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    SubjectRe: Wanted: simple, safe x86 stack overflow detection
    Bill Irwin wrote:
    >> Chuck, is any of this of any use to you?

    On Tue, Mar 06, 2007 at 01:59:39PM -0500, Chuck Ebbert wrote:
    > I said "simple." :)
    > In the 4k/4k stack i386 kernel, is there any fundamental reason it
    > can't be 4k/8k? We seem to be mostly hitting problems in overflowing
    > the IRQ stack... I think. Overhead would only be 4k per CPU for that.

    current is derived from current_thread_info()->task which is in turn
    derived from the stack pointer. Using segmentation to derive per-thread
    state would allow i386 to make IRQ and task stack-sizes independent.
    There are beginnings of this in the i386 PDA code.

    A patch to allow 8K/8K is relatively simple and may serve your
    purposes. vmalloc() of the task and IRQ stacks has negative
    performance and resource scalability implications, though it's not
    particularly involved. It also ensures no memory corruption can occur
    from stack overflow save in the case of stack frames larger than a
    page, where furthermore the procedure references distant locations
    prior to close ones, such as a reverse array walk. Allowing larger
    stacksizes than 8K doesn't seem to be of interest to you, probably due
    to the negative resource scalability implications, though it may
    stabilize some workloads.

    I'm not sure what to say on the simplicity front apart from patches
    consisting largely of Kconfig edits are demonstrably simple. There
    really wasn't much to any of this from my POV, but I do understand
    that others may differ particularly on the stack vmalloc() front.
    The individual patches of the series are mostly independent save for
    textual affairs, and I intended for users to cherrypick whatever
    strategies they preferred as per the config options provided. An
    individual patch for the preferred strategy can also be respun in
    isolation, if any of those provided are considered desirable.

    Otherwise I suppose I can actually make use of the PDA to make the
    IRQ stacksize independent of the task stacksize.


    -- wli
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