linux-kernel - Re: [RFC PATCH] rootfs: force mounting rootfs as tmpfs

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Date:   Wed, 31 Jan 2018 17:07:03 -0500
From:   Mimi Zohar <zohar@...ux.vnet.ibm.com>
To:     Rob Landley <rob@...dley.net>,
        initramfs <initramfs@...r.kernel.org>
Cc:     Taras Kondratiuk <takondra@...co.com>,
        Victor Kamensky <kamensky@...co.com>,
        linux-security-module <linux-security-module@...r.kernel.org>,
        Al Viro <viro@...IV.linux.org.uk>,
        linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [RFC PATCH] rootfs: force mounting rootfs as tmpfs

On Wed, 2018-01-31 at 13:32 -0600, Rob Landley wrote:
> On 01/30/2018 03:46 PM, Mimi Zohar wrote:
> > Commit 16203a7a9422 ("initmpfs: make rootfs use tmpfs when CONFIG_TMPFS
> > enabled") introduced using tmpfs as the rootfs filesystem.  The use of
> > tmpfs is limited to systems that do not specify "root=" on the boot
> > command line.
> > 
> > Without the check "!saved_root_name[0]", rootfs uses tmpfs.  As there
> > must be a valid reason for this check, this patch introduces a new boot
> > command line option named "noramfs" to force rootfs to use tmpfs.
> > 
> > Signed-off-by: Mimi Zohar <zohar@...ux.vnet.ibm.com>
> 
> How about just:
> 
> diff --git a/init/do_mounts.c b/init/do_mounts.c
> index 7cf4f6d..af66ede 100644
> --- a/init/do_mounts.c
> +++ b/init/do_mounts.c
> @@ -632,8 +632,8 @@ int __init init_rootfs(void)
>  	if (err)
>  		return err;
> 
> -	if (IS_ENABLED(CONFIG_TMPFS) && !saved_root_name[0] &&
> -		(!root_fs_names || strstr(root_fs_names, "tmpfs"))) {
> +	if (IS_ENABLED(CONFIG_TMPFS) && (!saved_root_name[0] ||
> +		!strcmp(saved_root_name, "tmpfs"))) {
>  		err = shmem_init();
>  		is_tmpfs = true;
>  	} else {
> 
> (Obviously-signed-off-by: Rob Landley <rob@...dley.net>)
> 
> I.E. if you somehow just can't stop yourself from specifying root= when
> using rootfs, have "root=tmpfs" do what you want.

I tried overloading "rootfstype=tmpfs", before posting this work
around, but for some reason that just doesn't work.

> 
> (The old "I configured in tmpfs and am using rootfs but I want that rootfs
> to be ramfs, not tmpfs" code doesn't seem to be a real-world concern, does
> it?)

I must be missing something.  Which systems don't specify "root=" on
the boot command line.  If we want to include and restore xattrs,
there needs to be a way of using tmpfs.

Mimi

> 
> > ---
> >  Documentation/admin-guide/kernel-parameters.txt |  2 ++
> >  init/do_mounts.c                                | 15 +++++++++++++--
> >  2 files changed, 15 insertions(+), 2 deletions(-)
> 
> I suppose I should do a documentation update too. Lemme send a proper one
> after work...
> 
> Rob
> 
> P.S. While I'm at it, I've meant to wire up rootflags= so you can specify
> a memory limit other than 50% forever, I should do that too. And resend
> my "make DEVTMPFS_MOUNT apply to initramfs" patch (with the debian bug
> workaround)...
>