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

[Al Viro]

On Fri, Oct 12, 2018 at 03:49:50PM +0100, Alan Jenkins wrote:
> > +SYSCALL_DEFINE3(fspick, int, dfd, const char __user *, path, unsigned int, flags)
> > +{
> > +	struct fs_context *fc;
> > +	struct path target;
> > +	unsigned int lookup_flags;
> > +	int ret;
> > +
> > +	if (!ns_capable(current->nsproxy->mnt_ns->user_ns, CAP_SYS_ADMIN))
> > +		return -EPERM;
> 
> 
> This seems to accept basically any mount.  Specifically: are you sure it's
> OK to return a handle to a SB_NO_USER superblock?

Umm...  As long as we don't try to do pathname resolution from its ->s_root,
shouldn't be a problem and I don't see anything that would do that.  I might've
missed something, but...