Fix #224: support for cmdline -- finit.fstab=/etc/fstab.secondary

This adds support for bringing up the system with an alternate fstab at
boot.  E.g., when using a primary/secondary setup for boot partitions.

By default /etc/fstab is read, like before, this can now be changed
using configure --with-fstab=/path/to/fstab.primary, which sets the
default that can be overridden using finit.fstab=/etc/fstab.secondary

If mounting, or fsck, fails in any way, Finit calls its own bundled
sulogin, or the system sulogin(8), to let the user handle the issue.
If there is no sulogin available, Finit will try to start up in its
rescue.conf boot mode.

Please note, in either of these rescue modes, use `reboot -f` to get the
system to reboot.  Finit is on pause in the background in rescue mode
and cannot be relied on (since there may not be any writable filesystems
available.).

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

src/finit.c

@@ -65,6 +65,7 @@ int   rescue    = 0;		/* rescue mode from kernel cmdline */
 int   single    = 0;		/* single user mode from kernel cmdline */
 int   bootstrap = 1;		/* set while bootrapping (for TTYs) */
 int   kerndebug = 0;		/* set if /proc/sys/kernel/printk > 7 */
+char *fstab     = FINIT_FSTAB;
 char *sdown     = NULL;
 char *network   = NULL;
 char *hostname  = NULL;
@@ -105,6 +106,35 @@ static void banner(void)
 #endif
 }
 
+static int sulogin(int do_reboot)
+{
+	int rc = EX_OSFILE;
+	char *cmd[] = {
+		_PATH_SULOGIN,
+		"sulogin",
+	};
+	size_t i;
+
+	for (i = 0; i < NELEMS(cmd); i++) {
+		char *path = which(cmd[i]);
+
+		if (access(path, X_OK)) {
+			free(path);
+			continue;
+		}
+
+		rc = systemf("%s", path);
+		break;
+	}
+
+	if (do_reboot) {
+		do_shutdown(SHUT_REBOOT);
+		exit(rc);
+	}
+
+	return rc;
+}
+
 /*
  * Check all filesystems in /etc/fstab with a fs_passno > 0
  */
@@ -114,10 +144,10 @@ static int fsck(int pass)
 	int rc = 0;
 	FILE *fp;
 
-	fp = setmntent("/etc/fstab", "r");
+	fp = setmntent(fstab, "r");
 	if (!fp) {
-		_pe("Failed opening fstab");
-		return 1;
+		_pe("Failed opening fstab: %s", fstab);
+		sulogin(1);
 	}
 
 	while ((mnt = getmntent(fp))) {
@@ -153,19 +183,8 @@ static int fsck(int pass)
 		 * errors were corrected but that the boot may proceed.
 		 */
 		if (fsck_rc > 1) {
-			char *sulogin[] = {
-				_PATH_SULOGIN,
-				"sulogin",
-			};
-			size_t i;
-
-			for (i = 0; i < NELEMS(sulogin); i++) {
-				if (systemf("%s", sulogin[i]))
-					continue;
-				break;
-			}
-
-			do_shutdown(SHUT_REBOOT);
+			logit(LOG_CONSOLE | LOG_ALERT, "Failed fsck of %s, attempting sulogin ...", mnt->mnt_fsname);
+			sulogin(1);
 		}
 		rc += fsck_rc;
 	}
@@ -208,7 +227,7 @@ static void fs_remount_root(int fsckerr)
 	struct mntent *mnt;
 	FILE *fp;
 
-	fp = setmntent("/etc/fstab", "r");
+	fp = setmntent(fstab, "r");
 	if (!fp)
 		return;
 
@@ -306,13 +325,29 @@ static void fs_finalize(void)
 
 static void fs_mount_all(void)
 {
+	char cmd[256] = "mount -na";
+
+	if (!fstab || !fexist(fstab)) {
+		logit(LOG_CONSOLE | LOG_NOTICE, "%s system fstab %s, trying fallback ...",
+		      !fstab ? "Missing" : "Cannot find", fstab ?: "\b");
+		fstab = FINIT_FSTAB;
+	}
+	if (!fstab || !fexist(fstab)) {
+		logit(LOG_CONSOLE | LOG_EMERG, "%s system fstab %s, attempting sulogin ...",
+		      !fstab ? "Missing" : "Cannot find", fstab ?: "\b");
+		sulogin(1);
+	}
+
 	if (!rescue)
 		fs_remount_root(fsck_all());
 
 	_d("Root FS up, calling hooks ...");
 	plugin_run_hooks(HOOK_ROOTFS_UP);
 
-	if (run_interactive("mount -na", "Mounting filesystems"))
+	if (fstab && strcmp(fstab, "/etc/fstab"))
+		snprintf(cmd, sizeof(cmd), "mount -na -T %s", fstab);
+
+	if (run_interactive(cmd, "Mounting filesystems from %s", fstab))
 		plugin_run_hooks(HOOK_MOUNT_ERROR);
 
 	_d("Calling extra mount hook, after mount -a ...");
@@ -595,21 +630,8 @@ int main(int argc, char *argv[])
 	/*
 	 * In case of emergency.
 	 */
-	if (rescue) {
-		char *sulogin[] = {
-			_PATH_SULOGIN,
-			"sulogin",
-		};
-		size_t i;
-
-		for (i = 0; i < NELEMS(sulogin); i++) {
-			if (systemf("%s", sulogin[i]))
-				continue;
-
-			rescue = 0;
-			break;
-		}
-	}
+	if (rescue)
+		rescue = sulogin(0);
 
 	/*
 	 * Load plugins early, the first hook is in banner(), so we
@@ -638,7 +660,10 @@ int main(int argc, char *argv[])
 	 */
 	cgroup_init(&loop);
 
-	/* Check and mount filesystems. */
+	/*
+	 * Check custom fstab from cmdline, including fallback, then run
+	 * fsck before mounting all filesystems, on error call sulogin.
+	 */
 	fs_mount_all();
 
 	/* Bootstrap conditions, needed for hooks */