Re: Compat 32-bit syscall entry from 64-bit task!?

[H. Peter Anvin]

On 02/08/2012 08:20 PM, Indan Zupancic wrote:
> 
> CS is already available to user space, but any other value than 0x23 or 0x33
> will confuse user space, as that is all they know about. Apparently Xen uses
> different values, but if those are static then user space can check for them
> separately. But if the values change dynamically then some other way may be
> needed.
> 
> But does it make much sense to pass the CPU mode of user space if that mode
> can be changed at any moment? I don't think it really does. Can you give an
> example of how that info can be used by a ptracer?
> 

Uh... you could make THAT argument about ANY register state!

I believe H.J. can fill you in about the usage.

> 
> Only confusion I can think of is someone following the register values
> across a systemcall instruction. Then the swizzling may be unexpected.
> But if they do that they could check how the sycall was entered and
> compensate for that. (I can't think of any requirement why this would
> need to be race-free.)
> 

You'd have to know how you'd entered, which right now you don't have any
way to know.

	-hpa
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