linux-kernel - Re: IMA: kernel reading files opened with O

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Message-ID: <CACE9dm_YF+aYSDXQg=JV2b4i7Uw9AcpsTt4DRCC+F7zUt_qO-w@mail.gmail.com>
Date:	Wed, 2 Jul 2014 21:12:03 +0300
From:	Dmitry Kasatkin <dmitry.kasatkin@...il.com>
To:	Jeff Moyer <jmoyer@...hat.com>
Cc:	Dmitry Kasatkin <d.kasatkin@...sung.com>, linux-mm@...ck.org,
	linux-fsdevel@...r.kernel.org,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	akpm@...ux-foundation.org, Al Viro <viro@...iv.linux.org.uk>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>,
	linux-security-module <linux-security-module@...r.kernel.org>,
	Greg KH <gregkh@...uxfoundation.org>
Subject: Re: IMA: kernel reading files opened with O_DIRECT

Hi Jeff,

Thanks for reply.

On 2 July 2014 18:55, Jeff Moyer <jmoyer@...hat.com> wrote:
> Hi, Dmitry,
>
> Dmitry Kasatkin <d.kasatkin@...sung.com> writes:
>
>> Hi,
>>
>> We are looking for advice on reading files opened for direct_io.
>
> [snip]
>
>> 2. Temporarily clear O_DIRECT in file->f_flags.
>
> [snip]
>
>> 3. Open another instance of the file with 'dentry_open'
>
> [snip]
>
>> Is temporarily clearing O_DIRECT flag really unacceptable or not?
>
> It's acceptable.  However, what you're proposing to do is read the
> entire file into the page cache to calculate your checksum.  Then, when
> the application goes to read the file using O_DIRECT, it will ignore the
> cached copy and re-read the portions of the file it wants from disk.  So
> yes, you can do that, but it's not going to be fast.  If you want to
> avoid polluting the cache, you can call invalidate_inode_pages2_range
> after you're done calculating your checksum.
>

Ok. If I understand correctly, after reading chunck/range like
kernel_read(offset, len),
just always drop loaded pages like

invalidate_inode_pages2_range(inode->i_mapping, offset, offset + len);

I see that generic_file_direct_write() calls this function too, before
doing direct IO and after...

Thanks!

>> Or may be there is a way to allocate "special" user-space like buffer
>> for kernel and use it with O_DIRECT?
>
> In-kernel O_DIRECT support has been proposed in the past, but there is
> no solution for that yet.
>
> Cheers,
> Jeff



-- 
Thanks,
Dmitry
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