Messages in this thread | | | Date | Tue, 16 May 2006 01:40:47 -0700 | From | Chris Wright <> | Subject | Re: [PATCH] Gerd Hoffman's move-vsyscall-into-user-address-range patch |
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* 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.
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