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    SubjectRe: [Fastboot] [PATCH 1/3] stack overflow safe kdump (2.6.18-rc1-i386) - safe_smp_processor_id
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    Fernando Luis Vázquez Cao <fernando@oss.ntt.co.jp> writes:

    > Hi Keith,
    >
    > Thank you for the comments.
    >
    > On Mon, 2006-07-10 at 18:27 +1000, Keith Owens wrote:
    >> Fernando Luis Vazquez Cao (on Mon, 10 Jul 2006 16:50:52 +0900) wrote:
    >> >On the event of a stack overflow critical data that usually resides at
    >> >the bottom of the stack is likely to be stomped and, consequently, its
    >> >use should be avoided.
    >> >
    >> >In particular, in the i386 and IA64 architectures the macro
    >> >smp_processor_id ultimately makes use of the "cpu" member of struct
    >> >thread_info which resides at the bottom of the stack. x86_64, on the
    >> >other hand, is not affected by this problem because it benefits from
    >> >the use of the PDA infrastructure.
    >> >
    >> >To circumvent this problem I suggest implementing
    >> >"safe_smp_processor_id()" (it already exists in x86_64) for i386 and
    >> >IA64 and use it as a replacement for smp_processor_id in the reboot path
    >> >to the dump capture kernel. This is a possible implementation for i386.
    >>
    >> I agree with avoiding the use of thread_info when the stack might be
    >> corrupt. However your patch results in reading apic data and scanning
    >> NR_CPU sized tables for each IPI that is sent, which will slow down the
    >> sending of all IPIs, not just dump.
    > This patch only affects IPIs sent using send_IPI_allbutself which is
    > rarely called, so the impact in performance should be negligible.

    Well smp_call_function uses it so I don't know if rarely called applies.

    However when called with the NMI vector every instance of send_IPI_allbutself
    transforms this into send_IPI_mask. Which is why we need to know our current
    cpu in the first place.

    Therefore why don't we just do that explicitly in crash.c
    i.e.

    static void smp_send_nmi_allbutself(void)
    {
    cpumask_t mask = cpu_online_map;
    cpu_clear(safe_smp_processor_id(), mask);
    send_IPI_mask(mask, NMI_VECTOR);
    }

    That will guarantee that any effects this code paranoia may have
    are only seen in the crash dump path.


    >> It would be far cheaper to define
    >> a per-cpu variable containing the logical cpu number, set that variable
    >> once as each cpu is brought up and just read it in cases where you
    >> might not trust the integrity of struct thread_info. safe_smp_processor_id()
    >> resolves to just a read of the per cpu variable.
    > But to read a per-cpu variable you need to index the corresponding array
    > with processor id of the current CPU (see code below), but that is
    > precisely what we are trying to figure out. Anyway as
    > send_IPI_allbutself is not a fast path (correct if this assumption is
    > wrong) the current implementation of safe_smp_processor_id should be
    > fine.
    >
    > #define get_cpu_var(var) (*({ preempt_disable();
    > &__get_cpu_var(var); }))
    > #define __get_cpu_var(var) per_cpu(var, smp_processor_id())
    >
    > Am I missing something obvious?

    No. Except that other architectures have cheaper per pointers so they
    don't have that problem.

    Eric
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