Re: [Xen-devel] [PATCH] xen/kexec: Clear unused registers before jumping into an image

[David Vrabel <david.vrabel@xxxxxxxxxx>]

On 18/11/13 09:29, Jan Beulich wrote:
>>>> On 15.11.13 at 21:07, David Vrabel <david.vrabel@xxxxxxxxxx> wrote:
>> On 15/11/13 15:56, Daniel Kiper wrote:
>>> Clear unused registers before jumping into an image. This way
>>> loaded image could not assume that any register has an specific
>>> info about earlier running Xen hypervisor. However, it also
>>> does not mean that the image may expect that a given register
>>> is zeroed. The image MUST assume that every register has a random
>>> value or in other words it is uninitialized or has undefined state.
>>
>> I think this, where the specification (registers undefined) differs from
>> the implementation (registers zeroed), is the worst option.
>>
>> I also think it is more likely for an image to inadvertently rely on a
>> zero value that whatever junk Xen has left behind.
> 
> Preventing users to rely on anything would likely make it
> desirable to put some random value into all unused registers.

I don't think we need to go that far.

I would just like to avoid someone looking that the implementation (and
not the documentation) and concluding that zero-ing of the registers is
part of the specified behaviour, or looking at the implementation and
documentation and wondering why they don't agree.

I really don't think there is anything more to be said on this.

David

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