Re: [PATCH] Fix argument checking in sched_setaffinity

[Paul Jackson <pj@sgi.com>]

Linus wrote:
>	/* We just assume that 8k CPU's aren't going to happen */

SGI doesn't so assume ;).


> but it's just a lot easier to do the "getaffinity" thing - if it fails,
> you can double the size of your buffer and try again. O(log(n)) rather
> than O(n) ;)

I agree.  That's what my cpumask sizing loop does.

Well ... did.

Now it reads /sys/devices/system/node/node0/cpumap and computes the
size of the cpumask as an arithmetic function of the number of bytes
read (the ascii format uses 9 chars for each 32 bits of mask).

Either way works ...

My nodemask sizing code loops on get_mempolicy() calls of increasing
size, until they stop failing -EINVAL.


> Well, historically we _have_ required sizes to match.

I'm not sure what history you're looking at here, Linus.

Last weeks sys_sched_setaffinity didn't seem to require matching size,
only that user size is >= kernel size.  The kernel ignored the extra
user bits.

For nodemask_t, well let me just say the mbind/mempolicy calls are different.

If we want to go in the direction of requiring sizes to match in the
'set' calls, then instead of this weeks changes to sys_sched_setaffinity
allowing user size < kernel size, shouldn't we be going the other way,
and tightening the check in kernel/sched.c:sys_sched_setaffinity(), from
what it was a week ago:

        if (len < sizeof(new_mask))
                return -EINVAL;

to:

        if (len != sizeof(new_mask))
                return -EINVAL;

Or at least reverting this last weeks changes back to the '<' check?


> I don't know how to sanely expose the damn things

How about:

	$ cd /proc/sys/kernel
	$ head sizeof*
	==> sizeof_cpumask <==
	64

	==> sizeof_nodemask <==
	32

-- 
                          I won't rest till it's the best ...
                          Programmer, Linux Scalability
                          Paul Jackson <pj@sgi.com> 1.650.933.1373