Re: [RFC]: mm,power: introduce MADV_WIPEONSUSPEND

Michal Hocko
2020-07-07T07:44:30Z

On Mon 06-07-20 14:52:07, Jann Horn wrote:
> On Mon, Jul 6, 2020 at 2:27 PM Alexander Graf <graf@amazon.com> wrote:
> > Unless we create a vsyscall that returns both the PID as well as the
> > epoch and thus handles fork *and* suspend. I need to think about this a
> > bit more :).
>
> You can't reliably detect forking by checking the PID if it is
> possible for multiple forks to be chained before the reuse check runs:
>
> - pid 1000 remembers its PID
> - pid 1000 forks, creating child pid 1001
> - pid 1000 exits and is waited on by init
> - the pid allocator wraps around
> - pid 1001 forks, creating child pid 1000
> - child with pid 1000 tries to check for forking, determines that its
> PID is 1000, and concludes that it is still the original process

I must be really missing something here because I really fail to see why
there has to be something new even invented. Sure, checking for pid is
certainly a suboptimal solution because pids are terrible tokens to work
with. We do have a concept of file descriptors which a much better and
supports signaling. There is a clear source of the signal IIUC
(migration) and there are consumers to act upon that (e.g. crypto
backends). So what does really prevent to use a standard signal delivery
over fd for this usecase?
--
Michal Hocko
SUSE Labs