linux-kernel - Re: [fuse-devel] fuse_get

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Date:	Fri, 22 May 2015 13:59:32 -0500
From:	Seth Forshee <seth.forshee@...onical.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	Miklos Szeredi <miklos@...redi.hu>, alexey@...nosov.spb.ru,
	Andy Lutomirski <luto@...capital.net>,
	Serge Hallyn <serge.hallyn@...ntu.com>,
	fuse-devel <fuse-devel@...ts.sourceforge.net>,
	Linux-Fsdevel <linux-fsdevel@...r.kernel.org>,
	Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [fuse-devel] fuse_get_context() and namespaces

On Fri, May 22, 2015 at 12:44:35PM -0500, Eric W. Biederman wrote:
> Seth Forshee <seth.forshee@...onical.com> writes:
> 
> > On Fri, May 22, 2015 at 04:23:55PM +0200, Miklos Szeredi wrote:
> >> On Sat, May 2, 2015 at 5:56 PM,  <alexey@...nosov.spb.ru> wrote:
> >> >
> >> > 3.10.0-229 form Scientific Linux and native 4.0.1-1 (from elrepo).
> >> > SL 7.1 on the host and SL 6.6 on the LXC guest. At least in 3.10
> >> > the 499dcf2024092e5cce41d05599a5b51d1f92031a is present.
> >> > Steps to reproduce:
> >> >
> >> > On first console:
> >> > [root@...test ~]# lxc-start  -n test-2 /bin/su -
> >> > [root@...t-2 ~]# diff -u  hello.py /usr/share/doc/fuse-python-0.2.1/example/hello.py
> >> > --- hello.py    2015-05-02 11:12:13.963093580 -0400
> >> > +++ /usr/share/doc/fuse-python-0.2.1/example/hello.py   2010-04-14 18:29:21.000000000 -0400
> >> > @@ -41,8 +41,6 @@
> >> >  class HelloFS(Fuse):
> >> >
> >> >      def getattr(self, path):
> >> > -        dic = Fuse.GetContext(self)
> >> > -        print dic
> >> >          st = MyStat()
> >> >          if path == '/':
> >> >              st.st_mode = stat.S_IFDIR | 0755
> >> > [root@...t-2 ~]# python hello.py -f  /mnt/
> >> >
> >> > On second console:
> >> > [root@...t-2 ~]# echo $$
> >> > 41
> >> > [root@...t-2 ~]# ls /mnt/
> >> > hello
> >> >
> >> > Output of first console:
> >> > {'gid': 0, 'pid': 12083, 'uid': 0}
> >> 
> >> Thanks.
> >> 
> >> Digging in mailbox...  There was a thread last year about adding
> >> support for running fuse daemon in a container:
> >> 
> >>   http://thread.gmane.org/gmane.linux.kernel/1811658
> >> 
> >> Not sure what happened, but no updated patches have been posted or
> >> maybe I just missed them.
> >
> > I haven't sent updated patches in a while. I still intend to, but I
> > shifted focus to first getting general support for mounts from user
> > namespaces into the vfs (which will give a clearer direction for some of
> > the concerns raised about the fuse patches).
> >
> > All of this code is available in the userns-mounts branch of
> > git://kernel.ubuntu.com/sforshee/linux.git, and I don't think the fuse
> > patches actually depend on any of the stuff that precedes them. I'm
> > planning to start submitting some of the earlier patches from that
> > branch soon, and eventually get back to resubmitting the fuse patches.
> >
> > This is about pid namespaces though, and the fuse pid namespace patch
> > from that series (see below) should be more or less independent of the
> > rest of the patches. Potentially that could be merged separately from
> > the user namespae stuff.
> 
> [snip]
> 
> > @@ -2076,7 +2077,15 @@ static int convert_fuse_file_lock(const struct fuse_file_lock *ffl,
> >  
> >  		fl->fl_start = ffl->start;
> >  		fl->fl_end = ffl->end;
> > -		fl->fl_pid = ffl->pid;
> > +
> > +		/*
> > +		 * Convert pid into the connection's pid namespace. If the
> > +		 * pid does not map into the namespace fl_pid will get set
> > +		 * to 0.
> > +		 */
> > +		rcu_read_lock();
> > +		fl->fl_pid = pid_vnr(find_pid_ns(ffl->pid, fc->pid_ns));
> > +		rcu_read_unlock();
> 
> Scratches my head.  This looks wrong.
> 
> I would have expected pid_nr_ns.  Am I missing something reading this
> patch quickly?

Here we're in the context of a F_GETLK operation. We've requested the
lock information from the fuse process, and ffl->pid is the pid number
in that process's pid namespace so it needs to be translated into
current's namespace. First we have to look up the struct pid, then
pid_vnr is just a wrapper for pid_nr_ns in the current pid namespace:

  pid_t pid_vnr(struct pid *pid)
  {
          return pid_nr_ns(pid, task_active_pid_ns(current));
  }

Oh, but the comment is wrong, so maybe that's what confused you.
s/connection/caller/ there and it should make more sense.

Seth
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