From owner-freebsd-hackers@FreeBSD.ORG  Wed Jun 22 21:52:59 2005
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Date: Wed, 22 Jun 2005 16:52:58 -0500
From: Dan Nelson <dnelson@allantgroup.com>
To: Mauser <mauser@poczta.fm>
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References: <20050622192127.05bff1b8.mauser@poczta.fm>
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Cc: freebsd-hackers@freebsd.org
Subject: Re: kld problem
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In the last episode (Jun 22), Mauser said:
> Unloading syscall kernel module can cause a system crash. It occurs when we 
> unload the module while a process is executing our syscall. Example:
> 
> $ cat kldtest.c
...
> static int test_syscall(struct thread *td, void *arg) {
>     struct timeval tv;
>     tv.tv_sec = 15;
>     tv.tv_usec = 0;
>     tsleep(&test_nw,PWAIT,"test",tvtohz(&tv));
>     return 0;
> }
...
> static int test_load(struct module *mod, int cmd, void *arg) {
>     if(cmd != MOD_LOAD && cmd != MOD_UNLOAD)
>         return EOPNOTSUPP;
>     return 0;
> }

In test_load, you can return a nonzero value on MOD_UNLOAD to abort an
unload request.  See the module(9) manpage for more details.  You may
need to increment a counter or hold a mutex while in the syscall to
make it easy for test_load to determine whether it's safe to unload or
not.

-- 
	Dan Nelson
	dnelson@allantgroup.com