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

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Date:   Tue, 28 Jun 2022 13:38:07 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Christian Brauner' <brauner@...nel.org>,
        Ralph Corderoy <ralph@...utplus.co.uk>
CC:     Matthew Wilcox <willy@...radead.org>,
        Nate Karstens <nate.karstens@...min.com>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        "Jeff Layton" <jlayton@...nel.org>,
        "J. Bruce Fields" <bfields@...ldses.org>,
        "Arnd Bergmann" <arnd@...db.de>,
        Richard Henderson <rth@...ddle.net>,
        "Ivan Kokshaysky" <ink@...assic.park.msu.ru>,
        Matt Turner <mattst88@...il.com>,
        "James E.J. Bottomley" <James.Bottomley@...senpartnership.com>,
        Helge Deller <deller@....de>,
        "David S. Miller" <davem@...emloft.net>,
        Jakub Kicinski <kuba@...nel.org>,
        Eric Dumazet <edumazet@...gle.com>,
        "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        "linux-arch@...r.kernel.org" <linux-arch@...r.kernel.org>,
        "linux-alpha@...r.kernel.org" <linux-alpha@...r.kernel.org>,
        "linux-parisc@...r.kernel.org" <linux-parisc@...r.kernel.org>,
        "sparclinux@...r.kernel.org" <sparclinux@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Changli Gao <xiaosuo@...il.com>
Subject: RE: [PATCH v2] Implement close-on-fork

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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