[Bug 1808928] Re: Bitmap Extra data is not supported

From: John Snow <1808928@bugs.launchpad.net>
To: qemu-devel@nongnu.org
Subject: [Bug 1808928] Re: Bitmap Extra data is not supported
Date: Tue, 20 Apr 2021 17:17:06 -0000	[thread overview]
Message-ID: <161893902671.22232.8113269026693095602.malone@gac.canonical.com> (raw)
In-Reply-To: 154511858636.8591.3431958243733477284.malonedeb@gac.canonical.com

my patches went in, ultimately, and my focus was since shifted
elsewhere. I just tried this by *manually* adding some extra data to a
bitmap by hand.

qemu-img create -f qcow2 foo.qcow2 64m
qemu-img bitmap --add foo.qcow2 mybitmap

This creates a bitmap extension header like this (starting at 0x1f8)

000001f0  00 00 00 00 00 00 00 00  23 85 28 75 00 00 00 18  |........#.(u....|
00000200  00 00 00 01 00 00 00 00  00 00 00 00 00 00 00 20  |............... |
00000210  00 00 00 00 00 05 00 00                           |........        |

And a bitmap table that looks like this:

00050000  00 00 00 00 00 04 00 00  00 00 00 01 00 00 00 02  |................|
00050010  01 10 00 08 00 00 00 00  6d 79 62 69 74 6d 61 70  |........mybitmap|

I modified the bitmap table to add eight bytes of bad data:

00050000  00 00 00 00 00 04 00 00  00 00 00 01 00 00 00 02  |................|
00050010  01 10 00 08 00 00 00 08  62 61 64 64 61 74 61 21  |........baddata!|
00050020  6d 79 62 69 74 6d 61 70                           |mybitmap|

And modified the header accordingly to add eight bytes to the table
(0x20f := 0x28):

000001f0  00 00 00 00 00 00 00 00  23 85 28 75 00 00 00 18  |........#.(u....|
00000200  00 00 00 01 00 00 00 00  00 00 00 00 00 00 00 28  |...............(|
00000210  00 00 00 00 00 05 00 00                           |........        |

And in these cases, QEMU refuses to load or work with the image even
slightly, rendering you unable to remove it:

> ./qemu-img bitmap --remove foo.qcow2 mybitmap
qemu-img: Could not open 'foo.qcow2': Bitmap extra data is not supported

So, it's still an open issue.

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https://bugs.launchpad.net/bugs/1808928

Title:
  Bitmap Extra data is not supported

Status in QEMU:
  Incomplete

Bug description:
  i am using dirty bitmaps and drive-backup. It works as aspected.

  Lately, i encounter a disastrous error. There is not any information
  about that situation. I cannot reach/open/attach/info or anything with
  a qcow2 file.

  virsh version
  Compiled against library: libvirt 4.10.0
  Using library: libvirt 4.10.0
  Using API: QEMU 4.10.0
  Running hypervisor: QEMU 2.12.0

  "qemu-img: Could not open '/var/lib/libvirt/images/test.qcow2': Bitmap
  extra data is not supported"

  what is that mean? what should i do?
  i cannot remove bitmap. i cannot open image or query.

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