Re: [pve-devel] [PATCH container 2/2] oci create: honor `User` from OCI image config

[Wolfgang Bumiller <w.bumiller@proxmox.com>]

On Mon, Dec 15, 2025 at 06:41:59PM +0100, Filip Schauer wrote:
> Honor a custom user and group specified for the entrypoint via the OCI
> image config `User` field instead of ignoring it.
> 
> User and group name lookups, including supplementary groups, are
> resolved from /etc/passwd and /etc/group in the container file system.
> This behaviour matches the OCI image spec. [0]
> 
> [0] https://specs.opencontainers.org/image-spec/config/?v=v1.1.1#properties
> 
> Signed-off-by: Filip Schauer <f.schauer@proxmox.com>
> ---
> This requires the following patch for LXC in order to work properly:
> https://github.com/lxc/lxc/pull/4626
> 
>  src/PVE/LXC/Create.pm | 59 +++++++++++++++++++++++++++++++++++++++++++
>  1 file changed, 59 insertions(+)
> 
> diff --git a/src/PVE/LXC/Create.pm b/src/PVE/LXC/Create.pm
> index 9956cf9..08e0386 100644
> --- a/src/PVE/LXC/Create.pm
> +++ b/src/PVE/LXC/Create.pm
> @@ -768,6 +768,65 @@ sub restore_oci_archive {
>          }
>      }
>  
> +    if (my $usercfg = $oci_config_get_checked_scalar->('User')) {

This code should be factored into a separate sub.

> +        my ($user, $group) = $usercfg =~ /^([^:]+)(?::([^:]+))?$/
> +            or die "OCI config value for 'User' has an invalid format\n";
> +
> +        my $etc_passwd = "$rootdir/etc/passwd";
> +        my $etc_group = "$rootdir/etc/group";
> +
> +        # Scan file, match column $match_index against $match_val, return value at $ret_index
> +        my $lookup_field = sub {
> +            my ($file, $match_index, $match_val, $ret_index) = @_;
> +
> +            open(my $fh, '<', $file) or return undef;
> +            while (my $line = <$fh>) {
> +                my @fields = split(/:/, $line);

A subtlety about this sub is that currently the last field also includes
the trailing `\n`. Should probably add a comment, or `chomp` it away,
even if we currently don't use the final field via `$ret_index`.

> +                if (defined($fields[$match_index]) && $fields[$match_index] eq $match_val) {
> +                    return $fields[$ret_index];
> +                }
> +            }
> +            return undef;
> +        };
> +
> +        my $get_supplementary_groups = sub {
> +            my ($username) = @_;
> +
> +            my @groups;
> +            open(my $fh, '<', $etc_group) or return undef;
> +            while (defined(my $line = <$fh>)) {
> +                push @groups, $1
> +                    if ($line =~ m/^[^:]*:[^:]*:([^:]*):(?:[^,]*,)*$username(?:,|$)/);

Username comes from the outside and is only limited to not containing a
colon. To use it inside a regex you'd have to quote it as
`\Q$username\E`.

Safer would be to do what you do in `$lookup_field` and just split on
`:` then `,` then grep with `eq`.

> +            }
> +            return join(',', @groups);
> +        };
> +
> +        my ($uid, $username, $gid, $groups);
> +
> +        if ($user =~ /^\d+$/) {
> +            $uid = $user;
> +            $username = $lookup_field->($etc_passwd, 2, $uid, 0);
> +        } else {
> +            $username = $user;
> +            $uid = $lookup_field->($etc_passwd, 0, $username, 2);
> +        }
> +
> +        if (defined($group)) {
> +            if ($group =~ /^\d+$/) {
> +                $gid = $group;
> +            } else {
> +                $gid = $lookup_field->($etc_group, 0, $group, 2);
> +            }
> +        } else {
> +            $gid = $lookup_field->($etc_passwd, 2, $uid, 3) if defined($uid);
> +            $groups = $get_supplementary_groups->($username) if defined($username);
> +        }
> +
> +        push $conf->{lxc}->@*, ['lxc.init.uid', $uid] if defined($uid);
> +        push $conf->{lxc}->@*, ['lxc.init.gid', $gid] if defined($gid);
> +        push $conf->{lxc}->@*, ['lxc.init.groups', $groups] if defined($groups);
> +    }
> +
>      if (my $working_dir = $oci_config_get_checked_scalar->('WorkingDir')) {
>          push $conf->{lxc}->@*, ['lxc.init.cwd', $working_dir];
>      }
> -- 
> 2.47.3


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