Subject: Re: Kernels v4.9+ cause short reads of block devices
To: Andreas Dilger <adilger@dilger.ca>,
        Linus Torvalds <torvalds@linux-foundation.org>
Cc: Al Viro <viro@zeniv.linux.org.uk>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        Wei Fang <fangwei1@huawei.com>,
        linux-fsdevel <linux-fsdevel@vger.kernel.org>,
        Mark Fasheh <mfasheh@versity.com>, Joel Becker <jlbec@evilplan.org>,
        Dave Kleikamp <shaggy@kernel.org>
References: <1ae53e17-e455-4f17-0280-b0dae183a449@nazar.ca>
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 <ae3c9d95-5af1-eb2d-c0c1-32ae622f6c54@nazar.ca>
 <CA+55aFzWCSU6A0xmG+TC+K-tN3tZTOo_0dxYy5k=5K+DeTEx8A@mail.gmail.com>
 <FCA4A132-E6E1-4568-8A89-3DE441F189AB@dilger.ca>
From: Doug Nazar <nazard@nazar.ca>
Message-ID: <75bc3c58-c699-f7f2-4fc2-a8e2aefeddb5@nazar.ca>
Date: Thu, 24 Aug 2017 06:20:05 -0400
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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.

 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