Bug 1599277

This is interesting.  This one’s not very easy to fix, in fact I don’t know an approach off the top of my hat.  I know what’s broken, though.

The qemu-internal string pointing to the backing file is updated when the backing file is actually opened.  So in this case it is modified from “base.qcow2” to “test/base.qcow2”.  However, when looking for the backing file, we try to make it an absolute filename, so now we interpret it relative to the overlay again.  This yields basically “test/test/base.qcow2”.  On top of that we do a realpath(), which fails, because that path does not exist.

So the base issue is that half of the block layer interprets bs->backing_file like it was the string in the image header (which it is before bdrv_open_backing_file()), and the other half interprets it as kind of a cache of bs->backing->bs->filename.  Those are different things, though.

The bottom line is that there probably won’t be a quick fix for this.

Max

Missed qemu-4.1, so we'll wait for the next rebase.

This is fixed by the same series as BZ 1594747.  Unfortunately, it is rather far from being merged upstream.

Max

QEMU has been recently split into sub-components and as a one-time operation to avoid breakage of tools, we are setting the QEMU sub-component of this BZ to "General". Please review and change the sub-component if necessary the next time you review this BZ. Thanks

The filter series has been merged upstream. Max will provide more details and reproducer here before we move it to POST.

The steps provided in comment 0 work well as a reproducer.

Result with qemu 5.2:

$ qemu-img commit -f qcow2 -b base.qcow2 test/sn2.qcow2
Image committed.

$ qemu-img commit -f qcow2 -b test/base.qcow2 test/sn2.qcow2
qemu-img: Did not find 'test/base.qcow2' in the backing chain of 'test/sn2.qcow2'

$ qemu-img commit -f qcow2 -b $PWD/test/base.qcow2 test/sn2.qcow2
Image committed.


(The second variant isn't expected to work.  The base filename either has to match the string that was given as the backing image during qemu-img create, i.e. "base.qcow2" in this case, or be an absolute path, as in the third variant.)

Tested with qemu-kvm-5.2.0-1.module+el8.4.0+9091+650b220a, not hit this issue.

Versions:
kernel-4.18.0-259.el8.x86_64
qemu-kvm-5.2.0-1.module+el8.4.0+9091+650b220a


Steps:

1.Create base.qcow2->sn1.qcow2->sn2.qcow2
# qemu-img create -f qcow2 base.qcow2 10G
# qemu-img create -f qcow2 -b base.qcow2 sn1.qcow2
# qemu-img create -f qcow2 -b sn1.qcow2 sn2.qcow2

2.Write data to the sn2.qcow2
# qemu-io -c 'write -P 1 0 1G' sn2.qcow2

3.Check the backing-chain info for sn2.qcow2
# qemu-img info bug_verify/sn2.qcow2 --backing-chain
image: bug_verify/sn2.qcow2
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 1 GiB
cluster_size: 65536
backing file: sn1.qcow2 (actual path: bug_verify/sn1.qcow2)
backing file format: qcow2
Format specific information:
    compat: 1.1
    compression type: zlib
    lazy refcounts: false
    refcount bits: 16
    corrupt: false
    extended l2: false

image: bug_verify/sn1.qcow2
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 196 KiB
cluster_size: 65536
backing file: base.qcow2 (actual path: bug_verify/base.qcow2)
backing file format: qcow2
Format specific information:
    compat: 1.1
    compression type: zlib
    lazy refcounts: false
    refcount bits: 16
    corrupt: false
    extended l2: false

image: bug_verify/base.qcow2
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 196 KiB
cluster_size: 65536
Format specific information:
    compat: 1.1
    compression type: zlib
    lazy refcounts: false
    refcount bits: 16
    corrupt: false
    extended l2: false


4.Commit sn2.qcow2 to base.qcow2
4.1 Specify the base.qcow2 by relative path
# qemu-img commit -f qcow2 -b base.qcow2 bug_verify/sn2.qcow2
Image committed.

# qemu-img info bug_verify/base.qcow2 
image: bug_verify/base.qcow2
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 1 GiB
cluster_size: 65536
Format specific information:
    compat: 1.1
    compression type: zlib
    lazy refcounts: false
    refcount bits: 16
    corrupt: false
    extended l2: false


4.2 repeat step 1 to step 3
# qemu-img commit -f qcow2 -b bug_verify/base.qcow2 bug_verify/sn2.qcow2 
qemu-img: Did not find 'bug_verify/base.qcow2' in the backing chain of 'bug_verify/sn2.qcow2'


4.3 Specify the base.qcow2 by full path
# qemu-img commit -f qcow2 -b /home/bug_verify/base.qcow2 /home/bug_verify/sn2.qcow2 
Image committed.

# qemu-img info /home/bug_verify/base.qcow2 
image: /home/bug_verify/base.qcow2
file format: qcow2
virtual size: 10 GiB (10737418240 bytes)
disk size: 1 GiB
cluster_size: 65536
Format specific information:
    compat: 1.1
    compression type: zlib
    lazy refcounts: false
    refcount bits: 16
    corrupt: false
    extended l2: false

According to Comment 11, set status to VERIFIED.

Since the problem described in this bug report should be
resolved in a recent advisory, it has been closed with a
resolution of ERRATA.

For information on the advisory (virt:av bug fix and enhancement update), and where to find the updated
files, follow the link below.

If the solution does not work for you, open a new bug report.

https://access.redhat.com/errata/RHBA-2021:2098