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man: document new nspawn ID mapping mounts features

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Lennart Poettering 2021-04-27 18:31:24 +02:00
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88
man/systemd-nspawn.xml
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@ -696,32 +696,41 @@
number of host UIDs/GIDs to assign to the container. If the second parameter is omitted, 65536 UIDs/GIDs are
assigned.</para></listitem>
<listitem><para>If the parameter is omitted, or true, user namespacing is turned on. The UID/GID range to
use is determined automatically from the file ownership of the root directory of the container's directory
tree. To use this option, make sure to prepare the directory tree in advance, and ensure that all files and
directories in it are owned by UIDs/GIDs in the range you'd like to use. Also, make sure that used file ACLs
exclusively reference UIDs/GIDs in the appropriate range. If this mode is used the number of UIDs/GIDs
assigned to the container for use is 65536, and the UID/GID of the root directory must be a multiple of
65536.</para></listitem>
<listitem><para>If the parameter is <literal>yes</literal>, user namespacing is turned on. The
UID/GID range to use is determined automatically from the file ownership of the root directory of
the container's directory tree. To use this option, make sure to prepare the directory tree in
advance, and ensure that all files and directories in it are owned by UIDs/GIDs in the range you'd
like to use. Also, make sure that used file ACLs exclusively reference UIDs/GIDs in the appropriate
range. In this mode, the number of UIDs/GIDs assigned to the container is 65536, and the owner
UID/GID of the root directory must be a multiple of 65536.</para></listitem>
<listitem><para>If the parameter is false, user namespacing is turned off. This is the default.</para>
<listitem><para>If the parameter is <literal>no</literal>, user namespacing is turned off. This is
the default.</para>
</listitem>
<listitem><para>The special value <literal>pick</literal> turns on user namespacing. In this case the UID/GID
range is automatically chosen. As first step, the file owner of the root directory of the container's
directory tree is read, and it is checked that it is currently not used by the system otherwise (in
particular, that no other container is using it). If this check is successful, the UID/GID range determined
this way is used, similar to the behavior if "yes" is specified. If the check is not successful (and thus
the UID/GID range indicated in the root directory's file owner is already used elsewhere) a new – currently
unused – UID/GID range of 65536 UIDs/GIDs is randomly chosen between the host UID/GIDs of 524288 and
1878982656, always starting at a multiple of 65536, and, if possible, consistently hashed from the machine
name. This setting implies
<option>--private-users-chown</option> (see below), which has the effect that the files and directories in
the container's directory tree will be owned by the appropriate users of the range picked. Using this option
makes user namespace behavior fully automatic. Note that the first invocation of a previously unused
container image might result in picking a new UID/GID range for it, and thus in the (possibly expensive) file
ownership adjustment operation. However, subsequent invocations of the container will be cheap (unless of
course the picked UID/GID range is assigned to a different use by then).</para></listitem>
<listitem><para>If the parameter is <literal>identity</literal>, user namespacing is employed with
an identity mapping for the first 65536 UIDs/GIDs. This is mostly equivalent to
<option>--private-users=0:65536</option>. While it does not provide UID/GID isolation, since all
host and container UIDs/GIDs are chosen identically it does provide process capability isolation,
and hence is often a good choice if proper user namespacing with distinct UID maps is not
appropriate.</para></listitem>
<listitem><para>The special value <literal>pick</literal> turns on user namespacing. In this case
the UID/GID range is automatically chosen. As first step, the file owner UID/GID of the root
directory of the container's directory tree is read, and it is checked that no other container is
currently using it. If this check is successful, the UID/GID range determined this way is used,
similar to the behavior if <literal>yes</literal> is specified. If the check is not successful (and
thus the UID/GID range indicated in the root directory's file owner is already used elsewhere) a
new – currently unused – UID/GID range of 65536 UIDs/GIDs is randomly chosen between the host
UID/GIDs of 524288 and 1878982656, always starting at a multiple of 65536, and, if possible,
consistently hashed from the machine name. This setting implies
<option>--private-users-ownership=auto</option> (see below), which possibly has the effect that the
files and directories in the container's directory tree will be owned by the appropriate users of
the range picked. Using this option makes user namespace behavior fully automatic. Note that the
first invocation of a previously unused container image might result in picking a new UID/GID range
for it, and thus in the (possibly expensive) file ownership adjustment operation. However,
subsequent invocations of the container will be cheap (unless of course the picked UID/GID range is
assigned to a different use by then).</para></listitem>
</orderedlist>
<para>It is recommended to assign at least 65536 UIDs/GIDs to each container, so that the usable UID/GID range in the
@ -747,31 +756,44 @@
</varlistentry>
<varlistentry>
<term><option>--private-users-chown</option></term>
<term><option>--private-users-ownership=</option></term>
<listitem><para>If specified, all files and directories in the container's directory tree will be
adjusted so that they are owned by the appropriate UIDs/GIDs selected for the container (see above).
This operation is potentially expensive, as it involves iterating through the full directory tree of
the container. Besides actual file ownership, file ACLs are adjusted as well.</para>
<listitem><para>Controls how to adjust the container image's UIDs and GIDs to match the UID/GID range
chosen with <option>--private-users=</option>, see above. Takes one of <literal>off</literal> (to
leave the image as is), <literal>chown</literal> (to recursively <function>chown()</function> the
container's directory tree as needed), <literal>map</literal> (in order to use transparent ID mapping
mounts) or <literal>auto</literal> for automatically using <literal>map</literal> where available and
<literal>chown</literal> where not.</para>
<para>This option is implied if <option>--private-users=pick</option> is used. This option has no effect if
user namespacing is not used.</para></listitem>
<para>If <literal>chown</literal> is selected, all files and directories in the container's directory
tree will be adjusted so that they are owned by the appropriate UIDs/GIDs selected for the container
(see above). This operation is potentially expensive, as it involves iterating through the full
directory tree of the container. Besides actual file ownership, file ACLs are adjusted as
well.</para>
<para>Typically <literal>map</literal> is the best choice, since it transparently maps UIDs/GIDs in
memory as needed without modifying the image, and without requiring an expensive recursive adjustment
operation. However, it is not available for all file systems, currently.</para>
<para>The <option>--private-users-ownership=auto</option> option is implied if
<option>--private-users=pick</option> is used. This option has no effect if user namespacing is not
used.</para></listitem>
</varlistentry>
<varlistentry>
<term><option>-U</option></term>
<listitem><para>If the kernel supports the user namespaces feature, equivalent to
<option>--private-users=pick --private-users-chown</option>, otherwise equivalent to
<option>--private-users=pick --private-users-ownership=auto</option>, otherwise equivalent to
<option>--private-users=no</option>.</para>
<para>Note that <option>-U</option> is the default if the
<filename>systemd-nspawn@.service</filename> template unit file is used.</para>
<para>Note: it is possible to undo the effect of <option>--private-users-chown</option> (or
<para>Note: it is possible to undo the effect of <option>--private-users-ownership=chown</option> (or
<option>-U</option>) on the file system by redoing the operation with the first UID of 0:</para>
<programlisting>systemd-nspawn … --private-users=0 --private-users-chown</programlisting>
<programlisting>systemd-nspawn … --private-users=0 --private-users-ownership=chown</programlisting>
</listitem>
</varlistentry>

10
man/systemd.nspawn.xml
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@ -452,12 +452,12 @@
</varlistentry>
<varlistentry>
<term><varname>PrivateUsersChown=</varname></term>
<term><varname>PrivateUsersOwnership=</varname></term>
<listitem><para>Configures whether the ownership of the files and directories in the container tree shall be
adjusted to the UID/GID range used, if necessary and user namespacing is enabled. This is equivalent to the
<option>--private-users-chown</option> command line switch. This option is privileged (see
above). </para></listitem>
<listitem><para>Configures whether the ownership of the files and directories in the container tree
shall be adjusted to the UID/GID range used, if necessary and user namespacing is enabled. This is
equivalent to the <option>--private-users-ownership=</option> command line switch. This option is
privileged (see above).</para></listitem>
</varlistentry>
</variablelist>