RE: [PATCH v2] Implement close-on-fork

David Laight
2022-06-28T13:41:01Z

From: Christian Brauner
> Sent: 28 June 2022 14:13
>
> On Sun, Jun 19, 2022 at 11:42:28AM +0100, Ralph Corderoy wrote:
> > Hi Matthew, thanks for replying.
> >
> > > > The need for O_CLOFORK might be made more clear by looking at a
> > > > long-standing Go issue, i.e. unrelated to system(3), which was started
> > > > in 2017 by Russ Cox when he summed up the current race-condition
> > > > behaviour of trying to execve(2) a newly created file:
> > > > https://github.com/golang/go/issues/22315.
> > >
> > > The problem is that people advocating for O_CLOFORK understand its
> > > value, but not its cost. Other google employees have a system which
> > > has literally millions of file descriptors in a single process.
> > > Having to maintain this extra state per-fd is a cost they don't want
> > > to pay (and have been quite vocal about earlier in this thread).
> >
> > So do you agree the userspace issue is best solved by *_CLOFORK and the
> > problem is how to implement *_CLOFORK at an acceptable cost?
> >
> > OTOH David Laight was making suggestions on moving the load to the
> > fork/exec path earlier in the thread, but OTOH Al Viro mentioned a
> > ‘portable solution’, though that could have been to a specific issue
> > rather than the more general case.
> >
> > How would you recommend approaching an acceptable cost is progressed?
> > Iterate on patch versions? Open a bugzilla.kernel.org for central
> > tracking and linking from the other projects? ..?
>
> Quoting from that go thread
>
> "If the OS had a "close all fds above x", we could use that. (I don't know of any that do, but it sure
> would help.)"
>
> So why can't this be solved with:
> close_range(fd_first, fd_last, CLOSE_RANGE_CLOEXEC | CLOSE_RANGE_UNSHARE)?
> e.g.
> close_range(100, ~0U, CLOSE_RANGE_CLOEXEC | CLOSE_RANGE_UNSHARE)?

That is a relatively recent linux system call.
Although it can be (mostly) emulated by reading /proc/fd
- but that may not be mounted.

In any case another thread can open an fd between the close_range()
and fork() calls.

(I can't remember what I said before :-)

David

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