linux-kernel - KVM overflows the stack

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Date:	Wed, 16 Jul 2008 15:48:47 -0700
From:	Dave Hansen <dave@...ux.vnet.ibm.com>
To:	Avi Kivity <avi@...ranet.com>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm-devel <kvm@...r.kernel.org>,
	"Anthony N. Liguori [imap]" <aliguori@...ibm.com>
Subject: KVM overflows the stack

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.

A 'make stackcheck' confirms this:

dave@...itz:~/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?

-- Dave

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