Openwall GNU/*/Linux - a small security-enhanced Linux distro for servers
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Date: Fri, 6 Sep 2019 18:06:25 +0200
From: Mickaël Salaün <mickael.salaun@....gouv.fr>
To: Florian Weimer <fweimer@...hat.com>, Mickaël Salaün
	<mic@...ikod.net>
CC: <linux-kernel@...r.kernel.org>, Aleksa Sarai <cyphar@...har.com>, Alexei
 Starovoitov <ast@...nel.org>, Al Viro <viro@...iv.linux.org.uk>, Andy
 Lutomirski <luto@...nel.org>, Christian Heimes <christian@...hon.org>, Daniel
 Borkmann <daniel@...earbox.net>, Eric Chiang <ericchiang@...gle.com>, James
 Morris <jmorris@...ei.org>, Jan Kara <jack@...e.cz>, Jann Horn
	<jannh@...gle.com>, Jonathan Corbet <corbet@....net>, Kees Cook
	<keescook@...omium.org>, Matthew Garrett <mjg59@...gle.com>, Matthew Wilcox
	<willy@...radead.org>, Michael Kerrisk <mtk.manpages@...il.com>, Mimi Zohar
	<zohar@...ux.ibm.com>, Philippe Trébuchet
	<philippe.trebuchet@....gouv.fr>, Scott Shell <scottsh@...rosoft.com>, Sean
 Christopherson <sean.j.christopherson@...el.com>, Shuah Khan
	<shuah@...nel.org>, Song Liu <songliubraving@...com>, Steve Dower
	<steve.dower@...hon.org>, Steve Grubb <sgrubb@...hat.com>, Thibaut Sautereau
	<thibaut.sautereau@....gouv.fr>, Vincent Strubel
	<vincent.strubel@....gouv.fr>, Yves-Alexis Perez
	<yves-alexis.perez@....gouv.fr>, <kernel-hardening@...ts.openwall.com>,
	<linux-api@...r.kernel.org>, <linux-security-module@...r.kernel.org>,
	<linux-fsdevel@...r.kernel.org>
Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on
 sys_open()


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.

>
> Or do I risk disabling this security feature if I do that?

It is only a security feature if the kernel support it, otherwise it is
a no-op.

>
> Do we need a different way for recognizing kernel support.  (Note that
> we cannot probe paths in /proc for various reasons.)

There is no need to probe for kernel support.

>
> Thanks,
> Florian
>

--
Mickaël Salaün

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