Re: [PATCH] Gerd Hoffman's move-vsyscall-into-user-address-range patch

[Chris Wright <chrisw@sous-sol.org>]

* Zachary Amsden (zach@vmware.com) wrote:
> Let's dive into it.  How do you get the randomization without 
> sacrificing syscall performance?  Do you randomize on boot, dynamically, 
> or on a per-process level?

The latter, on exec.

> Because I can see some issues with 
> per-process randomization that will certainly cost some amount of cycles 
> on the system call path.  Marginal perhaps, but that is exactly where 
> you don't want to shed cycles unnecessarily, and the complexity of the 
> whole thing will go up quite a bit I think.

The crux is here:

+       OFFSET(TI_sysenter_return, thread_info, sysenter_return);
...

-       pushl $SYSENTER_RETURN
-
+       /*
+        * Push current_thread_info()->sysenter_return to the stack.
+        * A tiny bit of offset fixup is necessary - 4*4 means the 4 words
+        * pushed above; +8 corresponds to copy_thread's esp0 setting.
+        */
+       pushl (TI_sysenter_return-THREAD_SIZE+8+4*4)(%esp)

...

and in binfmt_elf during exec thread_info->sysenter_return is setup
based on the randomized mapping it does for vdso

+               ti->sysenter_return = &SYSENTER_RETURN_OFFSET + addr;


I think it's not so bad, but I can't say I've benchmarked the cost.

thanks,
-chris