Fuz Hackerspace

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====== rozlav - Public Page ======
<markdown>
# Patcher du debian ! how to ? Avec des vrais morceaux de LXD dedans
## 1. Contexte : du LXD & du qemu-system
 - pour une machine toute neuve debian sous Bookworm (testing à l'époque), qui nécessite de faire fonctionner des machines virtuelles (dites « vm/VM ») grâce au logiciel de Canonical « LXD » (sorte de remplacant de lxc), il fallait patcher le paquet qemu-system.
 - en passant du côté de `irc.libera.chat#qemu` et `irc.libera.chat#lxc` j'ai été redirigé []sur l'issue qui va bien](https://gitlab.com/qemu-project/qemu/-/issues/1486) mais vu que ça n'avançait pas : l'idée a été de patcher w00t !
## 2. Reproduction du pépin (/¯◡ ‿ ◡)/¯ ~ ┻━┻
### 2.1 Install machine de base
```
sudo apt update
sudo apt install --no-install-recommends sudo vim etckeeper tig lnav screen htop borgbackup tcpstat nmap lsof iftop tcpdump tldr ncdu ripgrep unattended-upgrades gnupg debian-goodies apticron apt-listchanges ssmtp python3-pip
sudo apt install lxd
sudo apt install qemu-base qemu-utils qemu-system
sudo apt upgrade
```

### 2.2 Étapes avant le bug
 - lancement du bridge et création de la vm
```
lxd --version
5.0.2
lxc network create UPLINK --type=physical parent=lxdbr0
+-----------------+----------+---------+--------------+---------------------------+-------------+---------+---------+
|      NAME       |   TYPE   | MANAGED |     IPV4     |           IPV6            | DESCRIPTION | USED BY |  STATE  |
+-----------------+----------+---------+--------------+---------------------------+-------------+---------+---------+
| lxdbr0          | bridge   | YES     | 10.98.2.1/24 | fd42:f0a7:adc5:f886::1/64 |             | 3       | CREATED |
lxc launch images:debian/bullseye test0 --vm
Error: Failed setting up device via monitor: Failed setting up device "eth0": Failed adding NIC netdev: Monitor is disconnected
```
 - erreur, liée à qemu-system ( https://gitlab.com/qemu-project/qemu/-/issues/1486 )
```
root@Selachimorpha:~# lxc info --show-log local:test0
Name: test0
Status: STOPPED
Type: virtual-machine
Architecture: x86_64
Created: 2023/03/13 17:06 CET
Log:
warning: tap: open vhost char device failed: Permission denied
warning: tap: open vhost char device failed: Permission denied
qemu-system-x86_64: ../../net/net.c:1106: net_client_init1: Assertion `nc' failed.
```
## 3. Tentative de résolution : patcher du paquet debian（○゜ε＾○）
 - utilisation d'[]une ressource (maigre) de chez debian](https://wiki.debian.org/BuildingTutorial)
 - install des quelques paquets nécessaire à la (re)compilation
```
sudo apt install build-essential fakeroot devscripts dpkg-dev gcc make automake pbuilder
cd /home/roz/ && mkdir pkg-qemu-patching && cd pkg-qemu-patching/
sudo apt source qemu-system
```
 - création du patch, []téléchargé depuis la liste mail](https://www.mail-archive.com/qemu-devel@nongnu.org/msg924611.html) `vim debian/patches/0001-net-net.c-do-not-assert-nc-as-hotplug-vhost-net-may-.patch` :
```
From b2bca483212f04b9b021753aec1fcaeaf41385d1 Mon Sep 17 00:00:00 2001
From: Ming Yang <yangming73@huawei.com>
Date: Fri, 25 Nov 2022 10:45:52 +0800
Subject: [PATCH] net/net.c: Fix qemu crash when hot-pluging a vhost-net failed. 

Hot-pluging a vhost-net may cause virtual machine crash in following steps:
1. Starting a vm without net devices.
2. Hot-pluging 70 memory devices.
3. Hot-pluging a vhost-net device.

The reason is : if hotplug a vhost-net failed, the nc cannot be found via function qemu_find_netdev, as
it has been cleaned up through function qemu_cleanup_net_client. Which leads to the result
that assert(nc) failed, then qemu crashed.

While, the root reason is that, in commit 46d4d36d0bf2 if not both has_vhostforce and vhostforce flags
are true, the errp would not be set. Then net_init_tap would not return a negative value, fallowed by founding nc
and assert nc.

In this patch, asserting nc is replaced with setting an error message.

Fixes: 46d4d36d0bf2("tap: setting error appropriately when calling net_init_tap_one()")
Signed-off-by: Ming Yang <yangming73@huawei.com>
Signed-off-by: Liang Zhang <zhangliang5@huawei.com>
https://gitlab.com/qemu-project/qemu/-/issues/1486
---
 net/net.c | 11 ++++++++++-
 1 file changed, 10 insertions(+), 1 deletion(-)

diff --git a/net/net.c b/net/net.c
index 840ad9dca5..1d1d7e54c4 100644
--- a/net/net.c
+++ b/net/net.c
@@ -1103,7 +1103,16 @@ static int net_client_init1(const Netdev *netdev, bool is_netdev, Error **errp)
 
     if (is_netdev) {
         nc = qemu_find_netdev(netdev->id);
-        assert(nc);
+        /*
+         * If the tap of hotpluged net device do not has both has_vhostforce flag and vhostforce flags,
+         * when error occurs, the error messags will be report but not set to errp. Thus net_client_init_fun
+         * will not return a negatave value. Therefore the value of nc might be NULL. To make qemu robust,
+         * it is better to judge if nc is NULL.
+         */
+        if (!nc) {
+            error_setg(errp, "Device '%s' could not be initialized", netdev->id);
+            return -1;
+        }
         nc->is_netdev = true;
     }
 
-- 
2.33.0
```
 - ajout du patch au fichier de conf' des trucs à compiler : `vim qemu-7.2+dfsg/debian/patches/series` :
```
# Assertion failed au moment du lxc-start/launch
0001-net-net.c-do-not-assert-nc-as-hotplug-vhost-net-may-.patch
```
 - Compilation avec le patch, celui-ci devrait apparaître clairement via son nom (ici `0001-net-net.c-do-not-assert-nc-as-hotplug-vhost-net-may-.patch`) en début de compilation
```
cd /home/roz/pkg-qemu-patching/qemu-7.2+dfsg/
dpkg-buildpackage -us -uc
```
 - Vérification :
```
root@Selachimorpha:/home/roz/pkg-qemu-patching# ls -la
total 864740
drwxr-xr-x  3 root root      4096 Mar 12 00:56 .
drwx------  4 roz  roz       4096 Mar 11 23:40 ..
drwxr-xr-x 60 root root      4096 Mar 12 00:56 qemu-7.2+dfsg
-rw-r--r--  1 root root  30315205 Mar 12 01:01 qemu_7.2+dfsg-4_amd64.build
-rw-r--r--  1 root root     32454 Mar 12 00:55 qemu_7.2+dfsg-4_amd64.buildinfo
-rw-r--r--  1 root root     12577 Mar 12 00:56 qemu_7.2+dfsg-4_amd64.changes
-rw-r--r--  1 root root    121624 Mar 11 23:51 qemu_7.2+dfsg-4.debian.tar.xz
-rw-r--r--  1 root root      5892 Mar 11 23:51 qemu_7.2+dfsg-4.dsc
-rw-r--r--  1 root root  23523172 Dec 15 15:32 qemu_7.2+dfsg.orig.tar.xz
-rw-r--r--  1 root root     86428 Mar 12 00:52 qemu-block-extra_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root    292220 Mar 12 00:52 qemu-block-extra-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root    324884 Mar 12 00:53 qemu-guest-agent_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root    949128 Mar 12 00:53 qemu-guest-agent-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root     31504 Mar 12 00:52 qemu-system_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root   8981796 Mar 12 00:55 qemu-system-arm_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root  39860352 Mar 12 00:55 qemu-system-arm-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root   2138480 Mar 12 00:53 qemu-system-common_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root   7371508 Mar 12 00:53 qemu-system-common-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root   1276368 Mar 12 00:52 qemu-system-data_7.2+dfsg-4_all.deb
-rw-r--r--  1 root root    297904 Mar 12 00:53 qemu-system-gui_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root   1041592 Mar 12 00:53 qemu-system-gui-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root  13307612 Mar 12 00:53 qemu-system-mips_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root  55224640 Mar 12 00:54 qemu-system-mips-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root  46229776 Mar 12 00:54 qemu-system-misc_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root 193547560 Mar 12 00:55 qemu-system-misc-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root   7538304 Mar 12 00:55 qemu-system-ppc_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root  31417836 Mar 12 00:55 qemu-system-ppc-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root   5175268 Mar 12 00:53 qemu-system-sparc_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root  20431008 Mar 12 00:54 qemu-system-sparc-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root   9255868 Mar 12 00:55 qemu-system-x86_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root  37281972 Mar 12 00:55 qemu-system-x86-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root   2685276 Mar 12 00:53 qemu-system-xen_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root  10742216 Mar 12 00:53 qemu-system-xen-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root  12706240 Mar 12 00:54 qemu-user_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root      5344 Mar 12 00:55 qemu-user-binfmt_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root 135493288 Mar 12 00:55 qemu-user-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root  46142944 Mar 12 00:54 qemu-user-static_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root 132001404 Mar 12 00:55 qemu-user-static-dbgsym_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root   1281392 Mar 12 00:55 qemu-utils_7.2+dfsg-4_amd64.deb
-rw-r--r--  1 root root   8260576 Mar 12 00:52 qemu-utils-dbgsym_7.2+dfsg-4_amd64.deb
```
## 4. Test !
```
cd /home/roz/pkg-qemu-patching
dpkg -i *.deb
```
</markdown>