From f9d66802239e15434d7d0c01a8bc11afb2708a75 Mon Sep 17 00:00:00 2001

From: John Snow <jsnow@redhat.com>

Date: Wed, 27 Mar 2019 17:22:56 +0100

Subject: [PATCH 118/163] qemu-nbd: Deprecate qemu-nbd --partition

RH-Author: John Snow <jsnow@redhat.com>

Message-id: <20190327172308.31077-44-jsnow@redhat.com>

Patchwork-id: 85223

O-Subject: [RHEL-7.7 qemu-kvm-rhev PATCH 43/55] qemu-nbd: Deprecate qemu-nbd --partition

Bugzilla: 1691009

RH-Acked-by: Stefan Hajnoczi <stefanha@redhat.com>

RH-Acked-by: Max Reitz <mreitz@redhat.com>

RH-Acked-by: Miroslav Rezanina <mrezanin@redhat.com>

From: Eric Blake <eblake@redhat.com>

The existing qemu-nbd --partition code claims to handle logical

partitions up to 8, since its introduction in 2008 (commit 7a5ca86).

However, the implementation is bogus (actual MBR logical partitions

form a sort of linked list, with one partition per extended table

entry, rather than four logical partitions in a single extended

table), making the code unlikely to work for anything beyond -P5 on

actual guest images. What's more, the code does not support GPT

partitions, which are becoming more popular, and maintaining device

subsetting in both NBD and the raw device is unnecessary duplication

of effort (even if it is not too difficult).

Note that obtaining the offsets of a partition (MBR or GPT) can be

learned by using 'qemu-nbd -c /dev/nbd0 file.qcow2 && sfdisk --dump

/dev/nbd0', but by the time you've done that, you might as well

just mount /dev/nbd0p1 that the kernel creates for you instead of

bothering with qemu exporting a subset.  Or, keeping to just

user-space code, use nbdkit's partition filter, which has already

known both GPT and primary MBR partitions for a while, and was

just recently enhanced to support arbitrary logical MBR parititions.

Start the clock on the deprecation cycle, with examples of how

to accomplish device subsetting without using -P.

Signed-off-by: Eric Blake <eblake@redhat.com>

Message-Id: <20190125234837.2272-1-eblake@redhat.com>

Reviewed-by: Richard W.M. Jones <rjones@redhat.com>

Reviewed-by: Stefano Garzarella <sgarzare@redhat.com>

(cherry picked from commit 0ae2d54645eb2888af6dc7f701bc02ca18e4e656)

Signed-off-by: John Snow <jsnow@redhat.com>

Signed-off-by: Miroslav Rezanina <mrezanin@redhat.com>

---

 qemu-doc.texi | 33 +++++++++++++++++++++++++++++++++

 qemu-nbd.c    |  2 ++

 qemu-nbd.texi |  6 ++++--

 3 files changed, 39 insertions(+), 2 deletions(-)

diff --git a/qemu-doc.texi b/qemu-doc.texi

index 88358be..2acbec5 100644

--- a/qemu-doc.texi

+++ b/qemu-doc.texi

@@ -2987,6 +2987,39 @@ The ``xlnx-zcu102'' machine has the same features and capabilites in QEMU.

 In order to prevent QEMU from automatically opening an image's backing

 chain, use ``"backing": null'' instead.

+@section Related binaries

+

+@subsection qemu-nbd --partition (since 4.0.0)

+

+The ``qemu-nbd --partition $digit'' code (also spelled @option{-P})

+can only handle MBR partitions, and has never correctly handled

+logical partitions beyond partition 5.  If you know the offset and

+length of the partition (perhaps by using @code{sfdisk} within the

+guest), you can achieve the effect of exporting just that subset of

+the disk by use of the @option{--image-opts} option with a raw

+blockdev using the @code{offset} and @code{size} parameters layered on

+top of any other existing blockdev. For example, if partition 1 is

+100MiB long starting at 1MiB, the old command:

+

+@code{qemu-nbd -t -P 1 -f qcow2 file.qcow2}

+

+can be rewritten as:

+

+@code{qemu-nbd -t --image-opts driver=raw,offset=1M,size=100M,file.driver=qcow2,file.backing.driver=file,file.backing.filename=file.qcow2}

+

+Alternatively, the @code{nbdkit} project provides a more powerful

+partition filter on top of its nbd plugin, which can be used to select

+an arbitrary MBR or GPT partition on top of any other full-image NBD

+export.  Using this to rewrite the above example results in:

+

+@code{qemu-nbd -t -k /tmp/sock -f qcow2 file.qcow2 &}

+@code{nbdkit -f --filter=partition nbd socket=/tmp/sock partition=1}

+

+Note that if you are exposing the export via /dev/nbd0, it is easier

+to just export the entire image and then mount only /dev/nbd0p1 than

+it is to reinvoke @command{qemu-nbd -c /dev/nbd0} limited to just a

+subset of the image.

+

 @node License

 @appendix License

diff --git a/qemu-nbd.c b/qemu-nbd.c

index 1f7b2a0..00c07fd 100644

--- a/qemu-nbd.c

+++ b/qemu-nbd.c

@@ -787,6 +787,8 @@ int main(int argc, char **argv)

             flags &= ~BDRV_O_RDWR;

             break;

         case 'P':

+            warn_report("The '-P' option is deprecated; use --image-opts with "

+                        "a raw device wrapper for subset exports instead");

             if (qemu_strtoi(optarg, NULL, 0, &partition) < 0 ||

                 partition < 1 || partition > 8) {

                 error_report("Invalid partition '%s'", optarg);

diff --git a/qemu-nbd.texi b/qemu-nbd.texi

index 386bece..d0c5182 100644

--- a/qemu-nbd.texi

+++ b/qemu-nbd.texi

@@ -56,8 +56,10 @@ auto-detecting.

 @item -r, --read-only

 Export the disk as read-only.

 @item -P, --partition=@var{num}

-Only expose MBR partition @var{num}.  Understands physical partitions

-1-4 and logical partitions 5-8.

+Deprecated: Only expose MBR partition @var{num}.  Understands physical

+partitions 1-4 and logical partition 5. New code should instead use

+@option{--image-opts} with the raw driver wrapping a subset of the

+original image.

 @item -B, --bitmap=@var{name}

 If @var{filename} has a qcow2 persistent bitmap @var{name}, expose

 that bitmap via the ``qemu:dirty-bitmap:@var{name}'' context

-- 

1.8.3.1