xref: /illumos-gate/usr/src/lib/libgss/README.spi (revision ddcb6370)

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	The Service Provider Interface for libgss and its Mechanisms
	------------------------------------------------------------

1.  The libgss SPI upto 11/2004

    Prior to PSARC 2004/810 the libgss SPI consisted of a function
    provided by each mechanism whose return value is a pointer to a
    structure full of references to the mechanism's entry points
    (hereinafter: methods).

    This structure does not include any hooks for versioning, which
    means that additions of any mechanism methods at micro/patch
    releases require patching libgss.so.1 and all the GSS mechanisms
    shipped with Solaris (Kerberos V, DH, SPNEGO).

2.  The libgss SPI after PSARC 2004/810

    In order to avoid changing the gss_config struct and patching all
    three mechanisms (four, if the dummy mech counts) and libgss
    together and in anticipation of a cleaner SPI in the future (see
    next section) the SPI after PSARC 2004/810 will be as before but
    supplemented as follows:

     - any new SPI mechanism methods will NOT be placed in gss_config,
       instead there is a new gss_config_ext structure, which is to be
       used _only_ by libgss (to avoid struct versioning and/or patch
       issues), which should be extended to have a pointer to the new
       method;

     - there is a new libgss function, __gss_get_mechanism_ext(), which
       is used to get at the gss_config_ext for a mechanism;

     - __gss_get_mechanism_ext() uses dlsym() to build the
       gss_config_ext struct for the mech by individually loading each
       and every mechanism method that isn't part of the old gss_config
       struct -- this happens only once per-method, of course; the
       result is cached.

       The symbol names that are dlsym()ed are of the form gssspi_* and
       correspond to gss_*; e.g., gssspi_acquire_cred_with_password().

       New methods also have a corresponding typedef named
       <gss_func>_sfct -- the 's' in 'sfct' is for "SPI" and the 'fct'
       is for "function."  This is used to keep cast expressions short.

3.  The Future libgss SPI

    Once the Solaris krb5 source is resync'ed with MIT krb5 1.4 there
    will be no further need for the 'void *context' argument to all the
    libgss mechanisms' methods.

    At that point it will be possible to remove this 'void *context'
    argument from all the libgss SPI function prototypes, the main
    result of which will be that the mechanisms' methods will then have
    the same function signature as the corresponding GSS-API functions.

    We can then rename all mechanisms' methods from <mech>_<gss-func> to
    <gss-func>.  The corresponding typedefs will be renamed to
    <gss-func>_fct.

    The SPI, then, will be almost exactly the same as the API.

    There will be some minor differences, primarily that some API
    functions won't have a corresponding SPI method, such as
    gss_release_buffer(3GSS), for example.

    Some time later we may open the SPI to third party implementors;
    this could be particularly useful as a way to get access to 3rd
    party implementations of SPKM and LIPKEY (assuming any ever exist --
    SPKM's is a very problematic specification).

    Third party mechanisms should just export all the symbols for the
    GSS-API functions, like MIT krb5 does, but functions which libgss
    won't call (e.g., gss_release_buffer(3GSS)) should either not be
    implemented or should be weak symbols.

    Solaris native mechanisms may still provide the mechanism method
    registration function as usual for optimization purposes -- to
    reduce the number of calls to dlsym().

    Mechanisms that do not provide the old method registration function
    will be loaded as follows:

     - libgss will look for and find the mechanism's
       GSS_Indicate_mechs() method and will call it to discover the
       mechanism provider's mechanism OIDs.

     - libgss will dlsym() each mechanism provider SPI method.