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

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Date:   Sat, 25 Aug 2018 12:00:04 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     "Theodore Y. Ts'o" <tytso@....edu>,
        Eric Biggers <ebiggers@...nel.org>,
        <linux-fsdevel@...r.kernel.org>, <linux-ext4@...r.kernel.org>,
        <linux-f2fs-devel@...ts.sourceforge.net>,
        Dmitry Kasatkin <dmitry.kasatkin@...il.com>,
        Michael Halcrow <mhalcrow@...gle.com>,
        <linux-kernel@...r.kernel.org>, <linux-fscrypt@...r.kernel.org>,
        <linux-integrity@...r.kernel.org>,
        Mimi Zohar <zohar@...ux.vnet.ibm.com>,
        Victor Hsieh <victorhsieh@...gle.com>
Subject: Re: [f2fs-dev] [RFC PATCH 02/10] fs-verity: add data verification
 hooks for ->readpages()

Hi Ted,

On 2018/8/25 11:45, Theodore Y. Ts'o wrote:
> On Sat, Aug 25, 2018 at 10:29:26AM +0800, Gao Xiang wrote:
>> My first question is that 'Is there any way to skip to verify pages in a bio?'
>> I am thinking about
>> If metadata and data page are mixed in a filesystem of such kind, they could submit together in a bio, but metadata could be unsuitable for such kind of verification.
>>
>> The second question is related to the first question --- 'Is there any way to verify a partial page?'
>> Take scalability into consideration, some files could be totally inlined or partially inlined in metadata.
>> Is there any way to deal with them in per-file approach? at least --- support for the interface?
> A requirement of both fscrypt and fsverity is that is that block size
> == page size, and that all data is stored in blocks.  Inline data is
> not supported.
> 
> The files that are intended for use with fsverity are large files
> (such as APK files), so optimizing for files smaller than a block was
> not a design goal.

Thanks for your quickly reply. :)

I had seen the background of why Google/Android introduces fs-verity before.

> 

But I have some consideration than the current implementation.... (if it is suitable to discuss, thanks...)

1) Since it is the libfs-like library, I think bio-strict is too strict for its future fs users.

bios could be already organized in filesystem-specific way, which could include some other pages that is unnecessary to be verified.

I could give some example, if some filesystem organizes its bios for decompression, and some data exist in metadata.
It could be hard to use this libfs-like fsverity interface.

2) My last question
"At last, I hope filesystems could select the on-disk position of hash tree and 'struct fsverity_descriptor'
rather than fixed in the end of verity files...I think if fs-verity preparing such support and interfaces could be better....."

is also for some files partially or totally encoded (eg. compressed, or whatever ...)

I think the hash tree is unnecessary to be compressed...so I think it could be better that it can be selected by users (filesystems of course).

Thanks,
Gao Xiang.

> 					- Ted