From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Subject: Re: [PATCH] mm: adjust max read count in generic_file_buffered_read() To: Jan Kara , Andrew Morton Cc: mgorman@techsingularity.net, jlayton@redhat.com, ak@linux.intel.com, mawilcox@microsoft.com, tim.c.chen@linux.intel.com, linux-mm@kvack.org, linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org, Al Viro References: <20180719081726.3341-1-cgxu519@gmx.com> <20180719085812.sjup2odrjyuigt3l@quack2.suse.cz> <20180720161429.d63dccb9f66799dc0ff74dba@linux-foundation.org> <20180806102203.hmobd26cujmlfcsw@quack2.suse.cz> <20180806155927.4740babd057df9d5078281b1@linux-foundation.org> <20180807135453.nhatdtw25wa6dtzm@quack2.suse.cz> From: cgxu519 Message-ID: <7be05929-a5d0-e0b0-9d48-705c3840ee95@gmx.com> Date: Wed, 8 Aug 2018 08:57:13 +0800 MIME-Version: 1.0 In-Reply-To: <20180807135453.nhatdtw25wa6dtzm@quack2.suse.cz> Content-Type: text/plain; charset=utf-8; format=flowed Content-Transfer-Encoding: 7bit Content-Language: en-US Sender: owner-linux-mm@kvack.org List-ID: 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 wrote: >> >>> On Fri 20-07-18 16:14:29, Andrew Morton wrote: >>>> On Thu, 19 Jul 2018 10:58:12 +0200 Jan Kara 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 >>>>> Looks good to me. You can add: >>>>> >>>>> Reviewed-by: Jan Kara >>>> 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