On 07.07.20 09:44, Michal Hocko wrote:

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On Mon 06-07-20 14:52:07, Jann Horn wrote:

quoted

On Mon, Jul 6, 2020 at 2:27 PM Alexander Graf [off-list ref] wrote:

quoted

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?

I wasn't part of the discussions on why things like WIPEONFORK were 
invented instead of just using signalling mechanisms, but the main 
reason I can think of are libraries.

As a library, you are under no control of the main loop usually, which 
means you just don't have a way to poll for an fd. As a library author, 
I would usually try to avoid very hard to create such a dependency, 
because it makes it really hard to glue pieces together.

The same applies to signals btw, which would also be a possible way to 
propagate such events.


Alex



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