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[Qemu-devel] [PATCH 0/3] block/file-posix: File locking during creation
From: |
Max Reitz |
Subject: |
[Qemu-devel] [PATCH 0/3] block/file-posix: File locking during creation |
Date: |
Sat, 21 Apr 2018 00:09:10 +0200 |
Currently we do not take permissions on a file while it is being
created. That is a bit sad. The simplest way to test this is the
following:
$ qemu-img create -f qcow2 foo.qcow2 64M
Formatting 'foo.qcow2', fmt=qcow2 size=67108864 cluster_size=65536
lazy_refcounts=off refcount_bits=16
$ qemu-img convert -f qcow2 -O qcow2 foo.qcow2 foo.qcow2
qemu-img: foo.qcow2: error while converting qcow2: Failed to get "write"
lock
Is another process using the image?
$ qemu-img info foo.qcow2
image: foo.qcow2
file format: raw
virtual size: 0 (0 bytes)
disk size: 0
(See also https://bugzilla.redhat.com/show_bug.cgi?id=1519144)
Here is what's happening: file-posix opens the file with
O_CREAT | O_TRUNC, thus truncating the file to 0. Then qcow2 tries to
format it, but fails because it needs the WRITE permission to do so. So
it cannot format it and the file stays empty.
We should actually take a WRITE and a RESIZE permission before we
truncate the file, and this is what this series does.
I personally consider the solution taken here a bit of a hack, but it
works and we don't have any locking in any drivers but file-posix
anyway, so it isn't lacking anything in that regard. Integrating it in
blockdev-create might be possible, but there are two issues:
(1) It would be harder.
(2) What would we do anyway? We'd advise protocol drivers to take WRITE
and RESIZE permissions before they truncate a file to be empty...
Well, and then they'd do exactly what file-posix is made to do in
this series.
So basically what I consider a hack could be seen as exactly the right
way to tackle the issue: Protocol drivers have to ensure the correct
permissions (and they have to choose what those are) are applied before
changing any file -- which is what this series implements.
Max Reitz (3):
block/file-posix: Pass FD to locking helpers
block/file-posix: File locking during creation
iotests: Add creation test to 153
block/file-posix.c | 68 ++++++++++++++++++++++++++++++++++++----------
tests/qemu-iotests/153 | 18 ++++++++++++
tests/qemu-iotests/153.out | 13 +++++++++
3 files changed, 84 insertions(+), 15 deletions(-)
--
2.14.3
- [Qemu-devel] [PATCH 0/3] block/file-posix: File locking during creation,
Max Reitz <=