Chris Wright wrote:
* Zachary Amsden ([email protected]) 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));
Ok, I'm confused. What fixed math? The return EIP that is pushed here
is used when sysenter is active and you have to IRET back to userspace.
If that EIP is dynamically relocatable, you can't do fixed math unless
you patch the pushl site dynamically. Notable reasons for returning via
IRET on this fake exception frame were (until my recent submission) IOPL
changes, but I believe there were more. I will have to inspect the
source to determine if that is still the case.
Zach
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