XBTREE_RECPACKING_EMPTY)
+		xchk_btree_xref_set_corrupt(sc, sc->sr.refc_cur, 0);
+}
+
+/* xref check that the extent is not being used for CoW staging. */
+void
+xchk_xref_is_not_rt_cow_staging(
+	struct xfs_scrub	*sc,
+	xfs_rgblock_t		bno,
+	xfs_extlen_t		len)
+{
+	enum xbtree_recpacking	outcome;
+	int			error;
+
+	if (!sc->sr.refc_cur || xchk_skip_xref(sc->sm))
+		return;
+
+	error = xfs_refcount_has_records(sc->sr.refc_cur, XFS_REFC_DOMAIN_COW,
+			bno, len, &outcome);
+	if (!xchk_should_check_xref(sc, &error, &sc->sr.refc_cur))
+		return;
+	if (outcome != XBTREE_RECPACKING_EMPTY)
+		xchk_btree_xref_set_corrupt(sc, sc->sr.refc_cur, 0);
+}