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

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Message-ID: <20170307174909.GA24112@redhat.com>
Date:   Tue, 7 Mar 2017 18:49:09 +0100
From:   Oleg Nesterov <oleg@...hat.com>
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>,
        Al Viro <viro@...iv.linux.org.uk>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        Pavel Emelyanov <xemul@...allels.com>,
        James Bottomley <James.Bottomley@...senPartnership.com>
Subject: Re: [RFC] Add option to mount only a pids subset

I can't really review this... but in any case I think you should split
this patch to separate the vfs and proc changes.

On 03/07, Alexey Gladkov wrote:
>
> @@ -962,6 +963,14 @@ vfs_kern_mount(struct file_system_type *type, int flags, const char *name, void
>  	mnt->mnt.mnt_sb = root->d_sb;
>  	mnt->mnt_mountpoint = mnt->mnt.mnt_root;
>  	mnt->mnt_parent = mnt;
> +
> +	err = do_mount_sb(&mnt->mnt, flags, data);
> +	if(err) {
> +		mnt_free_id(mnt);
> +		free_vfsmnt(mnt);
> +		return ERR_PTR(err);
> +	}

This duplicates the error handling, we do the same if mount_fs() fails.
Perhaps you should move these 2 lines into cleanup block and add goto's.

> +int proc_getattrfs(struct vfsmount *mnt, struct dentry *dentry, struct kstat *stat)
> +{
> +	struct inode *inode = d_inode(dentry);
> +	struct pid *pid = proc_pid(dentry->d_inode);
> +	struct proc_options *opts = mnt->fs_data;
> +
> +	if (opts && opts->pid_only && mnt->mnt_root != dentry && !pid)
> +		return -ENOENT;

Hmm. I don't quite understand why do we need this, and how this should work.

Yes, "/bin/ls /pidonly-proc/sys" or opendir(/pidonly-proc/sys) should fail,
but only because they both do stat() ?

Afaics you still can do open("/pidonly-proc/sys") + getdents() and this should
work ?

I still think proc_dir_operations.open() makes more sense. Yes, as you pointed
out we also need to update proc_sys_dir_file_operations too and may be something
else...

> +
> +	if (!inode->i_op->getattr) {
> +		generic_fillattr(inode, stat);
> +		return 0;
> +	}
> +
> +	return inode->i_op->getattr(mnt, dentry, stat);
> +}

Oh, it would be nice to not duplicate the code from the caller, imo.

Oleg.