🚀 go-pugleaf

louie@sayshell.umd.edu (Louis A. Mamakos) writes:
>I'm designing a loadable kernel server which needs to communicate with
>user level processes.  

I hope you like a real challenge... I'm also writing a kernel server, and it's
certianly lots of fun and games.

Some of the big suprises I found that weren't mentioned in the "Writing
Loadable Kernel Servers" manual were that (at least in 2.1) only the superuser
can send out-of-line messages to a kernel server%, and that the message-based
interface was _40 times_ slower than ioctl#.

NeXT's documentation in this area leaves a _great deal_ to be desired; the
manual refers you to Egan & Teixeira for Unix-style drivers - but their book
is about System V, not BSD, and of course doesn't mention NeXT's changes or
anything about Mach. I've had to disassemble parts of the kernel numerous
times; the latest real beauty was disassembling selwakeup() to find that it
takes a thread *, not a proc * like it does in other BSD Unix systems. This
one took a real long time and lots of stabs in the dark to find. Quite
logical once you know about it, but it's documented _nowhere_.

And kgdb is next to useless, because you can't depend on the info it gives
you. I used it to single step a simple 'if' condition on an int in an
interrupt routine and it stepped down the _wrong_ path, so I wasted a whole
day until one of the hardware guys here suggested I put a printf on the line
I _knew_ it wasn't getting to. I did so to prove he was wrong and found kgdb
had lied to me. It always prints the values of local variables wrong too;
although it seems to get parameters of a function and info registers right.
We call it the Intel now. NeXT could not possibly have used it without
noticing some ef these problems.

Anyone else want to share their experiences?.

regards,
Graham

%: The kernel server needs access to the kernel task port to vm_write the
   out-of-line data into its (the kernel server's) address space; and the
   only way it can get the kernel task port is if the user process sends it
   in; and the only way the user process can get it is via task_by_unix_pid
   with pid 0; and only the superuser can do that. Fixed in 3.0 though.

#: On my NeXTstation color. Your milage may vary.
-- 
Graham Stoney                    | "a Perl script is correct if it's halfway
Flip Dibner fan club, "Hi Flip!" |  readable and gets the job done before your
Ph: +61 2 805-2909  Fax: -2929   |  boss fires you." L. Wall & R. Schwartz