tmpfiles: add new switch -E for quickly excluding /proc, /dev, /sys and /run

man/systemd-tmpfiles.xml

@@ -149,6 +149,7 @@
         the specified prefix. This option can be specified multiple
         times.</para></listitem>
       </varlistentry>
+
       <varlistentry>
         <term><option>--exclude-prefix=<replaceable>path</replaceable></option></term>
         <listitem><para>Ignore rules with paths that start with the
@@ -156,6 +157,16 @@
         times.</para></listitem>
       </varlistentry>
 
+      <varlistentry>
+        <term><option>-E</option></term>
+        <listitem><para>A shortcut for <literal>--exclude-prefix=/dev --exclude-prefix=/proc
+        --exclude-prefix=/run --exclude-prefix=/sys</literal>, i.e. exclude the hierarchies typically backed
+        by virtual or memory file systems. This is useful in combination with <option>--root=</option>, if
+        the specified directory tree contains an OS tree without these virtual/memory file systems mounted
+        in, as it is typically not desirable to create any files and directories below these subdirectories
+        if they are supposed to be overmounted during runtime.</para></listitem>
+      </varlistentry>
+
       <varlistentry>
         <term><option>--root=<replaceable>root</replaceable></option></term>
         <listitem><para>Takes a directory path as an argument. All paths will be prefixed with the given alternate
@@ -164,7 +175,11 @@
         <para>When this option is used, the libc Name Service Switch (NSS) is bypassed for resolving users
         and groups. Instead the files <filename>/etc/passwd</filename> and <filename>/etc/group</filename>
         inside the alternate root are read directly. This means that users/groups not listed in these files
-        will not be resolved, i.e. LDAP NIS and other complex databases are not considered.</para></listitem>
+        will not be resolved, i.e. LDAP NIS and other complex databases are not considered.</para>
+
+        <para>Consider combining this with <option>-E</option> to ensure the invocation does not create files
+        or directories below mount points in the OS image operated on that are typically overmounted during
+        runtime.</para></listitem>
       </varlistentry>
 
       <varlistentry>

src/tmpfiles/tmpfiles.c

@@ -2884,6 +2884,27 @@ static int cat_config(char **config_dirs, char **args) {
         return cat_files(NULL, files, 0);
 }
 
+static int exclude_default_prefixes(void) {
+        int r;
+
+        /* Provide an easy way to exclude virtual/memory file systems from what we do here. Useful in
+         * combination with --root= where we probably don't want to apply stuff to these dirs as they are
+         * likely over-mounted if the root directory is actually used, and it wouldbe less than ideal to have
+         * all kinds of files created/adjusted underneath these mount points. */
+
+        r = strv_extend_strv(
+                        &arg_exclude_prefixes,
+                        STRV_MAKE("/dev",
+                                  "/proc",
+                                  "/run",
+                                  "/sys"),
+                                 true);
+        if (r < 0)
+                return log_oom();
+
+        return 0;
+}
+
 static int help(void) {
         _cleanup_free_ char *link = NULL;
         int r;
@@ -2904,6 +2925,7 @@ static int help(void) {
                "     --boot                 Execute actions only safe at boot\n"
                "     --prefix=PATH          Only apply rules with the specified prefix\n"
                "     --exclude-prefix=PATH  Ignore rules with the specified prefix\n"
+               "  -E                        Ignore rules prefixed with /dev, /proc, /run, /sys\n"
                "     --root=PATH            Operate on an alternate filesystem root\n"
                "     --replace=PATH         Treat arguments as replacement for PATH\n"
                "     --no-pager             Do not pipe output into a pager\n"
@@ -2954,7 +2976,7 @@ static int parse_argv(int argc, char *argv[]) {
         assert(argc >= 0);
         assert(argv);
 
-        while ((c = getopt_long(argc, argv, "h", options, NULL)) >= 0)
+        while ((c = getopt_long(argc, argv, "hE", options, NULL)) >= 0)
 
                 switch (c) {
 
@@ -2998,6 +3020,13 @@ static int parse_argv(int argc, char *argv[]) {
                                 return log_oom();
                         break;
 
+                case 'E':
+                        r = exclude_default_prefixes();
+                        if (r < 0)
+                                return r;
+
+                        break;
+
                 case ARG_ROOT:
                         r = parse_path_argument_and_warn(optarg, /* suppress_root= */ false, &arg_root);
                         if (r < 0)