linux-kernel - Re: [PATCH 2/2] init: allow mounting arbitrary non-blockdevice filesystems as root

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Message-ID: <20210618132038.GA13406@lst.de>
Date:   Fri, 18 Jun 2021 15:20:38 +0200
From:   Christoph Hellwig <hch@....de>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     Christoph Hellwig <hch@....de>, viro@...iv.linux.org.uk,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        virtio-fs@...hat.com
Subject: Re: [PATCH 2/2] init: allow mounting arbitrary non-blockdevice
 filesystems as root

On Thu, Jun 17, 2021 at 12:26:10PM -0400, Vivek Goyal wrote:
> Not sure what FS_BINARY_MOUNTDATA is why fs should not have that set. nfs
> seems to set it too. So that means they can't use try_mount_nodev().

We can't really pass actual binary mountdata using the string separation
scheme used by the rootfstype= option.  But given that NFS only uses
binary mountdata for legacy reasons and people get what they ask for
using the option I think we can drop the check.

> In case of success err == 0, but we still panic(). We will need to
> check for success as well.

Indeed.

> root_fs_names can be NULL and it crashes with NULL pointer dereference.

True.

What do you think of this version?

---
>From 141caa79a619b5f5d100eeb8e94ecf8b3b1c9af7 Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@....de>
Date: Fri, 18 Jun 2021 15:10:39 +0200
Subject: init: allow mounting arbitrary non-blockdevice filesystems as root

Currently the only non-blockdevice filesystems that can be used as the
initial root filesystem are NFS and CIFS, which use the magic
"root=/dev/nfs" and "root=/dev/cifs" syntax that requires the root
device file system details to come from filesystem specific kernel
command line options.

Add a little bit of new code that allows to just pass arbitrary
string mount options to any non-blockdevice filesystems so that it can
be mounted as the root file system.

For example a virtiofs root file system can be mounted using the
following syntax:

"root=myfs rootfstype=virtiofs rw"

Based on an earlier patch from Vivek Goyal <vgoyal@...hat.com>.

Signed-off-by: Christoph Hellwig <hch@....de>
---
 init/do_mounts.c | 43 +++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 43 insertions(+)

diff --git a/init/do_mounts.c b/init/do_mounts.c
index ec32de3ad52b..66c47193e9ee 100644
--- a/init/do_mounts.c
+++ b/init/do_mounts.c
@@ -534,6 +534,45 @@ static int __init mount_cifs_root(void)
 }
 #endif
 
+static bool __init fs_is_nodev(char *fstype)
+{
+	struct file_system_type *fs = get_fs_type(fstype);
+	bool ret = false;
+
+	if (fs) {
+		ret = !(fs->fs_flags & FS_REQUIRES_DEV);
+		put_filesystem(fs);
+	}
+
+	return ret;
+}
+
+static int __init mount_nodev_root(void)
+{
+	char *fs_names, *fstype;
+	int err = -EINVAL;
+
+	fs_names = (void *)__get_free_page(GFP_KERNEL);
+	if (!fs_names)
+		return -EINVAL;
+	split_fs_names(fs_names, root_fs_names);
+
+	for (fstype = fs_names; *fstype; fstype += strlen(fstype) + 1) {
+		if (!fs_is_nodev(fstype))
+			continue;
+		err = do_mount_root(root_device_name, fstype, root_mountflags,
+				    root_mount_data);
+		if (!err)
+			break;
+		if (err != -EACCES && err != -EINVAL)
+			panic("VFS: Unable to mount root \"%s\" (%s), err=%d\n",
+			      root_device_name, fstype, err);
+	}
+
+	free_page((unsigned long)fs_names);
+	return err;
+}
+
 void __init mount_root(void)
 {
 #ifdef CONFIG_ROOT_NFS
@@ -550,6 +589,10 @@ void __init mount_root(void)
 		return;
 	}
 #endif
+	if (ROOT_DEV == 0 && root_fs_names) {
+		if (mount_nodev_root() == 0)
+			return;
+	}
 #ifdef CONFIG_BLOCK
 	{
 		int err = create_dev("/dev/root", ROOT_DEV);
-- 
2.30.2