[Xen-devel] Re: [RFC, PATCH 7/24] i386 Vmi memory hole

[Chris Wright <chrisw@xxxxxxxxxxxx>]

* Zachary Amsden (zach@xxxxxxxxxx) wrote:
> ENTRY(sysenter_entry)
>        movl TSS_sysenter_esp0(%esp),%esp
> sysenter_past_esp:
>        STI
>        pushl $(__USER_DS)
>        pushl %ebp
>        pushfl
>        pushl $(__USER_CS)
>        pushl $SYSENTER_RETURN
> 
> SYSENTER_RETURN is a link time constant that is defined based on the 
> location of the vsyscall page.  If the vsyscall page can move, this can 
> not be a constant.  The reason is, this "fake" exception frame is used 
> to return back to the EIP of the call site, and sysenter does not record 
> the EIP of the call site.

It's only real issue for something like execshield.  For this it's easy
to do the fixed math since it's still at fixed address.

+       DEFINE(VSYSCALL_BASE, (PAGE_OFFSET - 2*PAGE_SIZE));

But execshield has to make SYSENTER_RETURN context sensitive to current
since the vdso is mapped at random location.

thanks,
-chris

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