linux-kernel - Re: Kernels v4.9+ cause short reads of block devices

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Date:   Thu, 24 Aug 2017 10:22:11 -0500
From:   Dave Kleikamp <dave.kleikamp@...cle.com>
To:     Doug Nazar <nazard@...ar.ca>, Andreas Dilger <adilger@...ger.ca>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Al Viro <viro@...iv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Wei Fang <fangwei1@...wei.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>,
        Mark Fasheh <mfasheh@...sity.com>,
        Joel Becker <jlbec@...lplan.org>,
        Dave Kleikamp <shaggy@...nel.org>
Subject: Re: Kernels v4.9+ cause short reads of block devices

On 08/24/2017 05:20 AM, Doug Nazar wrote:
> On 8/23/17 5:01 PM, Andreas Dilger wrote:
>> Doug,
>> I noticed while checking for other implications of changing
>> MAX_LFS_FILESIZE
>> that fs/jfs/super.c is also working around this limit.  If you are going
>> to submit a patch for this, it also makes sense to fix
>> jfs_fill_super() to
>> use MAX_LFS_FILESIZE instead of JFS rolling its own, something like:
>>
>>     /* logical blocks are represented by 40 bits in pxd_t, etc.
>>      * and page cache is indexed by long. */
>>     sb->s_maxbytes = min((u64)sb->s_blocksize) << 40,
>>                               MAX_LFS_FILESIZE);
>>
>> It also looks like ocfs2_max_file_offset() is trying to avoid overflowing
>> the old 31-bit limit, and isn't using MAX_LFS_FILESIZE directly, so it
>> will
>> now be wrong.  It looks like it could use "bitshift = 32; trim = bytes;",
>> but Joel or Mark should confirm.
>>
>> Finally, there is a check in fs/super.c::mount_fs() that is verifying
>> s_maxbytes is not set too large, but this has been present since 2.6.32
>> and should probably be removed at this point, or changed to a BUG_ON()
>> (see commit 42cb56ae2ab for details).
> 
> I don't have any issue trying to write patches for those, but I have no
> domain knowledge
> in the area or any way to test them.

If you want to wrap the jfs change into this, I will be happy to test it
for you, or I could take care of jfs with a separate patch if you'd prefer.

> 
> From a quick glance, jfs is locked to PSIZE (4096) so should be ok.
> OCFS looks a little complex, and since it's a shared fs, little hesitant.
> 
> The check in fs/super.c, maybe that should be:
> 
> sb->s_maxbytes > MAX_LFS_FILESIZE
> 
> Actually, little confused, the comment says unsigned, but loff_t looks
> like its long long.
> Maybe cast to u64 and check greater than?
> 
> Doug
>