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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 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
- 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
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.