Re: kernel vm question

From: David Schultz (das_at_FreeBSD.ORG)
Date: 01/27/05

  • Next message: Jacques Fourie: "Re: kernel vm question" 
    Date: Thu, 27 Jan 2005 11:09:14 -0500
To: Jacques Fourie <jf@trispen.com>

    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.
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  • Next message: Jacques Fourie: "Re: kernel vm question"

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