Hello, Aleksa.
On Tue, Oct 15, 2019 at 02:59:31AM +1100, Aleksa Sarai wrote:
On 2019-10-14, Tejun Heo tj@kernel.org wrote:
On Sat, Oct 12, 2019 at 12:05:39PM +1100, Aleksa Sarai wrote:
Because pids->limit can be changed concurrently (but we don't want to take a lock because it would be needlessly expensive), use the appropriate memory barriers.
I can't quite tell what problem it's fixing. Can you elaborate a scenario where the current code would break that your patch fixes?
As far as I can tell, not using *_ONCE() here means that if you had a process changing pids->limit from A to B, a process might be able to temporarily exceed pids->limit -- because pids->limit accesses are not protected by mutexes and the C compiler can produce confusing intermediate values for pids->limit[1].
But this is more of a correctness fix than one fixing an actually exploitable bug -- given the kernel memory model work, it seems like a good idea to just use READ_ONCE() and WRITE_ONCE() for shared memory access.
READ/WRITE_ONCE provides protection against compiler generating multiple accesses for a single operation. It won't prevent split writes / reads of 64bit variables on 32bit machines. For that, you'd have to switch them to atomic64_t's.
Thanks.