From: Christian Brauner

Sent: 28 June 2022 14:13

On Sun, Jun 19, 2022 at 11:42:28AM +0100, Ralph Corderoy wrote:

quoted

Hi Matthew, thanks for replying.

quoted

quoted

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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