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From:   Alan Jenkins <alan.christopher.jenkins@gmail.com>
Subject: Re: [PATCH 31/34] vfs: syscall: Add fspick() to select a superblock
 for reconfiguration [ver #12]
To:     David Howells <dhowells@redhat.com>
Cc:     Al Viro <viro@ZenIV.linux.org.uk>, linux-api@vger.kernel.org,
        torvalds@linux-foundation.org, ebiederm@xmission.com,
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        mszeredi@redhat.com
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Message-ID: <240b2620-02cd-89de-4c22-293af6c5ac8b@gmail.com>
Date:   Wed, 17 Oct 2018 23:13:47 +0100
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[resent, hopefully with slightly less formatting damage]

On 17/10/2018 16:45, David Howells wrote:

> Alan Jenkins <alan.christopher.jenkins@gmail.com> wrote:
>
>> I agree. I'm happy to see this is using the same check as do_remount().
>>
>>
>> * change filesystem flags. dir should be a physical root of filesystem.
>> * If you've mounted a non-root directory somewhere and want to do remount
>> * on it - tough luck.
>> */
> Are you suggesting that it should only work at the ultimate root of a
> superblock and not a bind mount somewhere within?
>
> That's tricky to make work for NFS because s_root is a dummy dentry.
>
> David


Retro-actively: I do not suggest that.

I tried to answer this question in my reply to Eric correcting me. Eric 
was right to correct me.  I now understand the comment above 
do_remount() is incorrect.  I re-reviewed your diff in light of that.  I 
re-endorse your diff as a way to solve the problem I raised.

(I think it would be useful to remove the misleading comment above 
do_remount(), to avoid future confusion.)


> @@ -186,6 +186,10 @@ SYSCALL_DEFINE3(fspick, int, dfd, const char __user *, path, unsigned int, flags
>   	if (ret < 0)
>   		goto err;
>   
> +	ret = -EINVAL;
> +	if (target.mnt->mnt_root != target.dentry)
> +		goto err_path;
> +
>   	fc = vfs_new_fs_context(target.dentry->d_sb->s_type, target.dentry,
>   				0, 0, FS_CONTEXT_FOR_RECONFIGURE);
>   	if (IS_ERR(fc)) {


( the "if" statement it adds to fspick() is equivalent to the second 
"if" statement in do_remount(): )

static int do_remount(struct path *path, int ms_flags, int sb_flags,
		      int mnt_flags, void *data)
{
	int err;
	struct super_block *sb = path->mnt->mnt_sb;
	struct mount *mnt = real_mount(path->mnt);

	if (!check_mnt(mnt))
		return -EINVAL;

	if (path->dentry != path->mnt->mnt_root)
		return -EINVAL;

Thanks

Alan