core: rework syscall filter

- Allow configuration of an errno error to return from blacklisted
  syscalls, instead of immediately terminating a process.

- Fix parsing logic when libseccomp support is turned off

- Only keep the actual syscall set in the ExecContext, and generate the
  string version only on demand.

1 files changed, 39 insertions, 15 deletions

diff --git a/man/systemd.exec.xml b/man/systemd.exec.xml
index 0c6ca5acf..86ad7e223 100644
--- a/man/systemd.exec.xml
+++ b/man/systemd.exec.xml
@@ -1001,7 +1001,7 @@
                                 list of system call
                                 names. If this setting is used, all
                                 system calls executed by the unit
-                                process except for the listed ones
+                                processes except for the listed ones
                                 will result in immediate process
                                 termination with the
                                 <constant>SIGSYS</constant> signal
@@ -1031,23 +1031,47 @@
                                 prior assignments will have no
                                 effect.</para>
 
-                                <para>If you specify both types of this option
-                                (i.e. whitelisting and blacklisting) the first
-                                encountered will take precedence and will
-                                dictate the default action (termination
-                                or approval of a system call). Then the
-                                next occurrences of this option will add or
-                                delete the listed system calls from the set
-                                of the filtered system calls, depending of
-                                its type and the default action (e.g. You
-                                have started with a whitelisting of <function>
-                                read</function> and <function>write</function>
-                                and right after it add a blacklisting of
-                                <function>write</function>, then <function>
-                                write</function> will be removed from the set)
+                                <para>If you specify both types of
+                                this option (i.e. whitelisting and
+                                blacklisting) the first encountered
+                                will take precedence and will dictate
+                                the default action (termination or
+                                approval of a system call). Then the
+                                next occurrences of this option will
+                                add or delete the listed system calls
+                                from the set of the filtered system
+                                calls, depending of its type and the
+                                default action (e.g. You have started
+                                with a whitelisting of
+                                <function>read</function> and
+                                <function>write</function> and right
+                                after it add a blacklisting of
+                                <function>write</function>, then
+                                <function>write</function> will be
+                                removed from the set).
                                 </para></listitem>
                         </varlistentry>
 
+                        <varlistentry>
+                                <term><varname>SystemCallErrorNumber=</varname></term>
+
+                                <listitem><para>Takes an
+                                <literal>errno</literal> error number
+                                name to return when the system call
+                                filter configured with
+                                <varname>SystemCallFilter=</varname>
+                                is triggered, instead of terminating
+                                the process immediately. Takes an
+                                error name such as
+                                <literal>EPERM</literal>,
+                                <literal>EACCES</literal> or
+                                <literal>EUCLEAN</literal>. When this
+                                setting is not used, or when the empty
+                                string is assigned the process will be
+                                terminated immediately when the filter
+                                is triggered.</para></listitem>
+                        </varlistentry>
+
                 </variablelist>
         </refsect1>