From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from r3-24.sinamail.sina.com.cn (r3-24.sinamail.sina.com.cn [202.108.3.24]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C686617A2EA for ; Mon, 22 Jun 2026 01:46:07 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=202.108.3.24 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782092772; cv=none; b=i0MXut4SlAyLk+uORUezO3X1tZB5i60VTogdSejPzaK4l7+S0HNu60wMWYi2SSJZghShy3UYQOV5ATzHXJAjFYgMV982WNA+oaPJXTIJX2cXBHd8V0XmMMJup7/P0kp4dR+cxTODYrBceu1KX4H3o4LWw9JGew0ZEXwPpcHxgBQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1782092772; c=relaxed/simple; bh=Jz7yQGu/R1sslHSYnWigGeAaUrqXaUiXdzOCi9hGpuE=; h=From:To:Cc:Subject:Date:Message-ID:In-Reply-To:References: MIME-Version; b=fOYhIvLu8eXSrUiYrg8Wsr+qZpTpYBVrWbqE9u/N2e43S+Rb/fxjGcxnFGX6X7o8MZsLGvaSOwFEhuvR+qRCV0CXUvps0R/5iX2wTWicmvwA/0aTlLi+pC3rqzNw6Ppgng9ifnHtQlZzthinTFF2L0VcVSit1cgVlKbvvV7XWh8= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=sina.com; spf=pass smtp.mailfrom=sina.com; dkim=pass (1024-bit key) header.d=sina.com header.i=@sina.com header.b=0CS10NgC; arc=none smtp.client-ip=202.108.3.24 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=none dis=none) header.from=sina.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=sina.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=sina.com header.i=@sina.com header.b="0CS10NgC" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=sina.com; s=201208; t=1782092768; bh=GTi82fa48ZAdHtNIVE+MeEaCXBGvzEeerzWekigHzoE=; h=From:Subject:Date:Message-ID; b=0CS10NgC/4d6KUUBAgGU/21O3xhaoVvz4j/FObtaR2aiNex+nSm4Z5cmp9l/VCoD9 IeV65/j6OH/VIyzLUG8NsN6XonNCs3dN0xklK3hqsQ334lL++AkZhOIv+TPr9JB+ne snkhx5n82pMYE1Ol6nj0agYgfhYD7BK3Af0U1dqk= X-SMAIL-HELO: localhost.localdomain Received: from unknown (HELO localhost.localdomain)([114.249.62.144]) by sina.com (10.54.253.31) with ESMTP id 6A3893490000179A; Mon, 22 Jun 2026 09:43:39 +0800 (CST) X-Sender: hdanton@sina.com X-Auth-ID: hdanton@sina.com Authentication-Results: sina.com; spf=none smtp.mailfrom=hdanton@sina.com; dkim=none header.i=none; dmarc=none action=none header.from=hdanton@sina.com X-SMAIL-MID: 5437756816665 X-SMAIL-UIID: B2E31ED7DF9D4DB2B4EE62DA31D18A37-20260622-094339-1 From: Hillf Danton To: Deepanshu Kartikey Cc: edumazet@google.com, linux-block@vger.kernel.org, nbd@other.debian.org, linux-kernel@vger.kernel.org, syzbot+6b85d1e39a5b8ed9a954@syzkaller.appspotmail.com Subject: Re: [PATCH] nbd: don't warn when reclassifying a busy socket lock Date: Mon, 22 Jun 2026 09:43:26 +0800 Message-ID: <20260622014328.115-1-hdanton@sina.com> In-Reply-To: <20260621235255.66015-1-kartikey406@gmail.com> References: Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit On Mon, 22 Jun 2026 05:22:55 +0530 Deepanshu Kartikey wrote: > nbd_reclassify_socket() warns via WARN_ON_ONCE() if the socket lock is > held at the point of reclassification. That assertion was copied from > nvme-tcp, where the socket is created internally by the kernel > (sock_create_kern()) and is never visible to user space, so the lock > is guaranteed to be free. > > NBD is different: the socket is looked up from a user-supplied fd in > nbd_get_socket(), and user space retains that fd. A concurrent syscall > on the same socket (or softirq processing taking bh_lock_sock() on a > connected TCP socket) can legitimately hold the lock at the instant > NBD reclassifies it. sock_allow_reclassification() then returns false > and the WARN_ON_ONCE() fires, which turns into a crash under > panic_on_warn. This is reachable by simply racing NBD_CMD_CONNECT > against socket activity on the same fd, as reported by syzbot. > Given the syzbot report, if you are right (I suspect) then Eric delivered another half-baked croissant, and feel free to cut it off instead to make room for correct fix. > Hitting a held lock here is expected for an externally owned socket and > is not a kernel bug, so skip reclassification silently instead of > warning. Reclassification is a lockdep-only annotation, so skipping it > in the rare racing case is harmless. > > Reported-by: syzbot+6b85d1e39a5b8ed9a954@syzkaller.appspotmail.com > Closes: https://syzkaller.appspot.com/bug?extid=6b85d1e39a5b8ed9a954 > Fixes: d532cddb6c60 ("nbd: Reclassify sockets to avoid lockdep circular dependency") > Signed-off-by: Deepanshu Kartikey > --- > drivers/block/nbd.c | 2 +- > 1 file changed, 1 insertion(+), 1 deletion(-) > > diff --git a/drivers/block/nbd.c b/drivers/block/nbd.c > index 3a585a0c882a..8f10762e90ef 100644 > --- a/drivers/block/nbd.c > +++ b/drivers/block/nbd.c > @@ -1246,7 +1246,7 @@ static void nbd_reclassify_socket(struct socket *sock) > { > struct sock *sk = sock->sk; > > - if (WARN_ON_ONCE(!sock_allow_reclassification(sk))) > + if (!sock_allow_reclassification(sk)) > return; > > switch (sk->sk_family) { > -- > 2.43.0