On Thu, Jun 17, 2021 at 11:14:00AM +0100, Christoph Hellwig wrote:
> Why not something like the version below that should work for all nodev
> file systems?
Hi Christoph,
Thanks for this patch. It definitely looks much better. I had a fear
of breaking something if I were to go through this path of using
FS_REQUIRES_DEV.
This patch works for me with "root=myfs rootfstype=virtiofs rw". Have
few thoughts inline.
>
> diff --git a/init/do_mounts.c b/init/do_mounts.c
> index 74aede860de7..3c5676603fef 100644
> --- a/init/do_mounts.c
> +++ b/init/do_mounts.c
> @@ -530,6 +530,39 @@ static int __init mount_cifs_root(void)
> }
> #endif
>
> +static int __init mount_nodev_root(void)
> +{
> + struct file_system_type *fs = get_fs_type(root_fs_names);
get_fs_type() assumes root_fs_names is not null. So if I pass
"root=myfs rw", it crashes with null pointer dereference.
> + char *fs_names, *p;
> + int err = -ENODEV;
> +
> + if (!fs)
> + goto out;
> + if (fs->fs_flags & FS_REQUIRES_DEV)
> + goto out_put_filesystem;
> +
> + fs_names = (void *)__get_free_page(GFP_KERNEL);
> + if (!fs_names)
> + goto out_put_filesystem;
> + get_fs_names(fs_names);
I am wondering what use case we are trying to address by calling
get_fs_names() and trying do_mount_root() on all filesystems
returned by get_fs_names(). I am assuming following use cases
you have in mind.
A. User passes a single filesystem in rootfstype.
root=myfs rootfstype=virtiofs rw
B. User passes multiple filesystems in rootfstype and kernel tries all
of them one after the other
root=myfs, rootfstype=9p,virtiofs rw
C. User does not pass a filesystem type at all. And kernel will get a
list of in-built filesystems and will try these one after the other.
root=myfs rw
If that's the thought, will it make sense to call get_fs_names() first
and then inside the for loop call get_fs_type() and try mounting
only if FS_REQUIRES_DEV is not set, otherwise skip and move onto th
next filesystem in the list (fs_names).
Thanks
Vivek
> +
> + for (p = fs_names; *p; p += strlen(p) + 1) {
> + err = do_mount_root(root_device_name, p, 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, p, err);
> + }
> +
> + free_page((unsigned long)fs_names);
> +out_put_filesystem:
> + put_filesystem(fs);
> +out:
> + return err;
> +}
> +
> void __init mount_root(void)
> {
> #ifdef CONFIG_ROOT_NFS
> @@ -546,6 +579,8 @@ void __init mount_root(void)
> return;
> }
> #endif
> + if (ROOT_DEV == 0 && mount_nodev_root() == 0)
> + return;
> #ifdef CONFIG_BLOCK
> {
> int err = create_dev("/dev/root", ROOT_DEV);
>
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