[pve-devel] [PATCH container] create_vm: fix order of config creation/reading/locking

[Fabian Grünbichler]

On April 29, 2020 11:58 am, Fabian Ebner wrote:
> The update_pct_config call leads to a write_config call and so the
> configuration file was created before it was intended to be created.
> 
> When the CFS is updated in between the write_config call and the
> PVE::Cluster::check_vmid_unused call in create_and_lock_config,
> the container file would already exist and so creation would
> fail after writing out a basically empty config.
> (Meaning this is necessary for the proposed "update CFS after
> obtaining a configuration file lock" changes)
> 
> Even worse, a race was possible for two containers created with the
> same ID at the same time:
> Assuming the initial PVE::Cluster::check_vmid_unused check in the
> parameter verification passes for both create_vm calls, the later one
> would potentially overwrite the earlier configuration file with its
> update_pct_config call.
> 
> Additionally, the file read for $old_config was always the one written
> by update_pct_config. Meaning that for a create_vm call with force=1,
> already existing old volumes were not removed.
> 
> Signed-off-by: Fabian Ebner <f.ebner at proxmox.com>

hmm, update_pct_config there was not intended to write out the config, 
but just to fill the $conf hash.

three alternative approaches that would return to the original semantics:
1 skip hotplug/apply pending if pending section is empty (should always 
  be the case in this code path)
2 add explicit parameter to skip
3 drop the hotplug/apply pending calls in update_pct_config, add them to 
the update_vm API endpoint instead.

I'd prefer 3 - 1 - 2 in that order ;)

3 would be called with the same semantics in both call sites (create_vm 
and update_vm): here are add/delete/revert parameters, here's a conf 
hash; merge those operating solely on the conf hash.

> ---
> 
> Note that this should be applied before [0] to avoid temporary
> (further ;)) breakage of container creation.
> 
> [0]: https://pve.proxmox.com/pipermail/pve-devel/2020-April/043155.html
> 
>  src/PVE/API2/LXC.pm | 9 ++++-----
>  1 file changed, 4 insertions(+), 5 deletions(-)
> 
> diff --git a/src/PVE/API2/LXC.pm b/src/PVE/API2/LXC.pm
> index 148ba6a..46d2fd7 100644
> --- a/src/PVE/API2/LXC.pm
> +++ b/src/PVE/API2/LXC.pm
> @@ -334,10 +334,6 @@ __PACKAGE__->register_method({
>  	# check/activate default storage
>  	&$check_and_activate_storage($storage) if !defined($mp_param->{rootfs});
>  
> -	PVE::LXC::Config->update_pct_config($vmid, $conf, 0, $no_disk_param);
> -
> -	$conf->{unprivileged} = 1 if $unprivileged;
> -
>  	my $emsg = $restore ? "unable to restore CT $vmid -" : "unable to create CT $vmid -";
>  
>  	eval { PVE::LXC::Config->create_and_lock_config($vmid, $force) };
> @@ -346,8 +342,11 @@ __PACKAGE__->register_method({
>  	my $code = sub {
>  	    my $old_conf = PVE::LXC::Config->load_config($vmid);
>  	    my $was_template;
> -
>  	    my $vollist = [];
> +
> +	    PVE::LXC::Config->update_pct_config($vmid, $conf, 0, $no_disk_param);
> +	    $conf->{unprivileged} = 1 if $unprivileged;
> +
>  	    eval {
>  		my $orig_mp_param; # only used if $restore
>  		if ($restore) {
> -- 
> 2.20.1
> 
> 
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