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

From: David Laight <David.Laight@ACULAB.COM>
To: 'Christian Brauner' <brauner@kernel.org>,
	Ralph Corderoy <ralph@inputplus.co.uk>
Cc: Matthew Wilcox <willy@infradead.org>,
	Nate Karstens <nate.karstens@garmin.com>,
	Alexander Viro <viro@zeniv.linux.org.uk>,
	"Jeff Layton" <jlayton@kernel.org>,
	"J. Bruce Fields" <bfields@fieldses.org>,
	"Arnd Bergmann" <arnd@arndb.de>,
	Richard Henderson <rth@twiddle.net>,
	"Ivan Kokshaysky" <ink@jurassic.park.msu.ru>,
	Matt Turner <mattst88@gmail.com>,
	"James E.J. Bottomley" <James.Bottomley@hansenpartnership.com>,
	Helge Deller <deller@gmx.de>,
	"David S. Miller" <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Eric Dumazet <edumazet@google.com>,
	"linux-fsdevel@vger.kernel.org" <linux-fsdevel@vger.kernel.org>,
	"linux-arch@vger.kernel.org" <linux-arch@vger.kernel.org>,
	"linux-alpha@vger.kernel.org" <linux-alpha@vger.kernel.org>,
	"linux-parisc@vger.kernel.org" <linux-parisc@vger.kernel.org>,
	"sparclinux@vger.kernel.org" <sparclinux@vger.kernel.org>,
	"netdev@vger.kernel.org" <netdev@vger.kernel.org>,
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
	Changli Gao <xiaosuo@gmail.com>
Subject: RE: [PATCH v2] Implement close-on-fork
Date: Tue, 28 Jun 2022 13:38:07 +0000	[thread overview]
Message-ID: <35d0facc934748f995c2e7ab695301f7@AcuMS.aculab.com> (raw)
In-Reply-To: <20220628131304.gbiqqxamg6pmvsxf@wittgenstein>

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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