[WireGuard] Proposal: HAVE_SEPARATE_IRQ_STACK?

David Daney ddaney at caviumnetworks.com
Thu Nov 10 01:17:39 CET 2016

On 11/09/2016 01:27 PM, Jason A. Donenfeld wrote:
> Hi folks,
> I do some ECC crypto in a kthread. A fast 32bit implementation usually
> uses around 2k - 3k bytes of stack. Since kernel threads get 8k, I
> figured this would be okay. And for the most part, it is. However,
> everything falls apart on architectures like MIPS, which do not use a
> separate irq stack.

Easiest thing to do would be to select 16K page size in your .config, I 
think that will give you a similar sized stack.

>>From what I can tell, on MIPS, the irq handler uses whichever stack
> was in current at the time of interruption. At the end of the irq
> handler, softirqs trigger if preemption hasn't been disabled. When the
> softirq handler runs, it will still use the same interrupted stack. So
> let's take some pathological case of huge softirq stack usage: wifi
> driver inside of l2tp inside of gre inside of ppp. Now, my ECC crypto
> is humming along happily in its kthread, when all of the sudden, a
> wifi packet arrives, triggering the interrupt. Or, perhaps instead,
> TCP sends an ack packet on softirq, using my kthread's stack. The
> interrupt is serviced, and at the end of it, softirq is serviced,
> using my kthread's stack, which was already half full. When this
> softirq is serviced, it goes through our 4 layers of network device
> drivers. Since we started with a half full stack, we very quickly blow
> it up, and everything explodes, and users write angry mailing list
> posts.
> It seems to me x86, ARM, SPARC, SH, ParisC, PPC, Metag, and UML all
> concluded that letting the interrupt handler use current's stack was a
> terrible idea, and instead have a per-cpu irq stack that gets used
> when the handler is service. Whew!
> But for the remaining platforms, such as MIPS, this is still a
> problem. In an effort to work around this in my code, rather than
> having to invoke kmalloc for what should be stack-based variables, I
> was thinking I'd just disable preemption for those functions that use
> a lot of stack, so that stack-hungry softirq handlers don't crush it.
> This is generally unsatisfactory, so I don't want to do this
> unconditionally. Instead, I'd like to do some cludge such as:
>      preempt_disable();
>      #endif
>      some_func_that_uses_lots_of_stack();
>      preempt_enable();
>      #endif
> However, for this to work, I actual need that config variable. Would
> you accept a patch that adds this config variable to the relavent
> platforms? If not, do you have a better solution for me (which doesn't
> involve using kmalloc or choosing a different crypto primitive)?
> Thanks,
> Jason

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