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Subject: Re: [PATCH v4] Make initramfs honor CONFIG_DEVTMPFS_MOUNT
To:     Randy Dunlap <rdunlap@infradead.org>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
References: <25e7e777-19f9-6280-b456-6c9c782c7901@landley.net>
 <e522825e-b2da-3cfe-7655-20c1c1975be4@infradead.org>
From:   Rob Landley <rob@landley.net>
Message-ID: <5c597b68-4946-56ae-f5f8-a5f87bb22b44@landley.net>
Date:   Fri, 15 May 2020 20:14:42 -0500
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On 5/14/20 11:50 PM, Randy Dunlap wrote:
> Hi Rob,

Um, hi.

> You need to send this patch to some maintainer who could merge it.

The implicit "if" is "you expect the kernel bureaucracy to merge anything Not
Invented Here", and the 3 year gap since the last version is because I stopped:

  https://landley.net/notes-2017.html#14-09-2017

To be honest I didn't think anyone would notice this. They don't usually:

  http://lkml.iu.edu/hypermail/linux/kernel/2002.2/00083.html
  https://www.spinics.net/lists/linux-sh/msg56844.html

It just seems polite to post things that got shipped to customers.

> And it uses the wrong multi-line comment format.

Offending comment removed.

Rob

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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 +2417,16 @@  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"))
+		{
+			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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