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    On 08/26/2011 05:59 PM, Casey Schaufler wrote:
    > On 8/26/2011 10:01 AM, Eric Paris wrote:

    > Better to have a single question answered as required and with
    > complete accuracy than to carry around the baggage necessary to
    > maintain a cached duplicate of the kernel's rules and all the
    > bookkeeping that requires. SELinux libraries probably have to
    > make a system call just to determine if the caches they are
    > maintaining are out of date.

    Come on Casey, this is Linux. We have choices! I believe there are
    currently 3 solutions to solving the 'out of date caches' problem. The
    first and oldest is just a syscall per access request option (I think it
    does read() but don't remember). There is an option to get a netlink
    message informing you to flush your cache. SE-XWindows found that ONE
    syscall per check was WAY WAY WAY too much overhead and uses this
    method. There is also a magic selinuxfs file which a userspace process
    can mmap and just read the memory location to tell if a reload occurred.
    SE-PostgreSQL uses this method because their software architecture did
    not have a good way to handle a netlink socket.

    The value of the userspace cache is admittedly entirely a question of
    the willingness to accept the overhead of kernel lookups.

    /me checks out of thread.


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