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


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