Re: [PATCH 31/34] vfs: syscall: Add fspick() to select a superblock for reconfiguration [ver #12]

[Alan Jenkins]

On 17/10/2018 15:35, Eric W. Biederman wrote:
> Alan Jenkins <alan.christopher.jenkins@xxxxxxxxx> writes:
>
> > On 17/10/2018 14:20, David Howells wrote:
> > > David Howells <dhowells@xxxxxxxxxx> 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I cannot blame David for the do_remount() comment being out of date :-).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)) {