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    SubjectRe: User vs. Kernel (was: To be smug, or not to be smug, that is , the question)

    On Sat, 23 Jan 1999, Albert D. Cahalan wrote:

    > >> * Lack of generalized message passing
    > >
    > > (RT-signals in Linux, a feature of Linux 2.2)
    >
    > Could user acahalan send 42 bytes to user mmuscovi?

    sure, 'mail mmuscovi < ./42bytefile'. What are you trying to achieve and
    why?

    > >> * Lack of a decent privilege/capability model
    > >
    > > (i guess you missed include/linux/capability.h, a feature of 2.2.
    > > Not completely finished, but the main mechanizm is in there.)
    >
    > I believe he means "true" capability support. In any case,
    > Linux can't revoke normal user capabilities.

    what do you mean by 'true'. what is 'user capabilities'. If you mean
    military grade security, where one can restrict a user to be able only to
    execute the 'nop' assembly instruction, then you are right, Linux's doesnt
    want to do that. Linux has a capability model that splits up _system_
    priviledges (thus risks) so that eg. a security hole in 'ping' doesnt mean
    a full system compromise.

    > >> * Blocking I/O
    > >
    > > (whats your problem with that?)
    >
    > This limits concurrency. Within the kernel, IO events need not be
    > serialized. User-space can only get this ability with threads.
    > Threads are quite a bit of overhead for such a simple need, plus
    > the use of threads tends to promote bugs.

    take a look at async networking IO, fcntl(SETSIG), etc, implemented by
    Stephen Tweedie recently. (it's in 2.2)

    > >> * Interruptible system calls
    > >
    > > (what is your point here)
    >
    > It is not good to return to userspace, loop around, and make the system
    > call a second time. The overhead [...]

    Albert, do you have the slightest clue how often it happens to restart a
    system call, in a typical Linux system? Just for kicks, i've added a small
    hack to my kernel 5 minutes ago to count them:

    total syscalls: 28456
    restarted syscalls: 7

    what overhead exactly are you complaining about? ...

    > [...] and app code complexity are annoying.

    the application sees _nothing_ from a system call restart to begin with.
    It's handled in libc.

    -- mingo


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