From: Maoyi Xie <maoyixie.tju@gmail.com>
To: Takashi Iwai <tiwai@suse.de>, Jaroslav Kysela <perex@perex.cz>
Cc: linux-sound@vger.kernel.org, linux-kernel@vger.kernel.org
Subject: [PATCH 2/2] ALSA: seq: avoid past-the-end iterator in snd_seq_create_port()
Date: Tue, 19 May 2026 03:40:23 +0800	[thread overview]
Message-ID: <20260518194023.1667857-3-maoyixie.tju@gmail.com> (raw)
In-Reply-To: <20260518194023.1667857-1-maoyixie.tju@gmail.com>

snd_seq_create_port() walks client->ports_list_head looking for
the ordered insertion point and on loop fall-through passes
&p->list to list_add_tail():

    list_for_each_entry(p, &client->ports_list_head, list) {
            if (p->addr.port == port) {
                    kfree(new_port);
                    return -EBUSY;
            }
            if (p->addr.port > num)
                    break;
            ...
    }
    list_add_tail(&new_port->list, &p->list);

When the loop walks all entries without break (e.g., the new
port sorts last), p is past-the-end. &p->list aliases
&client->ports_list_head (the list head) via container_of offset
cancellation, so the insert lands at the list tail. That is the
intended behaviour, but the access is undefined per C11 even
though it works in practice.

Track an explicit insert_before pointer initialised to the list
head and overwritten to &p->list only when the loop breaks
early. The observable behaviour is unchanged.

Fixes: 9244b2c3079f ("[ALSA] alsa core: convert to list_for_each_entry*")
Signed-off-by: Maoyi Xie <maoyixie.tju@gmail.com>
---
 sound/core/seq/seq_ports.c | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

--- a/sound/core/seq/seq_ports.c	2026-05-18 19:17:08.278971518 +0800
+++ b/sound/core/seq/seq_ports.c	2026-05-18 19:18:44.006234339 +0800
@@ -144,18 +144,21 @@
 	num = max(port, 0);
 	guard(mutex)(&client->ports_mutex);
 	guard(write_lock_irq)(&client->ports_lock);
+	struct list_head *insert_before = &client->ports_list_head;
 	list_for_each_entry(p, &client->ports_list_head, list) {
 		if (p->addr.port == port) {
 			kfree(new_port);
 			return -EBUSY;
 		}
-		if (p->addr.port > num)
+		if (p->addr.port > num) {
+			insert_before = &p->list;
 			break;
+		}
 		if (port < 0) /* auto-probe mode */
 			num = p->addr.port + 1;
 	}
 	/* insert the new port */
-	list_add_tail(&new_port->list, &p->list);
+	list_add_tail(&new_port->list, insert_before);
 	client->num_ports++;
 	new_port->addr.port = num;	/* store the port number in the port */
 	sprintf(new_port->name, "port-%d", num);
-- 
2.34.1

next prev parent reply	other threads:[~2026-05-18 19:40 UTC|newest]

Thread overview: 6+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2026-05-18 16:00 ALSA: iterator used after loop end in timer/seq port registration? Maoyi Xie
2026-05-18 19:26 ` Takashi Iwai
2026-05-18 19:40   ` [PATCH 0/2] ALSA: avoid past-the-end iterators in timer/seq port registration Maoyi Xie
2026-05-18 19:40     ` [PATCH 1/2] ALSA: timer: avoid past-the-end iterator in snd_timer_dev_register() Maoyi Xie
2026-05-18 19:40     ` Maoyi Xie [this message]
2026-05-19  5:39     ` [PATCH 0/2] ALSA: avoid past-the-end iterators in timer/seq port registration Takashi Iwai

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=20260518194023.1667857-3-maoyixie.tju@gmail.com \
    --to=maoyixie.tju@gmail.com \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-sound@vger.kernel.org \
    --cc=perex@perex.cz \
    --cc=tiwai@suse.de \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link

This is an external index of several public inboxes,
see mirroring instructions on how to clone and mirror
all data and code used by this external index.