[PATCH RFC] cifs-utils: new plugin architecture for ID mapping code

[simo]

On Thu, 2012-12-06 at 07:37 -0500, Jeff Layton wrote:
> Currently, the ACL-related tools in cifs-utils call into the wbclient
> libs directly in order to do their bidding. The wbclient developers want
> to get away from needing to configure winbind on the clients and instead
> allow sssd to handle the id-mapping. Less muss, less fuss...
> 
> This patch represents an initial step in that direction. It adds a
> plugin architecture for cifs-utils, adds wrappers around the calls into
> libwbclient that find an idmap plugin library to use and then has it
> call into that plugin to do the actual ID mapping.
> 
> This patch should be considered an RFC on the overall design. Once I
> have some positive feedback (or lack of negative feedback), I'll do the
> same with cifs.idmap and setcifsacl.
> 
> This patch is still pretty rough, but should demonstrate the basic
> design:
> 
> The application will call into a set of routines that find the correct
> plugin and dlopen() it. Currently the plugin is located in a hardcoded
> location that will eventually be settable via autoconf. That location is
> intended to be a symlink that points to the real plugin (generally under
> %libdir/cifs-utils).
> 
> The plugin will export a number of functions with well-known names. The
> wrappers find those by using dlsym() and then call them.
> 
> I'm tracking progress on this work here:
> 
>     https://bugzilla.samba.org/show_bug.cgi?id=9203
> 
> Here are some questions to ponder:
> 
> 1/ Should we switch this code to use a config file of some sort instead
> of this symlink? The symlink would probably be more efficient, but it
> is a little odd and might confuse people. It also might make it hard to
> expand the idmapping interfaces later.

I think a symlink is ok for starters, a config file can always be added
later if needed.

> 2/ Should I switch this code to use libltdl for the plugin architecture?
> I started to use that initially, but it was awfully complex to get working.
> Since portability isn't really a concern with cifs-utils, I punted. Does
> anyone see issues with rolling our own here?

Well the cifs kernel module is Linux only, I would leave the hassle of
dealing with portability to whomever would like to port this.
Using dlopen/dlsym is simple, so roll-your-own seem fine to me.

One thing though I would name-space the symbol, so instead of
idmap_sid_to_str call it cifs_idmap_sid_to_str, in the plugin.
Internally you can call whatever you want.

Also I think you shouldn't resolve symbols each time,

Declare a function pointer in the data segment (so inited to NULL at
startup) and do a lazy init only if it is NULL, by assigning it.

#define RESOLVE_SYMBOL(name) \
do { \
    if (name == NULL) { \
        name = resolve_symbol("cifs_" # name); \
        if (name == NULL) \
            return -ENOSYS; \
    } \
} while(0);

sid_to_str()
{
    RESOLVE_SYMBOL(idmap_sid_to_str);
    return idmap_sid_to_str(..);
}

-- 
Simo Sorce
Samba Team GPL Compliance Officer <simo at samba.org>
Principal Software Engineer at Red Hat, Inc. <simo at redhat.com>