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

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Message-ID: <75bc3c58-c699-f7f2-4fc2-a8e2aefeddb5@nazar.ca>
Date:   Thu, 24 Aug 2017 06:20:05 -0400
From:   Doug Nazar <nazard@...ar.ca>
To:     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 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