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, 660 insertions, 0 deletions

diff --git a/fs/netfs/read_collect.c b/fs/netfs/read_collect.c
new file mode 100644
index 000000000..5cf22087d
--- /dev/null
+++ b/fs/netfs/read_collect.c
@@ -0,0 +1,660 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/* Network filesystem read subrequest result collection, assessment and
+ * retrying.
+ *
+ * Copyright (C) 2024 Red Hat, Inc. All Rights Reserved.
+ * Written by David Howells (dhowells@redhat.com)
+ */
+
+#include <linux/export.h>
+#include <linux/fs.h>
+#include <linux/mm.h>
+#include <linux/pagemap.h>
+#include <linux/slab.h>
+#include <linux/task_io_accounting_ops.h>
+#include "internal.h"
+
+/* Notes made in the collector */
+#define HIT_PENDING	0x01	/* A front op was still pending */
+#define MADE_PROGRESS	0x04	/* Made progress cleaning up a stream or the folio set */
+#define BUFFERED	0x08	/* The pagecache needs cleaning up */
+#define NEED_RETRY	0x10	/* A front op requests retrying */
+#define ABANDON_SREQ	0x80	/* Need to abandon untransferred part of subrequest */
+
+/*
+ * Clear the unread part of an I/O request.
+ */
+static void netfs_clear_unread(struct netfs_io_subrequest *subreq)
+{
+	netfs_reset_iter(subreq);
+	WARN_ON_ONCE(subreq->len - subreq->transferred != iov_iter_count(&subreq->io_iter));
+	iov_iter_zero(iov_iter_count(&subreq->io_iter), &subreq->io_iter);
+	if (subreq->start + subreq->transferred >= subreq->rreq->i_size)
+		__set_bit(NETFS_SREQ_HIT_EOF, &subreq->flags);
+}
+
+/*
+ * Cancel the copy-to-cache mark on a folio.
+ */
+void netfs_cancel_copy_to_cache(struct netfs_io_request *rreq, struct folio *folio)
+{
+	if (!test_bit(NETFS_RREQ_USE_PGPRIV2, &rreq->flags)) {
+		if (folio_get_private(folio) == NETFS_FOLIO_COPY_TO_CACHE) {
+			folio_detach_private(folio);
+			trace_netfs_folio(folio, netfs_folio_trace_cancel_copy);
+		} else if (netfs_folio_group(folio) == NETFS_FOLIO_COPY_TO_CACHE)  {
+			struct netfs_folio *finfo = netfs_folio_info(folio);
+
+			finfo->netfs_group = NULL;
+			trace_netfs_folio(folio, netfs_folio_trace_cancel_copy);
+		}
+	} else {
+		// TODO: Use of PG_private_2 is deprecated.
+		if (folio_test_private_2(folio)) {
+			folio_end_private_2(folio);
+			trace_netfs_folio(folio, netfs_folio_trace_cancel_copy);
+		}
+	}
+}
+
+/*
+ * Flush, mark and unlock a folio that's now completely read.  If we want to
+ * cache the folio, we set the group to NETFS_FOLIO_COPY_TO_CACHE, mark it
+ * dirty and let writeback handle it.
+ */
+static void netfs_unlock_read_folio(struct netfs_io_request *rreq,
+				    struct folio_queue *folioq,
+				    int slot)
+{
+	struct netfs_folio *finfo;
+	struct folio *folio = folioq_folio(folioq, slot);
+
+	if (unlikely(folio_pos(folio) < rreq->abandon_to)) {
+		trace_netfs_folio(folio, netfs_folio_trace_abandon);
+		netfs_cancel_copy_to_cache(rreq, folio);
+		goto just_unlock;
+	}
+
+	flush_dcache_folio(folio);
+	folio_mark_uptodate(folio);
+
+	if (unlikely(test_bit(NETFS_RREQ_CANCEL_CACHING, &rreq->flags)))
+		netfs_cancel_copy_to_cache(rreq, folio);
+
+	if (!test_bit(NETFS_RREQ_USE_PGPRIV2, &rreq->flags)) {
+		if (netfs_folio_group(folio) == NETFS_FOLIO_COPY_TO_CACHE)  {
+			trace_netfs_folio(folio, netfs_folio_trace_sched_copy);
+			folio_mark_dirty(folio);
+		} else {
+			finfo = netfs_folio_info(folio);
+			if (finfo) {
+				trace_netfs_folio(folio, netfs_folio_trace_filled_gaps);
+				if (finfo->netfs_group)
+					folio_change_private(folio, finfo->netfs_group);
+				else
+					folio_detach_private(folio);
+				kfree(finfo);
+			}
+			trace_netfs_folio(folio, netfs_folio_trace_read_done);
+		}
+
+		folioq_clear(folioq, slot);
+	} else {
+		// TODO: Use of PG_private_2 is deprecated.
+		if (folio_test_private_2(folio))
+			netfs_pgpriv2_copy_to_cache(rreq, folio);
+	}
+
+just_unlock:
+	if (folio == rreq->no_unlock_folio &&
+	    test_bit(NETFS_RREQ_NO_UNLOCK_FOLIO, &rreq->flags)) {
+		_debug("no unlock");
+	} else {
+		trace_netfs_folio(folio, netfs_folio_trace_read_unlock);
+		folio_unlock(folio);
+	}
+
+	folioq_clear(folioq, slot);
+}
+
+/*
+ * Determine how much to gather before unlocking more folios.
+ */
+void netfs_read_set_unlock_at(struct netfs_io_request *rreq)
+{
+	struct folio_queue *folioq = rreq->buffer.tail;
+	unsigned int slot = rreq->buffer.first_tail_slot;
+	size_t cleaned_to = rreq->cleaned_to - rreq->start;
+	size_t progress_at = cleaned_to;
+	size_t minimum = 256 * 1024;
+
+	while (progress_at < rreq->len) {
+		if (slot >= folioq_count(folioq)) {
+			folioq = folioq->next;
+			if (!folioq)
+				break;
+			slot = 0;
+		}
+
+		progress_at += folioq_folio_size(folioq, slot);
+		if (progress_at - cleaned_to >= minimum)
+			break;
+		slot++;
+	}
+
+	WRITE_ONCE(rreq->progress_at, progress_at);
+	trace_netfs_read_progress_at(rreq);
+}
+
+/*
+ * Unlock any folios we've finished with.
+ */
+static void netfs_read_unlock_folios(struct netfs_io_request *rreq,
+				     unsigned int *notes)
+{
+	struct folio_queue *folioq = rreq->buffer.tail;
+	unsigned long long collected_to = rreq->collected_to;
+	unsigned int slot = rreq->buffer.first_tail_slot;
+
+	if (rreq->cleaned_to >= rreq->collected_to)
+		return;
+
+	// TODO: Begin decryption
+
+	if (slot >= folioq_nr_slots(folioq)) {
+		folioq = rolling_buffer_delete_spent(&rreq->buffer);
+		if (!folioq) {
+			WRITE_ONCE(rreq->progress_at, rreq->len);
+			return;
+		}
+		slot = 0;
+	}
+
+	/* We have to wait for readahead refs to have been released before we
+	 * can unlock any folios as the ref-dropper walks i_pages and the only
+	 * thing preventing these folios from being removed is the folio lock.
+	 */
+	if (test_bit(NETFS_RREQ_NEED_PUT_RA_REFS, &rreq->flags))
+		netfs_wait_for_put_ra_refs(rreq);
+
+	for (;;) {
+		struct folio *folio;
+		unsigned long long fpos, fend;
+		size_t fsize;
+
+		folio = folioq_folio(folioq, slot);
+		if (WARN_ONCE(!folio_test_locked(folio),
+			      "R=%08x: folio %lx is not locked\n",
+			      rreq->debug_id, folio->index))
+			trace_netfs_folio(folio, netfs_folio_trace_not_locked);
+
+		fsize = folioq_folio_size(folioq, slot);
+		fpos = folio_pos(folio);
+		fend = fpos + fsize;
+
+		trace_netfs_collect_folio(rreq, folio, fend, collected_to);
+
+		/* Unlock any folio we've transferred all of. */
+		if (collected_to < fend)
+			break;
+
+		netfs_unlock_read_folio(rreq, folioq, slot);
+		WRITE_ONCE(rreq->cleaned_to, fpos + fsize);
+		*notes |= MADE_PROGRESS;
+
+		/* Clean up the head folioq.  If we clear an entire folioq, then
+		 * we can get rid of it provided it's not also the tail folioq
+		 * being filled by the issuer.
+		 */
+		folioq_clear(folioq, slot);
+		slot++;
+		if (slot >= folioq_nr_slots(folioq)) {
+			folioq = rolling_buffer_delete_spent(&rreq->buffer);
+			if (!folioq)
+				goto done;
+			slot = 0;
+			trace_netfs_folioq(folioq, netfs_trace_folioq_read_progress);
+		}
+
+		if (fpos + fsize >= collected_to)
+			break;
+	}
+
+	rreq->buffer.tail = folioq;
+done:
+	rreq->buffer.first_tail_slot = slot;
+
+	netfs_read_set_unlock_at(rreq);
+}
+
+/*
+ * Collect and assess the results of various read subrequests.  We may need to
+ * retry some of the results.
+ *
+ * Note that we have a sequence of subrequests, which may be drawing on
+ * different sources and may or may not be the same size or starting position
+ * and may not even correspond in boundary alignment.
+ */
+static void netfs_collect_read_results(struct netfs_io_request *rreq)
+{
+	struct netfs_io_subrequest *front, *remove;
+	struct netfs_io_stream *stream = &rreq->io_streams[0];
+	unsigned int notes;
+
+	_enter("%llx-%llx", rreq->start, rreq->start + rreq->len);
+	trace_netfs_rreq(rreq, netfs_rreq_trace_collect);
+	trace_netfs_collect(rreq);
+
+reassess:
+	if (rreq->origin == NETFS_READAHEAD ||
+	    rreq->origin == NETFS_READPAGE ||
+	    rreq->origin == NETFS_READ_FOR_WRITE)
+		notes = BUFFERED;
+	else
+		notes = 0;
+
+	/* Remove completed subrequests from the front of the stream and
+	 * advance the completion point.  We stop when we hit something that's
+	 * in progress.  The issuer thread may be adding stuff to the tail
+	 * whilst we're doing this.
+	 */
+	front = list_first_entry_or_null_acquire(&stream->subrequests,
+						 struct netfs_io_subrequest, rreq_link);
+	/* Read first subreq pointer before IN_PROGRESS flag. */
+
+	while (front) {
+		size_t transferred;
+
+		trace_netfs_collect_sreq(rreq, front);
+		_debug("sreq [%x] %llx %zx/%zx",
+		       front->debug_index, front->start, front->transferred, front->len);
+
+		if (stream->collected_to < front->start) {
+			trace_netfs_collect_gap(rreq, stream, front->start, 'F');
+			stream->collected_to = front->start;
+		}
+
+		if (netfs_check_subreq_in_progress(front))
+			notes |= HIT_PENDING;
+		smp_rmb(); /* Read counters after IN_PROGRESS flag. */
+		transferred = READ_ONCE(front->transferred);
+
+		/* If we can now collect the next folio, do so.  We don't want
+		 * to defer this as we have to decide whether we need to copy
+		 * to the cache or not, and that may differ between adjacent
+		 * subreqs.
+		 */
+		if (notes & BUFFERED) {
+			uoff_t unlock_at = rreq->start + rreq->progress_at;
+
+			/* Clear the tail of a short read. */
+			if (!(notes & HIT_PENDING) &&
+			    front->error == 0 &&
+			    transferred < front->len &&
+			    (test_bit(NETFS_SREQ_HIT_EOF, &front->flags) ||
+			     test_bit(NETFS_SREQ_CLEAR_TAIL, &front->flags))) {
+				netfs_clear_unread(front);
+				transferred = front->transferred = front->len;
+				trace_netfs_sreq(front, netfs_sreq_trace_clear);
+			}
+
+			stream->collected_to = front->start + transferred;
+			rreq->collected_to = stream->collected_to;
+
+			if (test_bit(NETFS_SREQ_FAILED, &front->flags)) {
+				rreq->abandon_to = front->start + front->len;
+				front->transferred = front->len;
+				transferred = front->len;
+				trace_netfs_rreq(rreq, netfs_rreq_trace_set_abandon);
+			}
+			if (front->start + transferred >= unlock_at ||
+			    test_bit(NETFS_SREQ_HIT_EOF, &front->flags))
+				netfs_read_unlock_folios(rreq, &notes);
+		} else {
+			stream->collected_to = front->start + transferred;
+			rreq->collected_to = stream->collected_to;
+		}
+
+		/* Stall if the front is still undergoing I/O. */
+		if (notes & HIT_PENDING)
+			break;
+
+		if (test_bit(NETFS_SREQ_FAILED, &front->flags)) {
+			if (!stream->failed) {
+				stream->error = front->error;
+				rreq->error = front->error;
+				set_bit(NETFS_RREQ_FAILED, &rreq->flags);
+				stream->failed = true;
+			}
+			notes |= MADE_PROGRESS | ABANDON_SREQ;
+		} else if (test_bit(NETFS_SREQ_NEED_RETRY, &front->flags)) {
+			stream->need_retry = true;
+			notes |= NEED_RETRY | MADE_PROGRESS;
+			break;
+		} else if (test_bit(NETFS_RREQ_SHORT_TRANSFER, &rreq->flags)) {
+			notes |= MADE_PROGRESS;
+		} else {
+			if (!stream->failed) {
+				stream->transferred += transferred;
+				stream->transferred_valid = true;
+			}
+			if (front->transferred < front->len)
+				set_bit(NETFS_RREQ_SHORT_TRANSFER, &rreq->flags);
+			notes |= MADE_PROGRESS;
+		}
+
+		/* Remove if completely consumed. */
+		stream->source = front->source;
+		spin_lock(&rreq->lock);
+
+		remove = front;
+		trace_netfs_sreq(front,
+				 notes & ABANDON_SREQ ?
+				 netfs_sreq_trace_abandoned : netfs_sreq_trace_consumed);
+		list_del_init(&front->rreq_link);
+		front = list_first_entry_or_null(&stream->subrequests,
+						 struct netfs_io_subrequest, rreq_link);
+		spin_unlock(&rreq->lock);
+		netfs_put_subrequest(remove,
+				     notes & ABANDON_SREQ ?
+				     netfs_sreq_trace_put_abandon :
+				     netfs_sreq_trace_put_done);
+	}
+
+	trace_netfs_collect_stream(rreq, stream);
+	trace_netfs_collect_state(rreq, rreq->collected_to, notes);
+
+	if (!(notes & BUFFERED))
+		rreq->cleaned_to = rreq->collected_to;
+
+	if (notes & NEED_RETRY)
+		goto need_retry;
+	if (notes & MADE_PROGRESS) {
+		netfs_wake_rreq_flag(rreq, NETFS_RREQ_PAUSE, netfs_rreq_trace_unpause);
+		//cond_resched();
+		goto reassess;
+	}
+
+out:
+	_leave(" = %x", notes);
+	return;
+
+need_retry:
+	/* Okay...  We're going to have to retry parts of the stream.  Note
+	 * that any partially completed op will have had any wholly transferred
+	 * folios removed from it.
+	 */
+	_debug("retry");
+	netfs_retry_reads(rreq);
+	goto out;
+}
+
+/*
+ * Do page flushing and suchlike after DIO.
+ */
+static void netfs_rreq_assess_dio(struct netfs_io_request *rreq)
+{
+	unsigned int i;
+
+	if (rreq->origin == NETFS_UNBUFFERED_READ ||
+	    rreq->origin == NETFS_DIO_READ) {
+		for (i = 0; i < rreq->direct_bv_count; i++) {
+			flush_dcache_page(rreq->direct_bv[i].bv_page);
+			// TODO: cifs marks pages in the destination buffer
+			// dirty under some circumstances after a read.  Do we
+			// need to do that too?
+			set_page_dirty(rreq->direct_bv[i].bv_page);
+		}
+	}
+
+	if (rreq->iocb) {
+		rreq->iocb->ki_pos += rreq->transferred;
+		if (rreq->iocb->ki_complete) {
+			trace_netfs_rreq(rreq, netfs_rreq_trace_ki_complete);
+			rreq->iocb->ki_complete(
+				rreq->iocb, rreq->error ? rreq->error : rreq->transferred);
+		}
+	}
+	if (rreq->netfs_ops->done)
+		rreq->netfs_ops->done(rreq);
+	if (rreq->origin == NETFS_UNBUFFERED_READ ||
+	    rreq->origin == NETFS_DIO_READ)
+		inode_dio_end(rreq->inode);
+}
+
+/*
+ * Do processing after reading a monolithic single object.
+ */
+static void netfs_rreq_assess_single(struct netfs_io_request *rreq)
+{
+	struct netfs_io_stream *stream = &rreq->io_streams[0];
+
+	if (!rreq->error && stream->source == NETFS_DOWNLOAD_FROM_SERVER &&
+	    fscache_resources_valid(&rreq->cache_resources)) {
+		trace_netfs_rreq(rreq, netfs_rreq_trace_dirty);
+		netfs_single_mark_inode_dirty(rreq->inode);
+	}
+
+	if (rreq->iocb) {
+		rreq->iocb->ki_pos += rreq->transferred;
+		if (rreq->iocb->ki_complete) {
+			trace_netfs_rreq(rreq, netfs_rreq_trace_ki_complete);
+			rreq->iocb->ki_complete(
+				rreq->iocb, rreq->error ? rreq->error : rreq->transferred);
+		}
+	}
+	if (rreq->netfs_ops->done)
+		rreq->netfs_ops->done(rreq);
+}
+
+/*
+ * Perform the collection of subrequests and folios.
+ *
+ * Note that we're in normal kernel thread context at this point, possibly
+ * running on a workqueue.
+ */
+bool netfs_read_collection(struct netfs_io_request *rreq)
+{
+	struct netfs_io_stream *stream = &rreq->io_streams[0];
+
+	netfs_collect_read_results(rreq);
+
+	/* We're done when the app thread has finished posting subreqs and the
+	 * queue is empty.
+	 */
+	if (!test_bit(NETFS_RREQ_ALL_QUEUED, &rreq->flags))
+		return false;
+	smp_rmb(); /* Read ALL_QUEUED before subreq lists. */
+
+	if (!list_empty(&stream->subrequests))
+		return false;
+
+	/* Okay, declare that all I/O is complete. */
+	rreq->transferred = stream->transferred;
+	trace_netfs_rreq(rreq, netfs_rreq_trace_complete);
+
+	//netfs_rreq_is_still_valid(rreq);
+
+	switch (rreq->origin) {
+	case NETFS_UNBUFFERED_READ:
+	case NETFS_DIO_READ:
+	case NETFS_READ_GAPS:
+		netfs_rreq_assess_dio(rreq);
+		break;
+	case NETFS_READ_SINGLE:
+		netfs_rreq_assess_single(rreq);
+		break;
+	default:
+		break;
+	}
+	task_io_account_read(rreq->transferred);
+
+	netfs_wake_rreq_flag(rreq, NETFS_RREQ_IN_PROGRESS, netfs_rreq_trace_wake_ip);
+	/* As we cleared NETFS_RREQ_IN_PROGRESS, we acquired its ref. */
+
+	trace_netfs_rreq(rreq, netfs_rreq_trace_done);
+	netfs_clear_subrequests(rreq);
+	netfs_unlock_abandoned_read_pages(rreq);
+	if (unlikely(rreq->copy_to_cache))
+		netfs_pgpriv2_end_copy_to_cache(rreq);
+	return true;
+}
+
+void netfs_read_collection_worker(struct work_struct *work)
+{
+	struct netfs_io_request *rreq = container_of(work, struct netfs_io_request, work);
+
+	netfs_see_request(rreq, netfs_rreq_trace_see_work);
+	if (netfs_check_rreq_in_progress(rreq)) {
+		if (netfs_read_collection(rreq))
+			/* Drop the ref from the IN_PROGRESS flag. */
+			netfs_put_request(rreq, netfs_rreq_trace_put_work_ip);
+		else
+			netfs_see_request(rreq, netfs_rreq_trace_see_work_complete);
+	}
+}
+
+/**
+ * netfs_read_subreq_progress - Note progress of a read operation.
+ * @subreq: The read request that has terminated.
+ *
+ * This tells the read side of netfs lib that a contributory I/O operation has
+ * made some progress and that it may be possible to unlock some folios.
+ *
+ * Before calling, the filesystem should update subreq->transferred to track
+ * the amount of data copied into the output buffer.
+ */
+void netfs_read_subreq_progress(struct netfs_io_subrequest *subreq)
+{
+	struct netfs_io_request *rreq = subreq->rreq;
+	struct netfs_io_stream *stream = &rreq->io_streams[subreq->stream_nr];
+	size_t progress_at = READ_ONCE(rreq->progress_at);
+	uoff_t update_at = rreq->start + progress_at;
+	uoff_t transferred_to = subreq->start + subreq->transferred;
+
+	/* If we are at the head of the queue, wake up the collector,
+	 * getting a ref to it if we were the ones to do so.
+	 */
+	if (progress_at < rreq->len &&
+	    transferred_to >= update_at &&
+	    (rreq->origin == NETFS_READAHEAD ||
+	     rreq->origin == NETFS_READPAGE ||
+	     rreq->origin == NETFS_READ_FOR_WRITE) &&
+	    list_is_first(&subreq->rreq_link, &stream->subrequests)
+	    ) {
+		trace_netfs_sreq(subreq, netfs_sreq_trace_progress);
+		__set_bit(NETFS_SREQ_MADE_PROGRESS, &subreq->flags);
+		netfs_wake_collector(rreq);
+	}
+}
+EXPORT_SYMBOL(netfs_read_subreq_progress);
+
+/**
+ * netfs_read_subreq_terminated - Note the termination of an I/O operation.
+ * @subreq: The I/O request that has terminated.
+ *
+ * This tells the read helper that a contributory I/O operation has terminated,
+ * one way or another, and that it should integrate the results.
+ *
+ * The caller indicates the outcome of the operation through @subreq->error,
+ * supplying 0 to indicate a successful or retryable transfer (if
+ * NETFS_SREQ_NEED_RETRY is set) or a negative error code.  The helper will
+ * look after reissuing I/O operations as appropriate and writing downloaded
+ * data to the cache.
+ *
+ * Before calling, the filesystem should update subreq->transferred to track
+ * the amount of data copied into the output buffer.
+ */
+void netfs_read_subreq_terminated(struct netfs_io_subrequest *subreq)
+{
+	struct netfs_io_request *rreq = subreq->rreq;
+
+	switch (subreq->source) {
+	case NETFS_READ_FROM_CACHE:
+		netfs_stat(&netfs_n_rh_read_done);
+		break;
+	case NETFS_DOWNLOAD_FROM_SERVER:
+		netfs_stat(&netfs_n_rh_download_done);
+		break;
+	default:
+		break;
+	}
+
+	/* Deal with retry requests, short reads and errors.  If we retry
+	 * but don't make progress, we abandon the attempt.
+	 */
+	if (!subreq->error && subreq->transferred < subreq->len) {
+		if (test_bit(NETFS_SREQ_HIT_EOF, &subreq->flags)) {
+			trace_netfs_sreq(subreq, netfs_sreq_trace_hit_eof);
+		} else if (test_bit(NETFS_SREQ_CLEAR_TAIL, &subreq->flags)) {
+			trace_netfs_sreq(subreq, netfs_sreq_trace_need_clear);
+		} else if (test_bit(NETFS_SREQ_NEED_RETRY, &subreq->flags)) {
+			trace_netfs_sreq(subreq, netfs_sreq_trace_need_retry);
+		} else if (test_bit(NETFS_SREQ_MADE_PROGRESS, &subreq->flags)) {
+			__set_bit(NETFS_SREQ_NEED_RETRY, &subreq->flags);
+			trace_netfs_sreq(subreq, netfs_sreq_trace_partial_read);
+		} else {
+			__set_bit(NETFS_SREQ_FAILED, &subreq->flags);
+			subreq->error = -ENODATA;
+			trace_netfs_sreq(subreq, netfs_sreq_trace_short);
+		}
+	}
+
+	/* If need retry is set, error should not matter unless we hit too many
+	 * retries. Pause the generation of new subreqs
+	 */
+	if (test_bit(NETFS_SREQ_NEED_RETRY, &subreq->flags)) {
+		trace_netfs_rreq(rreq, netfs_rreq_trace_set_pause);
+		set_bit(NETFS_RREQ_PAUSE, &rreq->flags);
+		goto skip_error_checks;
+	}
+
+	if (unlikely(subreq->error < 0)) {
+		trace_netfs_failure(rreq, subreq, subreq->error, netfs_fail_read);
+		if (subreq->source == NETFS_READ_FROM_CACHE) {
+			netfs_stat(&netfs_n_rh_read_failed);
+			__set_bit(NETFS_SREQ_NEED_RETRY, &subreq->flags);
+		} else {
+			netfs_stat(&netfs_n_rh_download_failed);
+			__set_bit(NETFS_SREQ_FAILED, &subreq->flags);
+		}
+		trace_netfs_rreq(rreq, netfs_rreq_trace_set_pause);
+		set_bit(NETFS_RREQ_PAUSE, &rreq->flags);
+	}
+
+skip_error_checks:
+	trace_netfs_sreq(subreq, netfs_sreq_trace_terminated);
+	netfs_subreq_clear_in_progress(subreq);
+	netfs_put_subrequest(subreq, netfs_sreq_trace_put_terminated);
+}
+EXPORT_SYMBOL(netfs_read_subreq_terminated);
+
+/*
+ * Cancel a read subrequest due to preparation failure.
+ */
+void netfs_cancel_read(struct netfs_io_subrequest *subreq, int error)
+{
+	trace_netfs_sreq(subreq, netfs_sreq_trace_cancel);
+	subreq->error = error;
+	__set_bit(NETFS_SREQ_FAILED, &subreq->flags);
+	netfs_read_subreq_terminated(subreq);
+}
+
+/*
+ * Handle termination of a read from the cache.
+ */
+void netfs_cache_read_terminated(void *priv, ssize_t transferred_or_error)
+{
+	struct netfs_io_subrequest *subreq = priv;
+
+	if (transferred_or_error > 0) {
+		subreq->error = 0;
+		if (transferred_or_error > 0) {
+			subreq->transferred += transferred_or_error;
+			__set_bit(NETFS_SREQ_MADE_PROGRESS, &subreq->flags);
+		}
+	} else {
+		subreq->error = transferred_or_error;
+	}
+	netfs_read_subreq_terminated(subreq);
+}