linux-kernel - Re: [PATCH] staging: erofs: harden inode lookup for 32-bit platforms

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Message-ID: <6318156d-4a3d-e5d0-93c1-90be2345565b@huawei.com>
Date:   Thu, 11 Oct 2018 18:49:57 +0800
From:   Gao Xiang <gaoxiang25@...wei.com>
To:     Dan Carpenter <dan.carpenter@...cle.com>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        <devel@...verdev.osuosl.org>, <linux-erofs@...ts.ozlabs.org>,
        Chao Yu <yuchao0@...wei.com>,
        LKML <linux-kernel@...r.kernel.org>, <weidu.du@...wei.com>,
        Miao Xie <miaoxie@...wei.com>
Subject: Re: [PATCH] staging: erofs: harden inode lookup for 32-bit platforms

Hi Dan,

On 2018/10/11 18:18, Dan Carpenter wrote:
> On Thu, Oct 11, 2018 at 05:46:26PM +0800, Gao Xiang wrote:
>>
>>
>> On 2018/10/11 16:44, Dan Carpenter wrote:
>>> On Tue, Oct 09, 2018 at 10:07:13PM +0800, Gao Xiang wrote:
>>>> This patch introduces inode hash function, test and set callbacks,
>>>> and iget5_locked to find the right inode for 32-bit platforms.
>>>>
>>>
>>> The way I read this changelog, we're trying to deal with corrupt file
>>> systems?  Is that correct?  Presumably in the current code it could lead
>>> to a Oops or something?
>>
>> No, this commit isn't trying to deal with corrupt file systems.
>> In EROFS, the nid is not continuous and it represents the inode offset
>> inode offset = nid * 32.
>> Therefore the nid is 64-bit both for 32-bit and 64-bit platforms. However,
>> i_ino is 'unsigned long', which means for 32-bit platforms, i_ino is not enough
>> to contain the nid.
>>
>> Therefore, we should use iget5_locked for this case.
> 
> I guess what I'm saying is, what are the user visible effects of this
> patch?  It's hard for me to tell from the patch description.  Could you
> please re-write the description and send a v2?

Sorry for my just hurry reply. It seems I really misunderstood your point :-(

If all nids are less than 32-bit, the user will see the correct inode/nid number
for all 32-bit platforms, but if some nids are greater than 32-bit, it could be
collisions without this patch for 32-bit platforms.

This patch switch the inode number by XOR the high 32-bit and the low 32-bit,
use customized tester to avoid collisions for 32-bit platforms.

I will send v2 to fix the commit message...but it seems Greg is already merged in
https://git.kernel.org/pub/scm/linux/kernel/git/gregkh/staging.git/commit/?h=staging-testing&id=2abd7814138774eb56319e9dcc0abd08ece45424

Thanks,
Gao Xiang

> 
> regards,
> dan carpenter
>