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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,
        linux-fsdevel@vger.kernel.org, linux-kernel@vger.kernel.org,
        mszeredi@redhat.com
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From:   Alan Jenkins <alan.christopher.jenkins@gmail.com>
Message-ID: <7b409325-9e23-aa5e-916f-cb362a6940aa@gmail.com>
Date:   Wed, 17 Oct 2018 18:41:24 +0100
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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;
> +

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

static  int  do_remount <https://elixir.bootlin.com/linux/v4.18/ident/do_remount>(struct  path <https://elixir.bootlin.com/linux/v4.18/ident/path>  *path <https://elixir.bootlin.com/linux/v4.18/ident/path>,  int  ms_flags,  int  sb_flags,
		int  mnt_flags,  void  *data)
{
	int  err;
	struct  super_block <https://elixir.bootlin.com/linux/v4.18/ident/super_block>  *sb  =  path <https://elixir.bootlin.com/linux/v4.18/ident/path>->mnt 
<https://elixir.bootlin.com/linux/v4.18/ident/mnt>->mnt_sb;
	struct  mount <https://elixir.bootlin.com/linux/v4.18/ident/mount>  *mnt <https://elixir.bootlin.com/linux/v4.18/ident/mnt>  =  real_mount 
<https://elixir.bootlin.com/linux/v4.18/ident/real_mount>(path 
<https://elixir.bootlin.com/linux/v4.18/ident/path>->mnt 
<https://elixir.bootlin.com/linux/v4.18/ident/mnt>);

	if  (!check_mnt <https://elixir.bootlin.com/linux/v4.18/ident/check_mnt>(mnt 
<https://elixir.bootlin.com/linux/v4.18/ident/mnt>))
		return  -EINVAL <https://elixir.bootlin.com/linux/v4.18/ident/EINVAL>;

	if  (path <https://elixir.bootlin.com/linux/v4.18/ident/path>->dentry  !=  path <https://elixir.bootlin.com/linux/v4.18/ident/path>->mnt 
<https://elixir.bootlin.com/linux/v4.18/ident/mnt>->mnt_root)
		return  -EINVAL <https://elixir.bootlin.com/linux/v4.18/ident/EINVAL>;

Thanks

Alan