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Re: [PATCH 3/3] block: fail on open when file size is unaligned to reque

From: Eric Blake
Subject: Re: [PATCH 3/3] block: fail on open when file size is unaligned to request_alignment
Date: Wed, 11 Mar 2020 07:29:28 -0500
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:68.0) Gecko/20100101 Thunderbird/68.5.0

On 3/11/20 6:06 AM, Max Reitz wrote:
On 30.01.20 16:22, Vladimir Sementsov-Ogievskiy wrote:
Prior to the commit the following command lead to crash:

   ./qemu-io --image-opts -c 'write 0 512' \

It failes on assertion in bdrv_aligned_pwritev:
   "end_sector <= bs->total_sectors || child->perm & BLK_PERM_RESIZE"

The problem is obvious: 512 is aligned to 4096 and becomes larger than
file size. And the core bad thing is that file size is unaligned to

Let's catch such case on bdrv_open_driver and fail.

I think we had a discussion on this before, but I can’t find it right
now.  (Although I think that had more to do with something in the
file-posix driver, because it wasn’t limited to alignments above 512.)

In any case, the file itself is totally valid.  Most importantly, qcow2
will regularly create files with unaligned file lengths.

So let me create a qcow2 image on a 4k-aligned device:

$ truncate 512M fs.img
$ sudo losetup -f --show -b 4096 fs.img
$ sudo mkfs.ext4 /dev/loop0
$ sudo mount /dev/loop0 /mnt/tmp

$ sudo ./qemu-img create -f qcow2 /mnt/tmp/foo.qcow2 64M
Formatting '/mnt/tmp/foo.qcow2', fmt=qcow2 size=67108864
cluster_size=65536 lazy_refcounts=off refcount_bits=16
$ sudo ./qemu-io -t none -c quit /mnt/tmp/foo.qcow2
qemu-io: can't open device /mnt/tmp/foo.qcow2: File size is unaligned to
request alignment

Which is too bad.

So the real solution would probably...  Be to align the file size up to
the alignment?

Or to bite the bullet and finally implement byte-accurate size everywhere (instead of our current insistence on rounding size up to 512-byte multiples). If we have to deal with unaligned tails anyways, it's better to make the code universally applicable whether that unaligned tail is to 512 or to 4k, than to have it work for 512 but to fail for 4k.

Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3226
Virtualization:  qemu.org | libvirt.org

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