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From:   ebiederm@xmission.com (Eric W. Biederman)
To:     Alan Jenkins <alan.christopher.jenkins@gmail.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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Date:   Wed, 17 Oct 2018 09:35:45 -0500
In-Reply-To: <f6f2553e-f64d-9c26-4290-baf43e5f533b@gmail.com> (Alan Jenkins's
        message of "Wed, 17 Oct 2018 15:31:26 +0100")
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Subject: Re: [PATCH 31/34] vfs: syscall: Add fspick() to select a superblock for reconfiguration [ver #12]
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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

>> ---
>> 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)) {
>>