Re: KVM overflows the stack

Avi Kivity
2008-07-17T05:52:40Z

Dave Hansen wrote:
> On Wed, 2008-07-16 at 14:44 -0700, Dave Hansen wrote:
>
>> On a suggestion of Anthony's, I tried a defconfig kernel.
>>
>> It is now bombing out on an assertion in the lapic code:
>>
>> http://sr71.net/~dave/linux/2.6.26-oops1.txt
>>
>
> I think I found it!!!
>
> $ (objdump -d kvm.ko ; objdump -d kvm-intel.ko ) | egrep 'sub.*0x...,.*esp|>:' | egrep sub -B1
> 00001a90 <kvm_vcpu_ioctl>:
> 1a9a: 81 ec 60 06 00 00 sub $0x660,%esp
> --
> 00004e90 <kvm_arch_vcpu_ioctl>:
> 4e9d: 81 ec 6c 08 00 00 sub $0x86c,%esp
> --
> 00005900 <kvm_arch_vm_ioctl>:
> 5903: 81 ec 34 05 00 00 sub $0x534,%esp
> --
> 0000d4f0 <paging64_prefetch_page>:
> d4f8: 81 ec 1c 01 00 00 sub $0x11c,%esp
> --
> 0000dfd0 <paging32_prefetch_page>:
> dfd8: 81 ec 1c 01 00 00 sub $0x11c,%esp
> --
> 0000f390 <kvm_pv_mmu_op>:
> f3a1: 81 ec 28 02 00 00 sub $0x228,%esp
>
> We're simply overflowing the stack. I changed all of the large on-stack
> allocations to 'static', and it actually boots now. I know 'static'
> isn't safe, but it was good for a quick test.
>
>

Yes! It's obvious, once you know it...

> A 'make stackcheck' confirms this:
>
> dave@nimitz:~/kernels/linux-2.6.git$ make checkstack
> objdump -d vmlinux $(find . -name '*.ko') | \
> perl /home/dave/kernels/linux-2.6.git-t61/scripts/checkstack.pl i386
> 0x000042d3 kvm_arch_vcpu_ioctl [kvm]: 2148
> 0x000012e3 kvm_vcpu_ioctl [kvm]: 1620
> 0x00004a83 kvm_arch_vm_ioctl [kvm]: 1332
> 0x00009a26 airo_get_aplist [airo]: 1140
> 0x00009b76 airo_get_aplist [airo]: 1140
> 0x00009c82 airo_get_aplist [airo]: 1140
> ...
>
> In other words, kvm has the top 3 stack users in my kernel. As you can
> see from my trace above, these things also get called with super-long
> stacks already. Man. That sucked to find.
>
> Avi, how would you like this fixed? I'd be happy to prepare some
> patches. Do you have a particular approach that you think we should
> use? Just make the big objects dynamically allocated?
>

Yes, things like kvm_lapic_state are way too big to be on the stack.
There's an additional problem here, that apparently your gcc (which
version?) doesn't fold objects in a switch statement into the same stack
slot:

switch (...) {
case x: {
struct medium a;
...
}
case y:
struct medium b;
...
}
};

These could be solved either by stack allocation, or by moving into
functions marked noinline. Whichever is easier.

--
I have a truly marvellous patch that fixes the bug which this
signature is too narrow to contain.