From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (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 3CE6328399; Tue, 4 Feb 2025 16:58:58 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738688339; cv=none; b=RVMKSCT9kSJVQ+KE1ciaeejqBQEK0dXmMZXnJay2mqEOAIj11XCf7Y35H53BrX94PfpUrWxd7mgbU3pap+x+Odf+16XaRHH2SZNUabuA7vstYlxNn4SXRS6nJ1zX7Cz856/s3hayCfVxKvT+N6HIJg9ZhgfX+aeutUz8ei4AHq8= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1738688339; c=relaxed/simple; bh=LbxYU7EuGREsbdwz5TUen7s+5wYYoVfOCHMiWXezq04=; h=Message-ID:Subject:From:To:Cc:Date:In-Reply-To:References: Content-Type:MIME-Version; b=OkMpZLRuBGopbfjyr5cme2a+PfeQtUwDOAjPKFobDxI6+OvsA6ffAv5REbXo3SGoIiNgUslC0vVwfD6oHKf/LOq6qD48yqUzwgKr0U69rWDG1LwgWKMIRva5nv20W1VaYauGHmkIN7oVw/VNI9wWKU+BeW4nLBr1KNJ2nx9omzo= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b=KHjBuKVY; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=kernel.org header.i=@kernel.org header.b="KHjBuKVY" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 27DACC4CEDF; Tue, 4 Feb 2025 16:58:58 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=kernel.org; s=k20201202; t=1738688338; bh=LbxYU7EuGREsbdwz5TUen7s+5wYYoVfOCHMiWXezq04=; h=Subject:From:To:Cc:Date:In-Reply-To:References:From; b=KHjBuKVYcRqQmhfwXpsBtdveFBXt+lTAJ4p5xykzvY6UhMFKkLHZqtFwJq3lWorEa cZ+9MtKG2qXzgWF/V5NFuUehP4GZYYVRkB8Ge3znTAYCKGqJq7HqWY6sSWRcnS7thi pwbF2JWyH2obLGi2YjWPzYDe8NBXvXmX19LkfM7BSOIaCfCDfVzbHFFd7dIrVb2clj pdMqCqe1XogvCyZvJI/G7vQRw7Yvd/1jio1IkMcP3709rPBJuVEYagIbGG4Cl1uBaZ IGzZE/ML8M8CW9pHn9lVzrMvRIzSJaNEExF5B4u57kOM90jBsx3wPS9sLWHwp0Omo/ uN30NockjpE6w== Message-ID: Subject: Re: [PATCH] statmount: add a new supported_mask field From: Jeff Layton To: Christian Brauner Cc: Alexander Viro , Jan Kara , linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org Date: Tue, 04 Feb 2025 11:58:56 -0500 In-Reply-To: <20250204-keimzelle-reibt-84877b62936c@brauner> References: <20250203-statmount-v1-1-871fa7e61f69@kernel.org> <20250204-stengel-lodern-8e6ce624cf77@brauner> <8d07cfbab7b1d70b6fc381ad065d55773f73d93f.camel@kernel.org> <20250204-vintage-sozusagen-33df0d66bd6c@brauner> <3a139a0cca7772935dbc7e376d19547191143ce5.camel@kernel.org> <20250204-keimzelle-reibt-84877b62936c@brauner> Autocrypt: addr=jlayton@kernel.org; prefer-encrypt=mutual; keydata=mQINBE6V0TwBEADXhJg7s8wFDwBMEvn0qyhAnzFLTOCHooMZyx7XO7dAiIhDSi7G1NPxw n8jdFUQMCR/GlpozMFlSFiZXiObE7sef9rTtM68ukUyZM4pJ9l0KjQNgDJ6Fr342Htkjxu/kFV1Wv 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linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 On Tue, 2025-02-04 at 17:45 +0100, Christian Brauner wrote: > On Tue, Feb 04, 2025 at 10:57:39AM -0500, Jeff Layton wrote: > > On Tue, 2025-02-04 at 16:09 +0100, Christian Brauner wrote: > > > On Tue, Feb 04, 2025 at 07:28:20AM -0500, Jeff Layton wrote: > > > > On Tue, 2025-02-04 at 12:07 +0100, Christian Brauner wrote: > > > > > On Mon, Feb 03, 2025 at 12:09:48PM -0500, Jeff Layton wrote: > > > > > > Some of the fields in the statmount() reply can be optional. If= the > > > > > > kernel has nothing to emit in that field, then it doesn't set t= he flag > > > > > > in the reply. This presents a problem: There is currently no wa= y to > > > > > > know what mask flags the kernel supports since you can't always= count on > > > > > > them being in the reply. > > > > > >=20 > > > > > > Add a new STATMOUNT_SUPPORTED_MASK flag and field that the kern= el can > > > > > > set in the reply. Userland can use this to determine if the fie= lds it > > > > > > requires from the kernel are supported. This also gives us a wa= y to > > > > > > deprecate fields in the future, if that should become necessary= . > > > > > >=20 > > > > > > Signed-off-by: Jeff Layton > > > > > > --- > > > > > > I ran into this problem recently. We have a variety of kernels = running > > > > > > that have varying levels of support of statmount(), and I need = to be > > > > > > able to fall back to /proc scraping if support for everything i= sn't > > > > > > present. This is difficult currently since statmount() doesn't = set the > > > > > > flag in the return mask if the field is empty. > > > > > > --- > > > > > > fs/namespace.c | 18 ++++++++++++++++++ > > > > > > include/uapi/linux/mount.h | 4 +++- > > > > > > 2 files changed, 21 insertions(+), 1 deletion(-) > > > > > >=20 > > > > > > diff --git a/fs/namespace.c b/fs/namespace.c > > > > > > index a3ed3f2980cbae6238cda09874e2dac146080eb6..7ec5fc436c4ff30= 0507c4ed71a757f5d75a4d520 100644 > > > > > > --- a/fs/namespace.c > > > > > > +++ b/fs/namespace.c > > > > > > @@ -5317,6 +5317,21 @@ static int grab_requested_root(struct mn= t_namespace *ns, struct path *root) > > > > > > return 0; > > > > > > } > > > > > > =20 > > > > > > +/* This must be updated whenever a new flag is added */ > > > > > > +#define STATMOUNT_SUPPORTED (STATMOUNT_SB_BASIC | \ > > > > > > + STATMOUNT_MNT_BASIC | \ > > > > > > + STATMOUNT_PROPAGATE_FROM | \ > > > > > > + STATMOUNT_MNT_ROOT | \ > > > > > > + STATMOUNT_MNT_POINT | \ > > > > > > + STATMOUNT_FS_TYPE | \ > > > > > > + STATMOUNT_MNT_NS_ID | \ > > > > > > + STATMOUNT_MNT_OPTS | \ > > > > > > + STATMOUNT_FS_SUBTYPE | \ > > > > > > + STATMOUNT_SB_SOURCE | \ > > > > > > + STATMOUNT_OPT_ARRAY | \ > > > > > > + STATMOUNT_OPT_SEC_ARRAY | \ > > > > > > + STATMOUNT_SUPPORTED_MASK) > > > > >=20 > > > > > Hm, do we need a separate bit for STATMOUNT_SUPPORTED_MASK? Afaiu= , this > > > > > is more of a convenience thing but then maybe we just do: > > > > >=20 > > > > > #define STATMOUNT_SUPPORTED_MASK STATMOUNT_MNT_BASIC > > > > >=20 > > > > > and be done with it? > > > > >=20 > > > > > Otherwise I think it is worth having support for this. > > > > >=20 > > > >=20 > > > > Are you suggesting that we should just add the ->supported_mask fie= ld > > > > without a declaring a bit for it? If so, how would that work on old > > > > kernels? You'd never know if you could trust the contents of that f= ield > > > > since the return mask wouldn't indicate any difference. > > >=20 > > > What I didn't realize because I hadn't read carefully enough in your > > > patch was that STATMOUNT_SUPPORTED_MASK is raised in ->mask and only > > > then is ->supported_mask filled in. > > >=20 > > > My thinking had been that ->supported_mask will simply always be fill= ed > > > in by the kernel going forward. Which is arguably not ideal but would > > > work: > > >=20 > > > So the kernel guarantees since the introduction of statmount() that w= hen > > > we copy out to userspace that anything the kernel doesn't know will b= e > > > copied back zeroed. So any unknown fields are zero. > > >=20 > > > (1) Say userspace passes a struct statmount with ->supported_mask to = the > > > kernel - even if it has put garbage in there or intentionally rai= sed > > > valid flags in there - the old kernel will copy over this and set= it > > > to zero. > > >=20 > > > (2) If you're on a new kernel but pass an old struct the kernel will > > > fill in ->supported_mask. Imho, that's fine. Userspace simply wil= l > > > not know about it. > > >=20 > > > But we can leave the explicit request in! > > >=20 > >=20 > >=20 > > I can respin without STATMOUNT_SUPPORTED_MASK. I was thinking it left > > that part of the return buffer untouched, but if it's zeroed, then that > > works as well. > >=20 > > If you see a supported_mask of 0, you know the kernel didn't fill it in > > (since it should at least support _something_). That'll need to be > > carefully documented though. >=20 > It's probably easier for userspace if that flag must be specifically rais= ed. Agreed. Should I assume you'll take this one as-is then? If you want to change it to always set the field though, then that's fine with me. --=20 Jeff Layton