Drop parsing of /proc/cmdline by default, instead use argc + argv[]

For most use-cases the kernel will give Finit its arguments as proper
command line args in argc + argv[], like any other program.  However,
for some users, most notably Alpine Linux, there is a slightly broken
initramfs that cannot forward more than one argument using init_args,
for such systems you can re-enable the old behavior with a configure
switch --enable-kernel-cmdline -- it's not ideal but what can you do.

The main reason for removing this feature by default is to support
use-cases where Finit runs as the init for container apps that can read
/proc -- we do not want them to use the init args from the host.

Signed-off-by: Joachim Wiberg <troglobit@gmail.com>

configure.ac

@@ -52,6 +52,10 @@ AC_ARG_ENABLE(auto_reload,
         AS_HELP_STRING([--enable-auto-reload], [Automatic reload on any .conf change]),,[
 	enable_auto_reload=no])
 
+AC_ARG_ENABLE(kernel_cmdline,
+        AS_HELP_STRING([--enable-kernel-cmdline], [Parse init args from /proc/cmdline (don't use!)]),,[
+	enable_kernel_cmdline=no])
+
 AC_ARG_ENABLE(redirect,
         AS_HELP_STRING([--disable-redirect], [Disable redirection of service output to /dev/null]),,[
 	enable_redirect=yes])
@@ -124,6 +128,9 @@ AC_ARG_WITH(watchdog,
 AS_IF([test "x$enable_auto_reload" = "xyes"], [
         AC_DEFINE(AUTO_RELOAD, 1, [Finit automatically does `initctl reload` on any .conf change])])
 
+AS_IF([test "x$enable_kernel_cmdline" = "xyes"], [
+        AC_DEFINE(KERNEL_CMDLINE, 1, [Dumpster diving after init args from /proc/cmdline])])
+
 AS_IF([test "x$enable_redirect" = "xyes"], [
 	AC_DEFINE(REDIRECT_OUTPUT, 1, [Enable redirection of service output to /dev/null])])
 

doc/build.md

@@ -53,6 +53,10 @@ Below are a few of the main switches to configure:
   built-ins (.o files) and all external libraries, except the C library
   will be linked statically.
 
+* `--enable-kernel-cmdline`: Enable Finit pre-4.1 parsing of init args from
+  `/proc/cmdline`, this is not recommended since Finit may be running as the
+  init for container apps that can see the host's `/proc` filesystem
+
 * `--enable-alsa-utils-plugin`: Enable the optional `alsa-utils.so` sound plugin.
 
 * `--enable-dbus-plugin`: Enable the optional D-Bus `dbus.so` plugin.

doc/cmdline.md

@@ -6,6 +6,8 @@ passed on the Linux kernel command line.  Not limited to Finit.
 
 The `bool` setting is one of `on, off, true false, 1, 0`.
 
+> **NOTE:** remember to use `--` to separate kernel parameters from
+> parameters to init.  E.g., `init=/sbin/finit -- finit.debug rescue`
 
 * `debug`: Enable kernel debug.  Debug messages are printed to the
     console until Finit starts up.

src/conf.c

@@ -145,24 +145,27 @@ static void parse_arg(char *arg)
 		single = 1;
 }
 
-void conf_parse_cmdline(int argc, char *argv[])
+#ifdef KERNEL_CMDLINE
+/*
+ * Parse /proc/cmdline to find args for init.  Don't use this!
+ *
+ * Instead, rely on the kernel to give Finit its arguments as
+ * regular argc + argv[].  Only use this if the system you run
+ * on has a broken initramfs system that cannot forward args
+ * to Finit properly.
+ */
+static void parse_kernel_cmdline(void)
 {
-	int dbg = 0;
-	FILE *fp;
 	char line[LINE_SIZE], *cmdline, *tok;
-
-	for (int i = 1; i < argc; i++)
-		parse_arg(argv[i]);
-
-	hide_args(argc, argv);
+	FILE *fp;
 
 	fp = fopen("/proc/cmdline", "r");
 	if (!fp)
-		goto done;
+		return;
 
 	if (!fgets(line, sizeof(line), fp)) {
 		fclose(fp);
-		goto done;
+		return;
 	}
 
 	cmdline = chomp(line);
@@ -173,8 +176,24 @@ void conf_parse_cmdline(int argc, char *argv[])
 		parse_arg(tok);
 	}
 	fclose(fp);
+}
+#else
+#define parse_kernel_cmdline()
+#endif
+
+/*
+ * Kernel gives us all non-kernel options on our cmdline
+ */
+void conf_parse_cmdline(int argc, char *argv[])
+{
+	int dbg = 0;
+
+	for (int i = 1; i < argc; i++)
+		parse_arg(argv[i]);
+
+	hide_args(argc, argv);
+	parse_kernel_cmdline();
 
-done:
 	log_init(dbg);
 }