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Date: Sat, 7 Sep 2019 03:13:35 +1000
From: Aleksa Sarai <>
To: Jeff Layton <>
Cc: Mickaël Salaün <>,
	Florian Weimer <>,
	Mickaël Salaün <>,, Alexei Starovoitov <>,
	Al Viro <>,
	Andy Lutomirski <>,
	Christian Heimes <>,
	Daniel Borkmann <>,
	Eric Chiang <>,
	James Morris <>, Jan Kara <>,
	Jann Horn <>, Jonathan Corbet <>,
	Kees Cook <>,
	Matthew Garrett <>,
	Matthew Wilcox <>,
	Michael Kerrisk <>,
	Mimi Zohar <>,
	Philippe Trébuchet <>,
	Scott Shell <>,
	Sean Christopherson <>,
	Shuah Khan <>, Song Liu <>,
	Steve Dower <>,
	Steve Grubb <>,
	Thibaut Sautereau <>,
	Vincent Strubel <>,
	Yves-Alexis Perez <>,,,,
Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on

On 2019-09-06, Jeff Layton <> wrote:
> On Fri, 2019-09-06 at 18:06 +0200, Mickaël Salaün wrote:
> > On 06/09/2019 17:56, Florian Weimer wrote:
> > > Let's assume I want to add support for this to the glibc dynamic loader,
> > > while still being able to run on older kernels.
> > > 
> > > Is it safe to try the open call first, with O_MAYEXEC, and if that fails
> > > with EINVAL, try again without O_MAYEXEC?
> > 
> > The kernel ignore unknown open(2) flags, so yes, it is safe even for
> > older kernel to use O_MAYEXEC.
> > 
> Well...maybe. What about existing programs that are sending down bogus
> open flags? Once you turn this on, they may break...or provide a way to
> circumvent the protections this gives.

It should be noted that this has been a valid concern for every new O_*
flag introduced (and yet we still introduced new flags, despite the
concern) -- though to be fair, O_TMPFILE actually does have a
work-around with the O_DIRECTORY mask setup.

The openat2() set adds O_EMPTYPATH -- though in fairness it's also
backwards compatible because empty path strings have always given ENOENT
(or EINVAL?) while O_EMPTYPATH is a no-op non-empty strings.

> Maybe this should be a new flag that is only usable in the new openat2()
> syscall that's still under discussion? That syscall will enforce that
> all flags are recognized. You presumably wouldn't need the sysctl if you
> went that route too.

I'm also interested in whether we could add an UPGRADE_NOEXEC flag to
how->upgrade_mask for the openat2(2) patchset (I reserved a flag bit for
it, since I'd heard about this work through the grape-vine).

Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH

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