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Date:   Sat, 13 Oct 2018 07:11:41 +0100
From:   Al Viro <viro@ZenIV.linux.org.uk>
To:     Alan Jenkins <alan.christopher.jenkins@gmail.com>
Cc:     David Howells <dhowells@redhat.com>, 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
Subject: Re: [PATCH 31/34] vfs: syscall: Add fspick() to select a superblock
 for reconfiguration [ver #12]
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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...