heimdal/lib/krb5/krb5-plugin.7

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.Dd December  21, 2011
.Dt KRB5-PLUGIN 7
.Os HEIMDAL
.Sh NAME
.Nm krb5-plugin
.Nd plugin interface for Heimdal
.Sh SYNOPSIS
.In krb5.h
.In krb5/an2ln_plugin.h
.In krb5/ccache_plugin.h
.In krb5/db_plugin.h
.In krb5/kuserok_plugin.h
.In krb5/locate_plugin.h
.In krb5/send_to_kdc_plugin.h
.Sh DESCRIPTION
Heimdal has a plugin interface.  Plugins may be statically linked into
Heimdal and registered via the
.Xr krb5_plugin_register 3
function, or they may be dynamically loaded from shared objects present
in the Heimdal plugins directories.
.Pp
Plugins consist of a C struct whose struct name is given in the
associated header file, such as, for example,
.Va krb5plugin_kuserok_ftable
and a pointer to which is either registered via
.Xr krb5_plugin_register 3
or via a plugin load function exported by a shared object.
Plugin load functions should be named by concatenating the name defined in the
associated header file with the string "plugin_load" (e.g.
"krb5_plugin_kuserok_plugin_load" for the plugin for
.Xr krb5_kuserok 3
).
The plugin load function must be of type
.Va heim_plugin_load_ft
which is:
.Bd -literal -offset indent
krb5_error_code HEIM_CALLCONV
my_plugin_load(heim_pcontext context,
               krb5_get_instance_func_t *get_instance,
               size_t *num_plugins,
               heim_plugin_common_ftable_cp **plugins);

.Ed
where
.Va HEIM_CALLCONV
is
.Va __stdcall
on Windows.
.Pp
The plugin should set the get_instance output parameter to the a
function that will return the instances of its library
dependencies.  For example:
.Bd -literal -offset indent
static uintptr_t HEIM_LIB_CALL
my_plugin_get_instance(const char *name)
{
    if (strcmp(name, "krb5") == 0)
        return krb5_get_instance(name);
    return 0;
}
.Ed
.Pp
The
.Va get_instance
function is used to check that dynamically-linked plugins are
linked with the same Heimdal shared objects as the one loading
and running the plugin.
.Pp
The output parameters
.Va plugins
and
.Va n_plugins
output an array of pointers to function tabls, and the number of
those, respectively.
.Pp
The plugin structs for all plugin types always begin with the same three
common fields:
.Bl -enum -compact
.It
.Va minor_version
, an int.  Plugin minor versions are defined in each plugin type's
associated header file.
.It
.Va init
, a pointer to a function with two arguments, a
.Va heim_pcontext
(which for krb5 plugins is actually a krb5_context),
and a
.Va void **
, returning a heim_error_code.  This function will be called to
initialize a plugin-specific context in the form of a
.Va void *
that will be output through the init function's second argument.
.It
.Va fini
, a pointer to a function of one argument, a
.Va void *
, consisting of the plugin's context to be destroyed, and
returning
.Va void.
.El
.Pp
Each plugin type may add fields to this struct following the above
three.  Plugins are typically invoked in no particular order until one
succeeds or fails, or all return a special return value that indicates
that the plugin was not applicable.  For krb5 plugins,
.Va KRB5_PLUGIN_NO_HANDLE
indicates that the plugin was not applicable.
.Pp
Heimdal plugin callers either invoke all plugins until one returns an
error or all return
.Va KRB5_PLUGIN_NO_HANDLE
, or invoke all plugins until one returns a value other than
.Va KRB5_PLUGIN_NO_HANDLE
with the expectation that only one plugin would return success and all
oters would return
.Va KRB5_PLUGIN_NO_HANDLE.
Thus Heimdal plugin invokation can be deterministic in spite of
non-deterministic invocation order.
.Pp
There is a database plugin system intended for many of the uses of
databases in Heimdal.  The plugin is expected to call
.Xr heim_db_register 3
from its
.Va init
entry point to register a DB type.  The DB plugin's
.Va fini
function must do nothing, and the plugin must not provide any other
entry points.
.Pp
The krb5_kuserok plugin adds a single field to its struct: a pointer to
a function that implements kuserok functionality with the following
form:
.Bd -literal -offset indent
static krb5_error_code
kuserok(void *plug_ctx, krb5_context context, const char *rule,
        unsigned int flags, const char *k5login_dir,
        const char *luser, krb5_const_principal principal,
        krb5_boolean *result)
.Ed
.Pp
The
.Va luser
,
.Va principal
and
.Va result
arguments are self-explanatory (see
.Xr krb5_kuserok 3
).  The
.Va plug_ctx
argument is the context output by the plugin's init function.  The
.Va rule
argument is a kuserok rule from the krb5.conf file; each plugin is invoked once
for each rule until all plugins fail or one succeeds.  The
.Va k5login_dir
argument provides an alternative k5login file location, if not NULL.
The
.Va flags
argument indicates whether the plugin may call
.Xr krb5_aname_to_localname 3
(KUSEROK_ANAME_TO_LNAME_OK), and whether k5login databases are expected to be
authoritative (KUSEROK_K5LOGIN_IS_AUTHORITATIVE).
.Pp
The plugin for
.Xr krb5_aname_to_localname 3
is named "an2ln" and has a single extra field for the plugin struct:
.Bd -literal -offset indent
typedef krb5_error_code (*set_result_f)(void *, const char *);

static krb5_error_code
an2ln(void *plug_ctx, krb5_context context, const char *rule,
      krb5_const_principal aname, set_result_f set_res_f, void *set_res_ctx)
.Ed
.Pp
The arguments for the
.Va an2ln
plugin are similar to those of the kuserok plugin, but the result, being
a string, is set by calling the
.Va set_res_f
function argument with the
.Va set_res_ctx
and result string as arguments.  The
.Va set_res_f
function will make a copy of the string.
.Sh FILES
.Bl -tag -compact
.It Pa libdir/plugin/krb5/*
Shared objects containing plugins for Heimdal.
.El
.Sh EXAMPLES
.Pp
An example an2ln plugin that maps principals to a constant "nouser"
follows:
.Pp
.Bd -literal -offset indent
#include <krb5/an2ln_plugin.h>

/* Note that `context' here is actually a krb5_context value */
static krb5_error_code
nouser_plug_init(heim_pcontext context, void **ctx)
{
    *ctx = NULL;
    return 0;
}

static void nouser_plug_fini(void *ctx) { }

static krb5_error_code
nouser_plug_an2ln(void *plug_ctx, krb5_context context,
                  const char *rule,
                  krb5_const_principal aname,
                  set_result_f set_res_f, void *set_res_ctx)
{   
    krb5_error_code ret;

    if (strcmp(rule, "NOUSER") != 0)
	return KRB5_PLUGIN_NO_HANDLE;

    ret = set_res_f(set_res_ctx, "nouser");

    return ret;
}

krb5plugin_an2ln_ftable an2ln = {
    KRB5_PLUGIN_AN2LN_VERSION_0,
    nouser_plug_init,
    nouser_plug_fini,
    nouser_plug_an2ln,
};

static const krb5plugin_an2ln_ftable *const plugins[] = {
    &an2ln
};

static uintptr_t
an2ln_get_instance(const char *libname)
{
    if (strcmp(libname, "krb5") == 0)
	return krb5_get_instance(libname);

    return 0;
}

/* Note that `context' here is actually a krb5_context value */
krb5_error_code
an2ln_plugin_load(heim_pcontext context,
                 krb5_get_instance_func_t *get_instance,
                 size_t *num_plugins,
                 const krb5plugin_an2ln_ftable * const **pplugins)
{
    *get_instance = an2ln_get_instance;
    *num_plugins = sizeof(plugins) / sizeof(plugins[0]);
    *pplugins = plugins;
    return 0;
}
.Ed
.Pp
An example kuserok plugin that rejects all requests follows.  (Note that
there exists a built-in plugin with this functionality; see
.Xr krb5_kuserok 3
).
.Pp
.Bd -literal -offset indent
#include <krb5/kuserok_plugin.h>

static krb5_error_code
reject_plug_init(heim_context context, void **ctx)
{
    *ctx = NULL;
    return 0;
}

static void reject_plug_fini(void *ctx) { }

static krb5_error_code
reject_plug_kuserok(void *plug_ctx, krb5_context context, const char *rule,
                    unsigned int flags, const char *k5login_dir,
                    const char *luser, krb5_const_principal principal,
                    krb5_boolean *result)
{
    if (strcmp(rule, "REJECT") != 0)
        return KRB5_PLUGIN_NO_HANDLE;

    *result = FALSE;
    return 0;
}

static krb5plugin_kuserok_ftable kuserok = {
    KRB5_PLUGIN_KUSEROK_VERSION_0,
    reject_plug_init,
    reject_plug_fini,
    reject_plug_kuserok,
};

static const krb5plugin_kuserok_ftable *const plugins[] = {
    &kuserok
};

static uintptr_t
kuserok_get_instance(const char *libname)
{
    if (strcmp(libname, "krb5") == 0)
	return krb5_get_instance(libname);

    return 0;
}

krb5_error_code
krb5_plugin_kuserok_plugin_load(
    heim_context context,
    krb5_get_instance_func_t *get_instance,
    size_t *num_plugins,
    const krb5plugin_kuserok_ftable * const **pplugins)
{
    *krb5_instance = kuserok_get_instance;
    *num_plugins = sizeof(plugins) / sizeof(plugins[0]);
    *pplugins = plugins;
    return 0;
}

.Ed
.Sh SEE ALSO
.Xr krb5_plugin_register 3
.Xr krb5_kuserok 3
.Xr krb5_aname_to_localname 3