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Date:   Sat, 7 Sep 2019 03:07:39 +1000
From:   Aleksa Sarai <cyphar@cyphar.com>
To:     =?utf-8?Q?Micka=C3=ABl_Sala=C3=BCn?= <mickael.salaun@ssi.gouv.fr>
Cc:     Florian Weimer <fweimer@redhat.com>,
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        linux-kernel@vger.kernel.org, Alexei Starovoitov <ast@kernel.org>,
        Al Viro <viro@zeniv.linux.org.uk>,
        Andy Lutomirski <luto@kernel.org>,
        Christian Heimes <christian@python.org>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Eric Chiang <ericchiang@google.com>,
        James Morris <jmorris@namei.org>, Jan Kara <jack@suse.cz>,
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        Matthew Garrett <mjg59@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Mimi Zohar <zohar@linux.ibm.com>,
        Philippe =?utf-8?Q?Tr=C3=A9buchet?= 
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        Song Liu <songliubraving@fb.com>,
        Steve Dower <steve.dower@python.org>,
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        Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>,
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        kernel-hardening@lists.openwall.com, linux-api@vger.kernel.org,
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        linux-fsdevel@vger.kernel.org
Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on
 sys_open()
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On 2019-09-06, Micka=EBl Sala=FCn <mickael.salaun@ssi.gouv.fr> wrote:
>=20
> 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?
>=20
> The kernel ignore unknown open(2) flags, so yes, it is safe even for
> older kernel to use O_MAYEXEC.

Depends on your definition of "safe" -- a security feature that you will
silently not enable on older kernels doesn't sound super safe to me.
Unfortunately this is a limitation of open(2) that we cannot change --
which is why the openat2(2) proposal I've been posting gives -EINVAL for
unknown O_* flags.

There is a way to probe for support (though unpleasant), by creating a
test O_MAYEXEC fd and then checking if the flag is present in
/proc/self/fdinfo/$n.

--=20
Aleksa Sarai
Senior Software Engineer (Containers)
SUSE Linux GmbH
<https://www.cyphar.com/>

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