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Date: Thu, 27 Jan 2005 11:09:14 -0500
From: David Schultz <das@FreeBSD.ORG>
To: Jacques Fourie <jf@trispen.com>
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cc: freebsd-hackers@FreeBSD.ORG
Subject: Re: kernel vm question
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On Thu, Jan 27, 2005, Jacques Fourie wrote:
> Hi,
> 
> I have a kernel module with the following entry point :
> 
> static int test_modevent(module_t mod, int type, void *unused)
> {
>   int s;
>   unsigned char *p = NULL;
>   unsigned char v = 0x55;
> 
>   switch (type)
>     {
>     case MOD_LOAD:
>       p = (unsigned char *)ip_output;
> 
>       s = splhigh();
> 
>       v = p[0];                   /* Page fault without this line */
>       p[0] = v;
[...]
> If I remove the line "Page fault without this line" line, I get a page 
> fault when loading this module. This crash seems to be hardware as well 
> as version specific - I can not reproduce the crash on 4.8 but on one 
> particular piece of hardware it crashes consistently on 4.9 and 4.10.

When the line is there, the compiler is probably smart enough to
realize that 'x=y; y=x' is (usually) a no-op, so it optimizes away
both statements.  Otherwise, you get a page fault because you're
trying to write to a non-writable page (namely, one in the
kernel's code segment).  If you're trying to do what I think
you're trying to do, you need to poke around in the VM system and
adjust the protection bits in the page you want to write to.