Re: [PATCH 3/20] Introduce MS_KERNMOUNT flag

[Christoph Hellwig <hch@infradead.org>]

On Fri, Aug 10, 2007 at 03:47:55PM +0400, xemul@openvz.org wrote:
> This flag tells the .get_sb callback that this is a kern_mount() call
> so that it can trust *data pointer to be valid in-kernel one. If this
> flag is passed from the user process, it is cleared since the *data
> pointer is not a valid kernel object.
> 
> Running a few steps forward - this will be needed for proc to create the
> superblock and store a valid pid namespace on it during the namespace
> creation. The reason, why the namespace cannot live without proc mount
> is described in the appropriate patch.

I don't like this at all.  We should never pass kernel and userspace
addresses through the same pointer.  Maybe add an additional argument
to the get_sb prototype instead.  But this whole idea of mounting /proc
from kernelspace sounds like a really bad idea to me.  /proc should
never be mounted from the kernel but always normally from userspace.

> 
> Signed-off-by: Pavel Emelyanov <xemul@openvz.org>
> Cc: Oleg Nesterov <oleg@tv-sign.ru>
> 
> ---
> 
>  fs/namespace.c     |    3 ++-
>  fs/super.c         |    6 +++---
>  include/linux/fs.h |    4 +++-
>  3 files changed, 8 insertions(+), 5 deletions(-)
> 
> diff -upr linux-2.6.23-rc1-mm1.orig/fs/namespace.c linux-2.6.23-rc1-mm1-7/fs/namespace.c
> --- linux-2.6.23-rc1-mm1.orig/fs/namespace.c	2007-07-26 16:34:45.000000000 +0400
> +++ linux-2.6.23-rc1-mm1-7/fs/namespace.c	2007-07-26 16:36:36.000000000 +0400
> @@ -1579,7 +1579,8 @@ long do_mount(char *dev_name, char *dir_
>  		mnt_flags |= MNT_NOMNT;
>  
>  	flags &= ~(MS_NOSUID | MS_NOEXEC | MS_NODEV | MS_ACTIVE |
> -		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME | MS_NOMNT);
> +		   MS_NOATIME | MS_NODIRATIME | MS_RELATIME |
> +		   MS_NOMNT | MS_KERNMOUNT);
>  
>  	/* ... and get the mountpoint */
>  	retval = path_lookup(dir_name, LOOKUP_FOLLOW, &nd);
> diff -upr linux-2.6.23-rc1-mm1.orig/fs/super.c linux-2.6.23-rc1-mm1-7/fs/super.c
> --- linux-2.6.23-rc1-mm1.orig/fs/super.c	2007-07-26 16:34:45.000000000 +0400
> +++ linux-2.6.23-rc1-mm1-7/fs/super.c	2007-07-26 16:36:36.000000000 +0400
> @@ -944,9 +944,9 @@ do_kern_mount(const char *fstype, int fl
>  	return mnt;
>  }
>  
> -struct vfsmount *kern_mount(struct file_system_type *type)
> +struct vfsmount *kern_mount_data(struct file_system_type *type, void *data)
>  {
> -	return vfs_kern_mount(type, 0, type->name, NULL);
> +	return vfs_kern_mount(type, MS_KERNMOUNT, type->name, data);
>  }
>  
> -EXPORT_SYMBOL(kern_mount);
> +EXPORT_SYMBOL_GPL(kern_mount_data);
> diff -upr linux-2.6.23-rc1-mm1.orig/include/linux/fs.h linux-2.6.23-rc1-mm1-7/include/linux/fs.h
> --- linux-2.6.23-rc1-mm1.orig/include/linux/fs.h	2007-07-26 16:34:45.000000000 +0400
> +++ linux-2.6.23-rc1-mm1-7/include/linux/fs.h	2007-07-26 16:36:36.000000000 +0400
> @@ -129,6 +129,7 @@ extern int dir_notify_enable;
>  #define MS_RELATIME	(1<<21)	/* Update atime relative to mtime/ctime. */
>  #define MS_SETUSER	(1<<23) /* set mnt_uid to current user */
>  #define MS_NOMNT	(1<<24) /* don't allow unprivileged submounts */
> +#define MS_KERNMOUNT	(1<<25) /* this is a kern_mount call */
>  #define MS_ACTIVE	(1<<30)
>  #define MS_NOUSER	(1<<31)
>  
> @@ -1459,7 +1460,8 @@ void unnamed_dev_init(void);
>  
>  extern int register_filesystem(struct file_system_type *);
>  extern int unregister_filesystem(struct file_system_type *);
> -extern struct vfsmount *kern_mount(struct file_system_type *);
> +extern struct vfsmount *kern_mount_data(struct file_system_type *, void *data);
> +#define kern_mount(type) kern_mount_data(type, NULL)
>  extern int may_umount_tree(struct vfsmount *);
>  extern int may_umount(struct vfsmount *);
>  extern void umount_tree(struct vfsmount *, int, struct list_head *);
> 
> -
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