Re: A Comment from User Space

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chrisat_private

* David Wagner (dawat_private) wrote:
> Chris Wright  wrote:
> >* Tim Hollebeek (thollebeekat_private) wrote:
> >> With the three-function interface, it is much simpler to guarantee the
> >> invariant that before_foo() runs iff check_foo() is true.
> >
> >i'm sorry, i don't understand this argument.
> >
> >static int do_check_foo(args);
> >static void do_before_foo(args);
> >int module_check_foo(args)
> >{
> >	int ret = do_check_foo(args);
> >	if (!ret)
> >		do_before_foo(args);
> >	return ret;
> >}
> 
> A few potential drawbacks:
>  * You have to know what method the caller of module_check_foo() uses
>    to determine whether the call will be allowed.  If it's always
>    "if (!ret)", then you're ok, but if it is even more complex than
>    that (e.g., MAC override; or if a call can be denied by Unix file
>    permissions even though the module said it was ok to allow it),
>    then the above fails.

Thanks for the explanation.  I have been complaining about this very issue
while working on the capabilities port and didn't see the correlation.

thanks,
-chris

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