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

Chris Wright
2006-05-16T08:37:56Z

* 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