linux-kernel - Re: [RFC] Add option to mount only a pids subset

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Message-ID: <20170312015430.GO29622@ZenIV.linux.org.uk>
Date:   Sun, 12 Mar 2017 01:54:38 +0000
From:   Al Viro <viro@...IV.linux.org.uk>
To:     Alexey Gladkov <gladkov.alexey@...il.com>
Cc:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        "Kirill A. Shutemov" <kirill@...temov.name>,
        Vasiliy Kulikov <segoon@...nwall.com>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Oleg Nesterov <oleg@...hat.com>,
        Pavel Emelyanov <xemul@...allels.com>,
        James Bottomley <James.Bottomley@...senPartnership.com>
Subject: Re: [RFC] Add option to mount only a pids subset

On Tue, Mar 07, 2017 at 12:05:16AM +0100, Alexey Gladkov wrote:

> diff --git a/include/linux/fs.h b/include/linux/fs.h
> index 83de8b6..5bd1b84 100644
> --- a/include/linux/fs.h
> +++ b/include/linux/fs.h
> @@ -1759,6 +1759,8 @@ struct super_operations {
>  	int (*thaw_super) (struct super_block *);
>  	int (*unfreeze_fs) (struct super_block *);
>  	int (*statfs) (struct dentry *, struct kstatfs *);
> +	int (*getattr_fs) (struct vfsmount *, struct dentry *, struct kstat *);
> +	int (*mount_fs) (struct vfsmount *, int *, char *);
>  	int (*remount_fs) (struct super_block *, int *, char *);
>  	void (*umount_begin) (struct super_block *);
>  
> diff --git a/include/linux/mount.h b/include/linux/mount.h
> index 1172cce..4bd364e 100644
> --- a/include/linux/mount.h
> +++ b/include/linux/mount.h
> @@ -67,6 +67,7 @@ struct vfsmount {
>  	struct dentry *mnt_root;	/* root of the mounted tree */
>  	struct super_block *mnt_sb;	/* pointer to superblock */
>  	int mnt_flags;
> +	void *fs_data;			/* fs-specific data */

	No.  This is really asking for trouble - you *can't* hang
fs-specific data structures off vfsmount.  Lifetimes make no
sense whatsoever.

	BTW, your patch leaks supreblock reference on failure, and
is kludgy as hell wrt to what it's doing in procfs itself, but
that's secondary - the quoted part is enough for a hard NAK on
architectural grounds.  Don't go there.