package/tinyinit: new package

This package contains a Linux init script suitable for resource-limited
systems which can be used as an alternative to the one provided by
Busybox.

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: Damien Le Moal <dlemoal@kernel.org>
[yann.morin.1998@free.fr: drop 'imply busybox']
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>

DEVELOPERS

@@ -793,6 +793,7 @@ F:	configs/stm32f769_disco_sd_defconfig
 F:	package/armadillo/
 F:	package/babeld/
 F:	package/sscep/
+F:	package/tinyinit/
 F:	package/uuu/
 
 N:	Dario Binacchi <dariobin@libero.it>

package/Config.in

@@ -2870,6 +2870,7 @@ menu "System tools"
 	source "package/tar/Config.in"
 	source "package/tealdeer/Config.in"
 	source "package/thermald/Config.in"
+	source "package/tinyinit/Config.in"
 	source "package/tpm-tools/Config.in"
 	source "package/tpm2-abrmd/Config.in"
 	source "package/tpm2-tools/Config.in"

package/busybox/busybox.mk

@@ -68,6 +68,7 @@ BUSYBOX_DEPENDENCIES = \
 	$(if $(BR2_PACKAGE_UNZIP),unzip) \
 	$(if $(BR2_PACKAGE_USBUTILS),usbutils) \
 	$(if $(BR2_PACKAGE_UTIL_LINUX),util-linux) \
+	$(if $(BR2_PACKAGE_TINYINIT),tinyinit) \
 	$(if $(BR2_PACKAGE_VIM),vim) \
 	$(if $(BR2_PACKAGE_WATCHDOG),watchdog) \
 	$(if $(BR2_PACKAGE_WGET),wget) \

package/tinyinit/Config.in

@@ -0,0 +1,7 @@
+config BR2_PACKAGE_TINYINIT
+	bool "tinyinit"
+	depends on BR2_INIT_TINYINIT
+	help
+	  A Linux tiny initialization script suitable for resource
+	  limited systems, which can be used as an alternative to the
+	  one provided by Busybox.

package/tinyinit/init

@@ -0,0 +1,32 @@
+#!/bin/sh
+
+# This script replaces the default busybox init process to avoid having that
+# process staying alive and sleeping in the background, (uselessly) consuming
+# precious memory.
+
+# Mount procfs and sysfs
+/bin/mount -t proc proc /proc
+/bin/mount -t sysfs sysfs /sys
+
+# When the kernel is directly booted, devtmpfs is not automatically mounted.
+# Manually mount it if needed.
+devmnt=$(mount | grep -c devtmpfs)
+if [ "${devmnt}" -eq 0 ]; then
+    /bin/mount -t devtmpfs devtmpfs /dev
+fi
+
+# Use the /dev/console device node from devtmpfs if possible to not
+# confuse glibc's ttyname_r().
+# This may fail (E.G. booted with console=), and errors from exec will
+# terminate the shell, so use a subshell for the test
+if (exec 0</dev/console) 2>/dev/null; then
+    exec 0</dev/console
+    exec 1>/dev/console
+    exec 2>/dev/console
+fi
+
+# Clear memory to reduce page fragmentation
+echo 3 > /proc/sys/vm/drop_caches
+
+# Finally, let's start an interactive shell
+exec /bin/sh

package/tinyinit/tinyinit.mk

@@ -0,0 +1,13 @@
+################################################################################
+#
+# tinyinit
+#
+################################################################################
+
+define TINYINIT_INSTALL_TARGET_CMDS
+	$(INSTALL) -m 0755 -D $(TINYINIT_PKGDIR)/init $(TARGET_DIR)/sbin/init
+	# Downside: In non-initramfs systems the symlink isn't used/needed
+	(cd $(TARGET_DIR); ln -sf /sbin/init init)
+endef
+
+$(eval $(generic-package))

system/Config.in

@@ -165,6 +165,15 @@ config BR2_INIT_TINI
 
 	  https://github.com/krallin/tini
 
+config BR2_INIT_TINYINIT
+	bool "tiny init"
+	select BR2_PACKAGE_TINYINIT
+	select BR2_PACKAGE_SKELETON_INIT_NONE if BR2_ROOTFS_SKELETON_DEFAULT
+	help
+	  A Linux tiny initialization script suitable for resource
+	  limited systems, which can be used as an alternative to the
+	  one provided by Busybox.
+
 config BR2_INIT_NONE
 	bool "Custom (none)"
 	select BR2_PACKAGE_SKELETON_INIT_NONE if BR2_ROOTFS_SKELETON_DEFAULT