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

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Message-ID: <CA+55aFzukw-6m0vtm6jDn4kXyfcUQbDFSwt1OOoa9nwG6toycQ@mail.gmail.com>
Date:   Wed, 23 Aug 2017 12:37:13 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Doug Nazar <nazard@...ar.ca>, Al Viro <viro@...iv.linux.org.uk>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Wei Fang <fangwei1@...wei.com>,
        linux-fsdevel <linux-fsdevel@...r.kernel.org>
Subject: Re: Kernels v4.9+ cause short reads of block devices

On Wed, Aug 23, 2017 at 12:15 PM, Doug Nazar <nazard@...ar.ca> wrote:
> The following commits cause short reads of block devices, however writes are
> still allowed.
>
> c2a9737f45e2 ("vfs,mm: fix a dead loop in truncate_inode_pages_range()")
> d05c5f7ba164 ("vfs,mm: fix return value of read() at s_maxbytes")
>
> When e2fsck sees this, it thinks it's a bad sector and tries to write a
> block of nulls which overwrites the valid data.

Hmm. Block devices shouldn't have issues with s_maxbytes, and I'm
surprised that nobody has seen that before.

> Device is LVM over 2 x RAID-5 on an old 32bit desktop.
>
> RO    RA   SSZ   BSZ   StartSec            Size   Device
> rw  4096   512  4096          0   9748044840960 /dev/Storage/Main

.. and the problem may be as simple as just a missing initialization
of s_maxbytes for blockdev_superblock.

Does the attcahed trivial one-liner fix things for you?

Al, if it really is this simple, how come nobody even noticed?

Also, I do wonder if that check in do_generic_file_read() should just
unconditionally use MAX_LFS_FILESIZE, since the whole point there is
really about the index wrap-around, not about any underlying
filesystem limits per se.

And that's exactly what MAX_LFS_FILESIZE is - the maximum size that
fits in the page index.

              Linus

View attachment "patch.diff" of type "text/plain" (425 bytes)