[PATCH] firewire: remove check of list iterator against head past the loop body

[PATCH] firewire: remove check of list iterator against head past the loop body

[Jakob Koschel]

When list_for_each_entry() completes the iteration over the whole list
without breaking the loop, the iterator value will be a bogus pointer
computed based on the head element.

While it is safe to use the pointer to determine if it was computed
based on the head element, either with list_entry_is_head() or
&pos->member == head, using the iterator variable after the loop should
be avoided.

In preparation to limit the scope of a list iterator to the list
traversal loop, use a dedicated pointer to point to the found element [1].

Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
---
 drivers/firewire/core-transaction.c | 30 +++++++++++++++--------------
 drivers/firewire/sbp2.c             | 13 +++++++------
 2 files changed, 23 insertions(+), 20 deletions(-)

diff --git a/drivers/firewire/core-transaction.c b/drivers/firewire/core-transaction.c
index ac487c96bb71..6c20815cc8d1 100644
--- a/drivers/firewire/core-transaction.c
+++ b/drivers/firewire/core-transaction.c
@@ -73,24 +73,25 @@ static int try_cancel_split_timeout(struct fw_transaction *t)
 static int close_transaction(struct fw_transaction *transaction,
 			     struct fw_card *card, int rcode)
 {
-	struct fw_transaction *t;
+	struct fw_transaction *t = NULL, *iter;
 	unsigned long flags;
 
 	spin_lock_irqsave(&card->lock, flags);
-	list_for_each_entry(t, &card->transaction_list, link) {
-		if (t == transaction) {
-			if (!try_cancel_split_timeout(t)) {
+	list_for_each_entry(iter, &card->transaction_list, link) {
+		if (iter == transaction) {
+			if (!try_cancel_split_timeout(iter)) {
 				spin_unlock_irqrestore(&card->lock, flags);
 				goto timed_out;
 			}
-			list_del_init(&t->link);
-			card->tlabel_mask &= ~(1ULL << t->tlabel);
+			list_del_init(&iter->link);
+			card->tlabel_mask &= ~(1ULL << iter->tlabel);
+			t = iter;
 			break;
 		}
 	}
 	spin_unlock_irqrestore(&card->lock, flags);
 
-	if (&t->link != &card->transaction_list) {
+	if (t) {
 		t->callback(card, rcode, NULL, 0, t->callback_data);
 		return 0;
 	}
@@ -935,7 +936,7 @@ EXPORT_SYMBOL(fw_core_handle_request);
 
 void fw_core_handle_response(struct fw_card *card, struct fw_packet *p)
 {
-	struct fw_transaction *t;
+	struct fw_transaction *t = NULL, *iter;
 	unsigned long flags;
 	u32 *data;
 	size_t data_length;
@@ -947,20 +948,21 @@ void fw_core_handle_response(struct fw_card *card, struct fw_packet *p)
 	rcode	= HEADER_GET_RCODE(p->header[1]);
 
 	spin_lock_irqsave(&card->lock, flags);
-	list_for_each_entry(t, &card->transaction_list, link) {
-		if (t->node_id == source && t->tlabel == tlabel) {
-			if (!try_cancel_split_timeout(t)) {
+	list_for_each_entry(iter, &card->transaction_list, link) {
+		if (iter->node_id == source && iter->tlabel == tlabel) {
+			if (!try_cancel_split_timeout(iter)) {
 				spin_unlock_irqrestore(&card->lock, flags);
 				goto timed_out;
 			}
-			list_del_init(&t->link);
-			card->tlabel_mask &= ~(1ULL << t->tlabel);
+			list_del_init(&iter->link);
+			card->tlabel_mask &= ~(1ULL << iter->tlabel);
+			t = iter;
 			break;
 		}
 	}
 	spin_unlock_irqrestore(&card->lock, flags);
 
-	if (&t->link == &card->transaction_list) {
+	if (!t) {
  timed_out:
 		fw_notice(card, "unsolicited response (source %x, tlabel %x)\n",
 			  source, tlabel);
diff --git a/drivers/firewire/sbp2.c b/drivers/firewire/sbp2.c
index 85cd379fd383..60051c0cabea 100644
--- a/drivers/firewire/sbp2.c
+++ b/drivers/firewire/sbp2.c
@@ -408,7 +408,7 @@ static void sbp2_status_write(struct fw_card *card, struct fw_request *request,
 			      void *payload, size_t length, void *callback_data)
 {
 	struct sbp2_logical_unit *lu = callback_data;
-	struct sbp2_orb *orb;
+	struct sbp2_orb *orb = NULL, *iter;
 	struct sbp2_status status;
 	unsigned long flags;
 
@@ -433,17 +433,18 @@ static void sbp2_status_write(struct fw_card *card, struct fw_request *request,
 
 	/* Lookup the orb corresponding to this status write. */
 	spin_lock_irqsave(&lu->tgt->lock, flags);
-	list_for_each_entry(orb, &lu->orb_list, link) {
+	list_for_each_entry(iter, &lu->orb_list, link) {
 		if (STATUS_GET_ORB_HIGH(status) == 0 &&
-		    STATUS_GET_ORB_LOW(status) == orb->request_bus) {
-			orb->rcode = RCODE_COMPLETE;
-			list_del(&orb->link);
+		    STATUS_GET_ORB_LOW(status) == iter->request_bus) {
+			iter->rcode = RCODE_COMPLETE;
+			list_del(&iter->link);
+			orb = iter;
 			break;
 		}
 	}
 	spin_unlock_irqrestore(&lu->tgt->lock, flags);
 
-	if (&orb->link != &lu->orb_list) {
+	if (orb) {
 		orb->callback(orb, &status);
 		kref_put(&orb->kref, free_orb); /* orb callback reference */
 	} else {

base-commit: f82da161ea75dc4db21b2499e4b1facd36dab275
-- 
2.25.1

Re: [PATCH] firewire: remove check of list iterator against head past the loop body

[Takashi Sakamoto]

Hi,

On Fri, Apr 01, 2022 at 12:36:01AM +0200, Jakob Koschel wrote:
> When list_for_each_entry() completes the iteration over the whole list
> without breaking the loop, the iterator value will be a bogus pointer
> computed based on the head element.
> 
> While it is safe to use the pointer to determine if it was computed
> based on the head element, either with list_entry_is_head() or
> &pos->member == head, using the iterator variable after the loop should
> be avoided.
> 
> In preparation to limit the scope of a list iterator to the list
> traversal loop, use a dedicated pointer to point to the found element [1].
> 
> Link: https://lore.kernel.org/all/CAHk-=wgRr_D8CB-D9Kg-c=EHreAsk5SqXPwr9Y7k9sA6cWXJ6w@mail.gmail.com/ [1]
> Signed-off-by: Jakob Koschel <jakobkoschel@gmail.com>
> ---
>  drivers/firewire/core-transaction.c | 30 +++++++++++++++--------------
>  drivers/firewire/sbp2.c             | 13 +++++++------
>  2 files changed, 23 insertions(+), 20 deletions(-)

I think it is a good catch.

Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>

For the changes in firewire-core module:
Tested-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>

Unfortunately the activity for maintenance in Linux FireWire subsystem
is quite low. No actions in past half a year. If the patch blocks
further work to improve list implementation, I think it better to look
for the other path to merge.


Regards

Takashi Sakamoto