Re: [kvm:next 1/1] arch/x86/kvm/emulate.c:232 writeback_registers() error: buffer overflow 'ctxt->_r

[Avi Kivity <avi@redhat.com>]

On 09/12/2012 09:07 AM, Fengguang Wu wrote:
> On Wed, Sep 12, 2012 at 01:58:22PM +0800, Amos Kong wrote:
>> On 11/09/12 22:31, Fengguang Wu wrote:
>> >Hi Avi,
>> >
>> >In the kvm/next branch, sparse warns about
>> >
>> >arch/x86/kvm/emulate.c:232 writeback_registers() error: buffer overflow 'ctxt->_regs' 9 <= 15
>> >
>> >This is because the array definition is ctxt._regs[NR_VCPU_REGS] where
>> >NR_VCPU_REGS=9 for i386 and 17 for x86_64.
>> >
>> >It could be fixed by changing the hard coded 16 to (NR_VCPU_REGS-1).
>> 
>> Hi Fengguang,
>> 
>> You replaced 16 to NR_VCPU_REGS in your patch, not (NR_VCPU_REGS-1).
>> I guess it's a mistake in your commitlog, right?
> 
> 16 = (NR_VCPU_REGS-1). So I mean, if replacing 16 with (NR_VCPU_REGS-1),
> there will be no behavior change for the x86_64 case. However I
> *suspect* the right value is (NR_VCPU_REGS), as I said in the below
> sentence.
> 
>> >And I wonder whether you actually want NR_VCPU_REGS here?
> 
> For your convenience, here is the relevant code for NR_VCPU_REGS:
> 
>         enum kvm_reg {
>                 VCPU_REGS_RAX = 0,
>                 VCPU_REGS_RCX = 1,
>                 VCPU_REGS_RDX = 2,
>                 VCPU_REGS_RBX = 3,
>                 VCPU_REGS_RSP = 4,
>                 VCPU_REGS_RBP = 5,
>                 VCPU_REGS_RSI = 6,
>                 VCPU_REGS_RDI = 7,
>         #ifdef CONFIG_X86_64
>                 VCPU_REGS_R8 = 8,
>                 VCPU_REGS_R9 = 9,
>                 VCPU_REGS_R10 = 10,
>                 VCPU_REGS_R11 = 11,
>                 VCPU_REGS_R12 = 12,
>                 VCPU_REGS_R13 = 13,
>                 VCPU_REGS_R14 = 14,
>                 VCPU_REGS_R15 = 15,
>         #endif 
>                 VCPU_REGS_RIP,
> =>             NR_VCPU_REGS
>         };

The right value is NR-1, since the loop excludes RIP.

Note the warning does not point to an actual error, since the high bits
of regs_dirty will be clear on i386.
-- 
error compiling committee.c: too many arguments to function