Re: [f2fs-dev] [RFC PATCH 02/10] fs-verity: add data verification hooks for ->readpages()

From: Gao Xiang <gaoxiang25@huawei.com>
To: "Theodore Y. Ts'o" <tytso@mit.edu>,
	Eric Biggers <ebiggers@kernel.org>,
	<linux-fsdevel@vger.kernel.org>, <linux-ext4@vger.kernel.org>,
	<linux-f2fs-devel@lists.sourceforge.net>,
	Dmitry Kasatkin <dmitry.kasatkin@gmail.com>,
	Michael Halcrow <mhalcrow@google.com>,
	<linux-kernel@vger.kernel.org>, <linux-fscrypt@vger.kernel.org>,
	<linux-integrity@vger.kernel.org>,
	Mimi Zohar <zohar@linux.vnet.ibm.com>,
	Victor Hsieh <victorhsieh@google.com>
Subject: Re: [f2fs-dev] [RFC PATCH 02/10] fs-verity: add data verification hooks for ->readpages()
Date: Sat, 25 Aug 2018 15:33:18 +0800	[thread overview]
Message-ID: <faf1fe45-e325-bae4-e60d-871f0b7b41b0@huawei.com> (raw)
In-Reply-To: <20180825050621.GA25031@thunk.org>

Hi Ted,

Thanks for your detailed reply. Sorry about my english, the words could not be logical.

Tiny pieces in B-tree to compose a page is too far from us too, and you are right,
fs-verity is complete for >99% cases for the existed file system, and no need to worry about currently.

As I mentioned in reply to Eric, I am actually curious about the Google fs-verity roadmap
for the future Android, I need to analyze if it is only limited to APKs for the read-write partitions
and not to replace dm-verity in the near future since fs-verity has some conflicts
to EROFS I am working on I mentioned in the email to Eric.

I think it is more than just to handle FILE_MAPPING and bio-strict for compression use.

On 2018/8/25 13:06, Theodore Y. Ts'o wrote:
> But I'd suggest worrying about it when such a file system
> comes out of the woodwork, and someone is willing to do the work to
> integrate fserity in that file system.
> 
Yes, we are now handling partial page due to compression use.

fs could submit bios in pages from different mapping(FILE_MAPPING[compress in-place and no caching
compressed page to reduce extra memory overhead] or META_MAPPING [for caching compressed page]) and
they could be decompressed into many full pages and (possible) a partial page (in-place or out-of-place).

so in principle, since we have BIO_MAX_PAGES limitation, a filemap page could be Uptodate
after two bios is ended and decompressed. and other runtime limitations could also divide a bio into two bios for encoded cases.

Therefore, I think in that case we could not just consider FILE_MAPPING and one bio, and as you said `In
that case, it could call fsverity after assembling the page in the page cache.' should be done in this way.

> Well, the userspace interface for instantiating a fs-verity file is
> that it writes the file data with the fs-verity metadata (which
> consists of the Merkle tree with a fs-verity header at the end of the
> file).  The program (which might be a package manager such as dpkg or
> rpm) would then call an ioctl which would cause the file system to
> read the fs-verity header and make only the file data visible, and the
> file system would the verify the data as it is read into the page
> cache.

Thanks for your reply again, I think fs-verity is good enough for now.
However, I need to think over about fs-verity itself more... :(

Thanks,
Gao Xiang