Re: [Fastboot] [PATCH 1/3] stack overflow safe kdump (2.6.18-rc1-i386) - safe_smp_processor_id

[Eric W. Biederman]

Fernando Luis Vázquez Cao <fernando@xxxxxxxxxxxxx> 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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