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Subject: Re: [PATCH 31/34] vfs: syscall: Add fspick() to select a superblock
 for reconfiguration [ver #12]
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     David Howells <dhowells@redhat.com>,
        Al Viro <viro@ZenIV.linux.org.uk>, linux-api@vger.kernel.org,
        torvalds@linux-foundation.org, 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: <8f5cf2a3-c522-d0b8-1257-1814e8149956@gmail.com>
Date:   Wed, 17 Oct 2018 15:55:11 +0100
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On 17/10/2018 15:35, Eric W. Biederman wrote:
> Alan Jenkins <alan.christopher.jenkins@gmail.com> writes:
>
>> On 17/10/2018 14:20, David Howells wrote:
>>> David Howells <dhowells@redhat.com> wrote:
>>>
>>>> I should probably check that the picked point is actually a mountpoint.
>>> The root of the mount object at the path specified, that is, perhaps with
>>> something like the attached.
>>>
>>> David
>>
>> 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.
>> */
> Davids check will work for bind mounts as well.  It just won't work it
> just won't work for files or subdirectories of some mountpoint.
>
> Eric


I see.  Then I am still happy to see the fspick() check match a check in 
do_remount() (and it still solves the problem I was worried about).

I cannot blame David for the do_remount() comment being out of date :-).

# uname -r
4.18.10-200.fc.28.x86_64
# mount --bind /mnt /mnt
# mount -oremount,debug /mnt
# findmnt /mnt; findmnt /
[findmnt shows / has been remounted, adding the ext4 "debug" mount option]


>
>>> ---
>>> diff --git a/fs/fsopen.c b/fs/fsopen.c
>>> index f673e93ac456..aaaaa17a233c 100644
>>> --- a/fs/fsopen.c
>>> +++ b/fs/fsopen.c
>>> @@ -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)) {
>>>