linux-kernel - Re: KVM overflows the stack

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Message-ID: <487F53FB.20708@qumranet.com>
Date:	Thu, 17 Jul 2008 17:15:23 +0300
From:	Avi Kivity <avi@...ranet.com>
To:	Dave Hansen <dave@...ux.vnet.ibm.com>
CC:	Roland Dreier <rdreier@...co.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	kvm-devel <kvm@...r.kernel.org>,
	"Anthony N. Liguori [imap]" <aliguori@...ibm.com>
Subject: Re: KVM overflows the stack

Dave Hansen wrote:
> On Wed, 2008-07-16 at 23:08 -0700, Roland Dreier wrote:
>   
>>  > Yes, things like kvm_lapic_state are way too big to be on the
>> stack.
>>
>> I had a quick look at the code, and my worry about dynamic allocation
>> would be that handling allocation failure seems like it might get
>> tricky.  Eg for handling struct kvm_pv_mmu_op_buffer (which is 528 bytes
>> on the stack in kvm_pv_mmu_op()) can you deal with an mmu op failing?
>>     
>
> Well, you *better* be able to deal with it.  :)
>
> This code is also doing a *ton* of copy_to/from_user().  If userspace
> had one of its input buffers swapped out (or one of its output buffers
> not faulted in yet) and we're out of memory enough to be failing
> kmallocs() then we're sure as heck also going to failing the user
> copies.
>
> I think it's a non-issue.
>
>   

Yes, it's designed to be restartable.  Returning 0 should be fine.

We can reduce the buffer size to 256 though.  I wouldn't want an 
allocation in this hot path.

-- 
error compiling committee.c: too many arguments to function

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