Messages in this thread | | | From | (Eric W. Biederman) | Subject | Re: [Fastboot] [PATCH 1/3] stack overflow safe kdump (2.6.18-rc1-i386) - safe_smp_processor_id | Date | Mon, 10 Jul 2006 05:37:26 -0600 |
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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.
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