On Fri, Jun 18, 2021 at 03:20:38PM +0200, Christoph Hellwig wrote:
> 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?
[ Cc Dominique Martinet ]
Hi Christoph,
This one works well for me. Just one minor nit. root_device_name, can
be NULL as well if user passes following.
"rootfstype=virtiofs rw".
And currently mount_nodev_root() is assuming root_device_name is not NULL
and we crash with null pointer dereference.
May be something like this.
if (ROOT_DEV == 0 && root_device_name && root_fs_names) {
if (mount_nodev_root() == 0)
return;
}
Strangely people are using "rootfstype=virtiofs rw" to mount virtiofs
as rootfs. They give their filesystem a tag named "/dev/root". And
currently it works and they can mount virtiofs as rootfs.
With above change, current hackish way will also continue to work and
not break existing setups.
Thanks
Vivek
>
> ---
> From 141caa79a619b5f5d100eeb8e94ecf8b3b1c9af7 Mon Sep 17 00:00:00 2001
> From: Christoph Hellwig <hch@xxxxxx>
> 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@xxxxxxxxxx>.
>
> Signed-off-by: Christoph Hellwig <hch@xxxxxx>
> ---
> 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
>
