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Date:   Wed, 8 Aug 2018 08:57:13 +0800
From:   cgxu519 <cgxu519@....com>
To:     Jan Kara <jack@...e.cz>, Andrew Morton <akpm@...ux-foundation.org>
Cc:     mgorman@...hsingularity.net, jlayton@...hat.com,
        ak@...ux.intel.com, mawilcox@...rosoft.com,
        tim.c.chen@...ux.intel.com, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org, linux-fsdevel@...r.kernel.org,
        Al Viro <viro@...IV.linux.org.uk>
Subject: Re: [PATCH] mm: adjust max read count in generic_file_buffered_read()



On 08/07/2018 09:54 PM, Jan Kara wrote:
> On Mon 06-08-18 15:59:27, Andrew Morton wrote:
>> On Mon, 6 Aug 2018 12:22:03 +0200 Jan Kara <jack@...e.cz> wrote:
>>
>>> On Fri 20-07-18 16:14:29, Andrew Morton wrote:
>>>> On Thu, 19 Jul 2018 10:58:12 +0200 Jan Kara <jack@...e.cz> wrote:
>>>>
>>>>> On Thu 19-07-18 16:17:26, Chengguang Xu wrote:
>>>>>> When we try to truncate read count in generic_file_buffered_read(),
>>>>>> should deliver (sb->s_maxbytes - offset) as maximum count not
>>>>>> sb->s_maxbytes itself.
>>>>>>
>>>>>> Signed-off-by: Chengguang Xu <cgxu519@....com>
>>>>> Looks good to me. You can add:
>>>>>
>>>>> Reviewed-by: Jan Kara <jack@...e.cz>
>>>> Yup.
>>>>
>>>> What are the runtime effects of this bug?
>>> Good question. I think ->readpage() could be called for index beyond
>>> maximum file size supported by the filesystem leading to weird filesystem
>>> behavior due to overflows in internal calculations.
>>>
>> Sure.  But is it possible for userspace to trigger this behaviour?
>> Possibly all callers have already sanitized the arguments by this stage
>> in which case the statement is arguably redundant.
> So I don't think there's any sanitization going on before
> generic_file_buffered_read(). E.g. I don't see any s_maxbytes check on
> ksys_read() -> vfs_read() -> __vfs_read() -> new_sync_read() ->
> call_read_iter() -> generic_file_read_iter() ->
> generic_file_buffered_read() path... However now thinking about this again:
> We are guaranteed i_size is within s_maxbytes (places modifying i_size
> are checking for this) and generic_file_buffered_read() stops when it
> should read beyond i_size. So in the end I don't think there's any breakage
> possible and the patch is not necessary?
>
I think most of time i_size is within s_maxbytes in local filesystem,
but consider network filesystem, write big file in 64bit client and
read in 32bit client, in this case maybe generic_file_buffered_read()
can read more than s_maxbytes, right?


Thanks,
Chengguang

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