RE: Per-Processor Data Page

Bret Indrelee (bindrele@sbs.com)
Thu, 9 Dec 1999 16:32:31 -0600

Linus Torvalds [mailto:torvalds@transmeta.com] wrote:
> On 9 Dec 1999, Andi Kleen wrote:
> >
> > I've seen gettimeofday() often as a example here in this
> thread for magic
> > user process mappings, and I think that is wrong.
> gettimeofday() nowadays
> > uses the CPU internal cycle counter to get very accurate
> timings. You would
> > never be able to implement a counter in some memory
> location as accurately
> > as the CPU does. Reading it requires some internal
> knowledge though (e.g.
> > the delta between the counters on SMP systems and thus the
> current CPU cpu),
> > which are better left in a system call.
>
> Note that I will NOT EVER accept a patch that exports kernel data
> structures to user space this way. No way will I have
> something that says
> "at magic address 0xffff002 can you find the current time".
> That is just
> broken.

The other thing to consider is if you return time as accurate as the CPU
cycle counter, you have just created a mechanism for a covert information
channel. One of the things that surprised me was that the higher security
levels do not want a process to be able to get a highly accurate time.

If processes can get a highly accurate time value from some sort of global
clock, it allows a pair of processes to create a covert channel for passing
information. The less secure program monitors the time variences of the
high-security program in order to get information about or from them.

-Bret

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