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    SubjectRe: [uml-devel] SYSCALL, ptrace and syscall restart breakages (Re: [RFC] weird crap with vdso on uml/i386)
    On 08/23/2011 09:48 AM, Al Viro wrote:
    > Um... How would it know which syscall variant had that been, to start
    > with? For int 0x80 it would need to use registers as-is. For SYSENTER
    > it also could use them as-is - ebp will differ from what we put there
    > when entering the sucker, but not critically so; on the way out of
    > syscall we'll overwrite it anyway immediately (either by pop or mov).
    > For SYSCALL... we don't really care about ecx contents prior to entering
    > the kernel (and it'll be blown out anyway), and ebp one could be found in
    > regs.ecx. So yes, we can do it that way, but... how to tell what variant
    > had been triggered? Examining two bytes prior to user eip? Sounds bloody
    > brittle...

    We could drop that information in a metaregister. It's not backward
    compatible, but at least it will be obvious when that information is
    available and not.


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