Re: [PATCH] Gerd Hoffman's move-vsyscall-into-user-address-range patch
From: Chris Wright
Date: Tue May 16 2006 - 04:37:18 EST
* Zachary Amsden (zach@xxxxxxxxxx) 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.
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