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To:     "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
From:   Rob Landley <rob@landley.net>
Subject: [PATCH v4] Make initramfs honor CONFIG_DEVTMPFS_MOUNT
Message-ID: <25e7e777-19f9-6280-b456-6c9c782c7901@landley.net>
Date:   Thu, 14 May 2020 23:04:54 -0500
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FYI I dug up my old https://lkml.org/lkml/2017/9/13/651 and ported it to
current, because I needed it for a thing.

From: Rob Landley <rob@landley.net>

Make initramfs honor CONFIG_DEVTMPFS_MOUNT, and move
/dev/console open after devtmpfs mount.

Add workaround for Debian bug that was copied by Ubuntu.

Signed-off-by: Rob Landley <rob@landley.net>

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diff --git a/drivers/base/Kconfig b/drivers/base/Kconfig
index f046d21..97352d4 100644
--- a/drivers/base/Kconfig
+++ b/drivers/base/Kconfig
@@ -48,16 +48,10 @@  config DEVTMPFS_MOUNT
 	bool "Automount devtmpfs at /dev, after the kernel mounted the rootfs"
 	depends on DEVTMPFS
 	help
-	  This will instruct the kernel to automatically mount the
-	  devtmpfs filesystem at /dev, directly after the kernel has
-	  mounted the root filesystem. The behavior can be overridden
-	  with the commandline parameter: devtmpfs.mount=0|1.
-	  This option does not affect initramfs based booting, here
-	  the devtmpfs filesystem always needs to be mounted manually
-	  after the rootfs is mounted.
-	  With this option enabled, it allows to bring up a system in
-	  rescue mode with init=/bin/sh, even when the /dev directory
-	  on the rootfs is completely empty.
+	  Automatically mount devtmpfs at /dev on the root filesystem, which
+	  lets the system come up in rescue mode with [rd]init=/bin/sh.
+	  Override with devtmpfs.mount=0 on the commandline. Initramfs can
+	  create a /dev dir as needed, other rootfs needs the mount point.
 
 config STANDALONE
 	bool "Select only drivers that don't need compile-time external firmware"
diff --git a/fs/namespace.c b/fs/namespace.c
index f8893dc..06057d7 100644
--- a/fs/namespace.c
+++ b/fs/namespace.c
@@ -2417,7 +2417,21 @@  static int do_add_mount(struct mount *newmnt, struct path *path, int mnt_flags)
 	err = -EBUSY;
 	if (path->mnt->mnt_sb == newmnt->mnt.mnt_sb &&
 	    path->mnt->mnt_root == path->dentry)
+	{
+		if (IS_ENABLED(CONFIG_DEVTMPFS_MOUNT) &&
+		    !strcmp(path->mnt->mnt_sb->s_type->name, "devtmpfs"))
+		{
+			/* Debian's kernel config enables DEVTMPFS_MOUNT, then
+			   its initramfs setup script tries to mount devtmpfs
+			   again, and if the second mount-over-itself fails
+			   the script overmounts a tmpfs on /dev to hide the
+			   existing contents, then boot fails with empty /dev. */
+			printk(KERN_WARNING "Debian bug workaround for devtmpfs overmount.");
+
+			err = 0;
+		}
 		goto unlock;
+	}
 
 	err = -EINVAL;
 	if (d_is_symlink(newmnt->mnt.mnt_root))
diff --git a/init/main.c b/init/main.c
index 0ee9c686..0d8e5ec 100644
--- a/init/main.c
+++ b/init/main.c
@@ -1065,12 +1065,6 @@  static noinline void __init kernel_init_freeable(void)
 
 	do_basic_setup();
 
-	/* Open the /dev/console on the rootfs, this should never fail */
-	if (ksys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
-		pr_err("Warning: unable to open an initial console.\n");
-
-	(void) ksys_dup(0);
-	(void) ksys_dup(0);
 	/*
 	 * check if there is an early userspace init.  If yes, let it do all
 	 * the work
@@ -1082,8 +1076,17 @@  static noinline void __init kernel_init_freeable(void)
 			ramdisk_execute_command, 0) != 0) {
 		ramdisk_execute_command = NULL;
 		prepare_namespace();
+	} else if (IS_ENABLED(CONFIG_DEVTMPFS_MOUNT)) {
+		sys_mkdir("/dev", 0755);
+		devtmpfs_mount("/dev");
 	}
 
+	/* Open the /dev/console on the rootfs, this should never fail */
+	if (ksys_open((const char __user *) "/dev/console", O_RDWR, 0) < 0)
+		pr_err("Warning: unable to open an initial console.\n");
+	(void) ksys_dup(0);
+	(void) ksys_dup(0);
+
 	/*
 	 * Ok, we have completed the initial bootup, and
 	 * we're essentially up and running. Get rid of the

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