Re: [RFC][PATCH] rbind across namespaces

[Miklos Szeredi]

(Sorry for replying in two parts, I missed this in the first reading)

> >>walking mountpoints in userspace: 
> >>http://marc.theaimsgroup.com/?l=linux-kernel&m=109875012510262&w=2
> > 
> > 
> > Is this needed?  Userspace can find out mountpoints from /proc/mounts
> > (or something similar for detached trees).
> > 
> 
> With detached mountpoints (and especially with detached mountpoint
> _trees_) it can become very difficult to assess which trees are which.
> 
> Also, just like /proc/PID/fd/*, /proc/mounts is built according to
> _current_'s root.  This only gives a skewed view of what is going on.

I'm thinking of /proc/PID/fdinfo/FD/mounts or similar.

> >>attaching mountpoints in userspace:
> >>http://marc.theaimsgroup.com/?l=linux-fsdevel&m=109875063100111&w=2
> > 
> > 
> > Again, bind from/to /proc/PID/fd/FD should work without any new
> > interfaces.
> 
> No..  It wouldn't.  Pathname resolution is doing everything according to
> the ->readlink information provided by this magic proc files, again in
> current's namespace.  If you care to hijack ->follow_link, prepare
> yourself for a slew of corner cases.

No need to hijack, it's already done.  Removing calls to
proc_check_root() will allow access to different namespaces detached
mounts, etc.  It's been tried and actually works.

What's more you don't even need /proc, just pass a file descriptor
(with reference to mount in different namespace, etc.) to another
process with SCM_RIGHTS, do fchrdir(fd), and then do mount --bind,
etc.  This actually works with an unmodified kernel.

> >>detaching mountpoints in userspace:
> >>http://marc.theaimsgroup.com/?l=linux-kernel&m=109880051800963&w=2
> > 
> > 
> > What's wrong with sys_umount()?
> 
> sys_umount only works with paths in current's namespace. It doesn't
> allow you to handle vfsmounts as primaries in userspace.

But it does.  Again, either with fchdir() or /proc.

fchdir() has the drawback of only working on directories, and that a
process has only one CWD.  The /proc approach has no such limitations.

Miklos
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