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

Matthew Wilcox
2022-06-18T19:41:19Z

On Sat, Jun 18, 2022 at 12:41:11PM +0100, Ralph Corderoy wrote:
> Hi Nate,
>
> > One manifestation of this is a race conditions in system(), which
> > (depending on the implementation) is non-atomic in that it first calls
> > a fork() and then an exec().
>
> 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. I raised it on linux-kernel
> in 2017, https://marc.info/?l=linux-kernel&m=150834137201488, and linked
> to a proposed patch from 2011, ‘[PATCH] fs: add FD_CLOFORK and
> O_CLOFORK’ by Changli Gao. As I said, long-standing.

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).

Fundamentally, fork()+exec() is a terrible model. Mind you, so is
spawn(). I haven't seen a good model yet.