Re: crash in entry.S restore_all, 2.6.12-rc2, x86, PAGEALLOC

From: Stas Sergeev (stsp_at_aknet.ru)
Date: 04/07/05

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    Date:	Thu, 07 Apr 2005 20:11:43 +0400
To: Ingo Molnar <mingo@elte.hu>

    Hello.

    Ingo Molnar wrote:
    > now if an interrupt hits at this point, it will set up a 'same privilege
    > level' stackframe, which has eip/xcs/eflags, i.e. no esp/xss.
    Yes, that's something I tried to say
    when talking about the interrupt gates
    (sorry if I wasn't clear).

    > If upon
    > irq-return we then examine the stack due to your patch, it will be an
    > incorrect stackframe -> kaboom.
    Yes, and that's where I think my patch is
    at fault, i.e. it just shouldn't do this.
    Another option is to make it always possible
    to access OLDSS(%esp), but I think it is just
    my patch have to be fixed to not do this at all.

    > your patch doesnt remove the condition, it only removes the crash,
    No, that wasn't my point at all. That example
    with moving "sti" was *only* to answer Linus's
    question of where we have an empty stack.
    And I guess I wasn't clear enough also here,
    I was in a hurry :(
    The real patch I meant, is this one:
    http://www.uwsg.iu.edu/hypermail/linux/kernel/0504.0/1287.html
    This, I am sure, fixes a real bug. But there
    can be the other approaches too.

    > because it adds the 2 words space that is needed - but the information
    > relied on by your irq-return test is still bogus.
    But as an example for demonstrating the problem,
    I thought, it could do:)

    > At this point i'd
    > suggest to remove the ESP patch altogether.
    That's probably too heavy-handed. The fix is
    really simple, we can either store the right
    values by hands (as you propose), or fix my
    patch (as I propose).

    > the correct solution is to always let the sysenter path set up a full
    > and correct stackframe,
    But what will this solve? If I understand you
    correctly, you will push the %ss/%esp of the
    user-space process that did sysenter. Then
    you enable the interrupts and get pre-empted.
    Now what we have: OLDSS/OLDESP are of the
    user-space process, but the EFLAGS/CS/EIP
    are of the kernel (where it got pre-empted
    on a sysenter path). This will avoid the crash,
    but the information on stack is still wrong.
    Or am I missing something?

    > before allowing preemption (see the attached
    > patch).
    Hmm, will it work also for NMIs? You move
    the sti, you can't be pre-empted, but the
    NMI uses the restore_all too, no?
    Also, it seems that Linus wants only the
    *some* values available on stack, just to
    make it not to crash. I think we can simply
    adjust the tss.esp0 to point 8 bytes below
    the real stack top, and so we are always safe.

    -
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