From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from us-smtp-delivery-124.mimecast.com (us-smtp-delivery-124.mimecast.com [170.10.129.124]) (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 C625837F735 for ; Wed, 8 Jul 2026 13:44:25 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=170.10.129.124 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783518267; cv=none; b=OrzNRyYspApN/lk+3X+FMQMwTs3Di57pcrbdqK5z+tKyGQFAFKn7uz0FnZPT6T69dsSCgHrM8v8bhoG1TRO/dYPUmFRtrYTofCc8cUc929MrK3saG1ygAi0SI20Q9iKwvA44pX0+H/x3S7XhPu4JtyKgR0Vyyrdmcp4IejqH5lQ= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1783518267; c=relaxed/simple; bh=YsBaplu3SMyqS5ChwAscmwg7/sP4A3KtcN3vsRP2+do=; h=From:To:Cc:Subject:In-Reply-To:References:Date:Message-ID: MIME-Version:Content-Type; b=tfeA83bpcX7sjAHlck+LO1uS5ucV+GrIsNhBoKJP0M/3Rrmtd5TFQkc3p/xjotGo1NwjN7gDMXrtaWEcSDUdbW7mLsV2df4FsCPoVvq8k9JHY3dyGu8SkRiDpXwKLAmZZSaLVl+mhiHeXEYftFfaPghkbAdnQPlSxUuWM7/hXfo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com; spf=pass smtp.mailfrom=redhat.com; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b=NCYXOyDn; arc=none smtp.client-ip=170.10.129.124 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=quarantine dis=none) header.from=redhat.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=redhat.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=redhat.com header.i=@redhat.com header.b="NCYXOyDn" DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=redhat.com; s=mimecast20190719; t=1783518264; h=from:from:reply-to:subject:subject:date:date:message-id:message-id: to:to:cc:cc:mime-version:mime-version:content-type:content-type: content-transfer-encoding:content-transfer-encoding: in-reply-to:in-reply-to:references:references; bh=6MQQ1awMEWZQP75HQCTbVtUf+Uu2HSLb/C+A8UOw06U=; b=NCYXOyDnUxvJlV2LZlJFNSbhx0llEArqSVAULdMNShSzp6OBMVJmDzFVQqdUaC0sPbwL5y H3fe7qmeYkPZlPoGz/Cz7RbQ0fZCdk6nNhd/OkiFhLWtxIt+ZSP23/QmqQQXY2tyhfnxSr 5fCRz0zBqE8GhjIqaAbBJOLxAscNkNI= Received: from mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (ec2-54-186-198-63.us-west-2.compute.amazonaws.com [54.186.198.63]) by relay.mimecast.com with ESMTP with STARTTLS (version=TLSv1.3, cipher=TLS_AES_256_GCM_SHA384) id us-mta-433-koEbMCitOZKS5lRLZMc2EQ-1; Wed, 08 Jul 2026 09:44:19 -0400 X-MC-Unique: koEbMCitOZKS5lRLZMc2EQ-1 X-Mimecast-MFC-AGG-ID: koEbMCitOZKS5lRLZMc2EQ_1783518258 Received: from mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com [10.30.177.111]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature RSA-PSS (2048 bits) server-digest SHA256) (No client certificate requested) by mx-prod-mc-03.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTPS id 2F56A19560B7; Wed, 8 Jul 2026 13:44:18 +0000 (UTC) Received: from localhost (unknown [10.44.33.4]) by mx-prod-int-08.mail-002.prod.us-west-2.aws.redhat.com (Postfix) with ESMTP id 54FEC1800591; Wed, 8 Jul 2026 13:44:17 +0000 (UTC) From: Giuseppe Scrivano To: Amir Goldstein Cc: Miklos Szeredi , linux-unionfs@vger.kernel.org, linux-fsdevel@vger.kernel.org, linux-api@vger.kernel.org Subject: Re: [PATCH] ovl: add ioctls to retrieve layer file descriptors In-Reply-To: (Amir Goldstein's message of "Wed, 8 Jul 2026 15:23:46 +0200") References: <20260708095831.3381978-1-gscrivan@redhat.com> <878q7l8y4y.fsf@redhat.com> Date: Wed, 08 Jul 2026 15:44:14 +0200 Message-ID: <871pdd8ukx.fsf@redhat.com> User-Agent: Gnus/5.13 (Gnus v5.13) Precedence: bulk X-Mailing-List: linux-unionfs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Transfer-Encoding: quoted-printable X-Scanned-By: MIMEDefang 3.4.1 on 10.30.177.111 Amir Goldstein writes: > On Wed, Jul 8, 2026 at 2:31=E2=80=AFPM Giuseppe Scrivano wrote: >> >> Miklos Szeredi writes: >> >> > On Wed, 8 Jul 2026 at 12:00, Giuseppe Scrivano w= rote: >> >> >> >> Add two ioctls to overlay filesystem to allow userspace to retrieve >> >> information about the overlay layers: >> >> >> >> OVL_IOC_OPEN_LAYER: return an O_PATH fd to the root of a layer. >> >> arg =3D=3D 0 returns the upper layer (-ENOENT= if >> >> no upper is configured), arg >=3D 1 returns >> >> lower layers (-ENOENT if index is out of >> >> range). >> > >> > We could do this with a plain open() call. Something like the magic >> > symlinks we have under /proc/PID/fd/. Question is where could these >> > live... >> >> is there any existing user of such a mechanism? I don't see any mount >> specific info under /proc or /sys. >> >> > >> >> OVL_IOC_GET_LAYERS_INFO: copy a struct ovl_layers_info to userspace >> >> with numlower, numlowerdata, and has_upper. >> > >> > Isn't this info obtainable via statmount(2) already? If not, it >> > should be there, instead of a specialized ioctl. >> >> no that is not exposed by statmount and I don't see any way to export >> file system specific data through it. Do you've anything in mind? >> >> >> --- a/fs/overlayfs/ovl_entry.h >> >> +++ b/fs/overlayfs/ovl_entry.h >> >> @@ -35,6 +35,8 @@ struct ovl_layer { >> >> struct vfsmount *mnt; >> >> /* Trap in ovl inode cache */ >> >> struct inode *trap; >> >> + /* Keeps the original fsmount file alive for OVL_IOC_OPEN_LAY= ER */ >> >> + struct file *origin; >> > >> > Don't need to keep the file open: the only info missing is the >> > original vfsmount, everything else is already there to reconstruct the >> > file. >> >> I didn't manage to get that to work. As soon as the userspace process >> closes the mount fd that was passed to fsconfig, the anonymous mount >> namespace is destroyed and dissolve_on_fput sets mnt->mnt_ns to NULL. >> >> So whenever I try to use this mount again from userspace, it is not >> usable because the mount namespace is empty, causing check_mnt() to >> fail. >> >> Do you have any suggestions on how to solve this problem? > > The suggestion was to store origin->f_path->mnt instead of storing origin= file, > because you only end up using the origin vfsmount. the reason I am keeping the file and not just the vfsmount is that the file is what keeps the mount namespace alive (preventing dissolve_on_fput from fire). Should we export open_detached_copy from fs/namespace.c? I've not tested it, but it might work. Are there other ways to solve it that I am not seeing? Thanks, Giuseppe