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man: reword DeviceAllow= documentation 

Don't claim we'd use cgroup.deny much. It's just a way to remove stuff
from device lists, which is nothing we allow users to explicitly
configure.

Also, extend documentation when wildcards may be used, and when not.
This commit is contained in:
Lennart Poettering 2019-07-31 10:20:16 +02:00 committed by Zbigniew Jędrzejewski-Szmek
parent 5c76ca6093
commit 3ff668cb9a
1 changed files with 25 additions and 30 deletions
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55
man/systemd.resource-control.xml
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@ -655,37 +655,32 @@
<term><varname>DeviceAllow=</varname></term>
<listitem>
<para>Control access to specific device nodes by the
executed processes. Takes two space-separated strings: a
device node specifier followed by a combination of
<constant>r</constant>, <constant>w</constant>,
<constant>m</constant> to control
<emphasis>r</emphasis>eading, <emphasis>w</emphasis>riting,
or creation of the specific device node(s) by the unit
(<emphasis>m</emphasis>knod), respectively. This controls
the <literal>devices.allow</literal> and
<literal>devices.deny</literal> control group
attributes. For details about these control group
attributes, see <ulink
url="https://www.kernel.org/doc/Documentation/cgroup-v1/devices.txt">devices.txt</ulink>.</para>
<para>Control access to specific device nodes by the executed processes. Takes two space-separated
strings: a device node specifier followed by a combination of <constant>r</constant>,
<constant>w</constant>, <constant>m</constant> to control <emphasis>r</emphasis>eading,
<emphasis>w</emphasis>riting, or creation of the specific device node(s) by the unit
(<emphasis>m</emphasis>knod), respectively. On cgroup-v1 this controls the
<literal>devices.allow</literal> control group attribute. For details about this control group
attribute, see <ulink
url="https://www.kernel.org/doc/Documentation/cgroup-v1/devices.txt">devices.txt</ulink>. On
cgroup-v2 this functionality is implemented using eBPF filtering.</para>
<para>The device node specifier is either a path to a device
node in the file system, starting with
<filename>/dev/</filename>, or a string starting with either
<literal>char-</literal> or <literal>block-</literal>
followed by a device group name, as listed in
<filename>/proc/devices</filename>. The latter is useful to
whitelist all current and future devices belonging to a
specific device group at once. The device group is matched
according to filename globbing rules, you may hence use the
<literal>*</literal> and <literal>?</literal>
wildcards. Examples: <filename>/dev/sda5</filename> is a
path to a device node, referring to an ATA or SCSI block
device. <literal>char-pts</literal> and
<literal>char-alsa</literal> are specifiers for all pseudo
TTYs and all ALSA sound devices,
respectively. <literal>char-cpu/*</literal> is a specifier
matching all CPU related device groups.</para>
<para>The device node specifier is either a path to a device node in the file system, starting with
<filename>/dev/</filename>, or a string starting with either <literal>char-</literal> or
<literal>block-</literal> followed by a device group name, as listed in
<filename>/proc/devices</filename>. The latter is useful to whitelist all current and future
devices belonging to a specific device group at once. The device group is matched according to
filename globbing rules, you may hence use the <literal>*</literal> and <literal>?</literal>
wildcards. (Note that such globbing wildcards are not available for device node path
specifications!) In order to match device nodes by numeric major/minor, use device node paths in
the <filename>/dev/char/</filename> and <filename>/dev/block/</filename> directories. However,
matching devices by major/minor is generally not recommended as assignments are neither stable nor
portable between systems or different kernel versions.</para>
<para>Examples: <filename>/dev/sda5</filename> is a path to a device node, referring to an ATA or
SCSI block device. <literal>char-pts</literal> and <literal>char-alsa</literal> are specifiers for
all pseudo TTYs and all ALSA sound devices, respectively. <literal>char-cpu/*</literal> is a
specifier matching all CPU related device groups.</para>
<para>Note that whitelists defined this way should only reference device groups which are
resolvable at the time the unit is started. Any device groups not resolvable then are not added to