mtab: make /etc/mtab as a file configurable

NEWS.md

@@ -3,6 +3,29 @@
 This file will contain a list of notable changes for each release. Note
 the information in this file is in reverse order.
 
+## OpenRC-0.18.3
+
+Modern Linux systems expect /etc/mtab to be a symbolic link to
+/proc/self/mounts. Reasons for this change include support for mount
+namespaces, which will not work if /etc/mtab is a file.
+By default, the mtab service enforces this on each reboot.
+
+If you find that this breaks your system in some way, please do the
+following:
+
+- Set mtab_is_file=yes in /etc/conf.d/mtab.
+
+- Restart mtab. This will recreate the /etc/mtab file.
+
+- Check for an issue on https://github.com/openrc/openrc/issues
+  explaining why you need /etc/mtab to be a file. If there isn't one,
+  please open one and explain in detail why you need this to be a file.
+  If there is one, please add your comments to it. Please give concrete
+  examples of why  it is important that /etc/mtab be a file instead of a
+  symbolic link. Those comments will be taken into consideration for how
+  long to keep supporting mtab as a file or when the support can be
+  removed.
+
 ## OpenRC-0.18
 
 The behaviour of localmount and netmount in this version is changing. In

conf.d/Makefile

@@ -15,7 +15,7 @@ include ${MK}/os.mk
 
 CONF-FreeBSD=	ipfw moused powerd rarpd savecore syscons
 
-CONF-Linux=	consolefont devfs dmesg hwclock keymaps killprocs modules
+CONF-Linux=	consolefont devfs dmesg hwclock keymaps killprocs modules mtab
 
 CONF-NetBSD=	moused rarpd savecore
 

conf.d/mtab

@@ -0,0 +1,5 @@
+# This setting controls whether /etc/mtab is a file or symbolic link.
+# Most of the time, you shouldn't touch this. However, if the default
+# breaks your system in some way, please see the NEWS.md file that comes
+# with OpenRC for the actions to take.
+# mtab_is_file=no

init.d/mtab.in

@@ -14,16 +14,17 @@ start()
 {
 	local rc=0
 	ebegin "Updating /etc/mtab"
-	if [ -L /etc/mtab ] ; then
+	if ! checkpath -W /etc; then
-		ewarn "/etc/mtab is a symlink, not updating."
-		ewarn "FYI - You can safely remove mtab from your runlevels."
-	elif ! checkpath -W /etc; then
 		rc=1
-	elif [ ! -e /etc/mtab ]; then
+	elif ! yesno ${mtab_is_file:-no}; then
-		einfo "Creating mtab symlink"
+		[ ! -L /etc/mtab ] && [ -f /etc/mtab] &&
+			ewarn "Removing /etc/mtab file"
+		einfo "Creating mtab symbolic link"
 		ln -snf /proc/self/mounts /etc/mtab
 	else
-		einfo "Updating mtab file"
+		[ -L /etc/mtab ] && ewarn "Removing /etc/mtab symbolic link"
+		rm -f /etc/mtab
+		einfo "Creating mtab file"
 		# With / as tmpfs we cannot umount -at tmpfs in localmount as that
 		# makes / readonly and dismounts all tmpfs even if in use which is
 		# not good. Luckily, umount uses /etc/mtab instead of /proc/mounts