Hello, Aleksa. On Tue, Oct 15, 2019 at 02:59:31AM +1100, Aleksa Sarai wrote: > On 2019-10-14, Tejun Heo <tj@xxxxxxxxxx> 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. -- tejun
