linux-kernel - Re: [PATCH] fs: Remove implicit nodev for new mounts in non-root userns

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Date:	Fri, 15 Aug 2014 19:37:47 +0000
From:	Serge Hallyn <serge.hallyn@...ntu.com>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Linux Containers <containers@...ts.linux-foundation.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	stable <stable@...r.kernel.org>,
	Kenton Varda <kenton@...dstorm.io>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	Linux FS Devel <linux-fsdevel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH] fs: Remove implicit nodev for new mounts in non-root
 userns

Quoting Andy Lutomirski (luto@...capital.net):
> On Fri, Aug 15, 2014 at 12:05 PM, Serge Hallyn <serge.hallyn@...ntu.com> wrote:
> > Quoting Andy Lutomirski (luto@...capital.net):
> >> Currently, creating a new mount (as opposed to bindmount) in a
> >> non-root userns will implicitly set nodev unless the fs is devpts.
> >> Something like this will be necessary for file systems that allow
> >> the mounter to create device nodes without using mknod (e.g. FUSE
> >> if/when that is allowed), but none of the currently allowed
> >> filesystems do this.
> >
> > Hi,
> >
> > Sorry, I'm probably thinking stupidly, but I don't see this restriction
> > being the case
> >
> > serge@sl:~$ mount | grep tmp
> > [...]
> > tmpfs on /run type tmpfs (rw,noexec,nosuid,size=10%,mode=0755)
> > serge@sl:~$ sudo mknod /run/kvm c 10 232
> > [sudo] password for serge:
> > serge@sl:~$ echo $?
> > 0
> > serge@sl:~$ ls -l /run/kvm
> > crw-r--r-- 1 root root 10, 232 Aug 15 14:04 /run/kvm
> >
> > But you seem to be saying I shouldn't be allowed to create a device inside
> > a tmpfs.  What am I overlooking?
> 
> I assume you're in the root userns.  This patch is unnecessary, and
> has no effect, if you're in the root userns.

Right, but I thought you were justifying adding FS_USERNS_DEV_MOUNT by saying
that you cannot mknod in those filesystems.  But I see you actually said
"without using mknod".  I guess I don't understand that caveat.

-serge
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