Re: [PATCH 3/4] autofs4 - track uid and gid of last mount requestor

[Andrew Morton <akpm@linux-foundation.org>]

On Thu, 28 Feb 2008 16:08:20 +0900 Ian Kent <raven@themaw.net> wrote:

> 
> > 
> > What problem are you actually trying to solve here?
> 
> The basic problem arises only when we want to restart the user space
> daemon and there are active autofs managed mounts in place at exit (ie.
> autofs mounts that have busy user mounts). They are left mounted and
> processes using them continue to function. But then, when we startup
> autofs we need to reconnect to these autofs mounts, some of which can
> covered the by mounted file systems, and hence the need for another way
> to open an ioctl descriptor to them.

So we want to store persistant state in the kernel across userspace process
invokations.  That's normally done with a thing called a "file" ;) Could we
stick all the necessary state into files in a pseudo-fs and have the daemon
go and open and read them all when it restarts?

> It may have been overkill to re-implement all the current ioctls (and
> add a couple of other much needed ones)

I don't understand that bit.  But then, I don't have a clue how autofs
works.

> but I though it sensible for
> completeness, and we get to identify any possible problems the current
> ioctls might have had due to the use of the BKL (by the VFS when calling
> the ioctls).

It isn't a good idea to wait for races to reveal themselves.  It will take
years, especially with a system which has as low a call frequency as autofs
mounting.  And once a bug _does_ reveal itself, by then we'll have tens of
millions of machines out there running that bug.

> So, why do we need the uid and gid? When someone walks over an autofs
> dentry that is meant to cause a mount we send a request packet to the
> daemon via a pipe

connector or genetlink would be more fashionable transports.

> which includes the process uid and gid, and as part of
> the lookup we set macros for several mount map substitution variables,
> derived from the uid and gid of the process requesting the mount and
> they can be used within autofs maps.

yeah, could be a problem.  Hopefully the namespace people can advise. 
Perhaps we need a concept of an exportable-to-userspace namespace-id+uid,
namespace-id+gid, namespace-id+pid, etc for this sort of thing.  It has
come up before.  Recently, but I forget what the context was.

> This is all fine as long as we don't need to re-connect to these mounts
> when starting up, since we don't get kernel requests for the mounts, we
> need to obtain that information from the active mount itself.