[Qemu-devel] Improved shared folders support (was: Re: slirp-smb broken with Samba 4.1)

[Peter Wu]

On Monday 03 November 2014 15:58:35 Michael Tokarev wrote:
> 03.11.2014 15:22, Peter Wu wrote:
> []
> > As an aside, would it be possible to override the samba binary at runtime,
> > without compiling? Either an envvar (SMBD) or an option (-net
> > user,smb=...,smbd=...).
> 
> The whole thing needs to be rewritten to use a shell script to create
> smb.conf and to call smbd.  It's something I wanted to do for very long
> time, the only question is where to put this script so a user can
> easily customize it.  In debian we have a rule that anything in /etc
> is a conffile, and it gets upgraded (when upgrading the corresponding
> package) only if it hasn't been modified by the user.  Everything else
> (in /usr/lib, /usr/share etc) gets upgraded unconditionally.  So maybe
> storing it in /etc/qemu/run-smbd.sh or somehting like that makes sense.

I do not think that everyone is (or wants to be) a Samba wizard. For
that reason, I see little value in giving the user too much control over
the creation of the samba config file. There exists helpers for setting up
bridges, but that is because it may need more privileges, and starting samba is
not such a case.

> The rest is a shell code to create smb.conf in a temp dir -- the same
> as current code does in qemu, plus all your additions,

If by "my additions" you mean specifying a custom samba path, that is
just a single program, why not allow to override it at runtime?

> plus whatever else might be needed (that script can even query which
> samba version is in use and enable one or another option).

That single program could be a wrapper script that modifies the files as
needed.

I propose to keep a simple interface which do not need users to modify a
shell script or learn Samba. Consider Virtual Box' and VMWare' shared
folders functionality. It allows multiple folders to be specified with
these properties:

 - share name,
 - folder to be shared,
 - read-only,
 - automount it in the guest (feature of VBox Guest additions),
 - disable after session (after reboot/poweroff, VMWare feature).

Only the first three options are interesting enough for QEMU's
interface. What about extending the current '-net user,smb' option as
follows (lines are wrapped for readability):

    smb=[sharename:]folder,
    [smb=sharename2:folder2[:readonly]] (etc)

If 'sharename' is omitted, 'qemu' is used. Duplicate share names must
result in a fatal error. 'readonly' requires specification of the
sharename to avoid an ambiguity. Further share options could be added
in the future as needed using ':' as separator (the comma is already
parsed by -net).

Alternative option (defaults to rw:):

    smb=[rw:|ro:][sharename:]folder,
    smb=[rw:|ro:][sharename2:]folder2 (etc)

It would also be nice if a QMP/HMP was also available to add shares on
runtime. In the current implementation, that could be done by writing a
new config file, renaming it to smb.conf when done. Samba appears to
have support to do this without restarting samba, but I have no idea
whether it can be used in the current implementation where
communication happens through stdin/stdout.

By the way, how well does '-net user,smb' work with multiple
connections? The documentation for guestfwd suggests that the command
gets executed for each connection. Won't that cause concurrency issues
in the access to databases?
-- 
Kind regards,
Peter
https://lekensteyn.nl