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Subject: Re: [PATCH v2 1/5] fs: Add support for an O_MAYEXEC flag on
 sys_open()
To:     James Morris <jmorris@namei.org>
CC:     Jeff Layton <jlayton@kernel.org>,
        Florian Weimer <fweimer@redhat.com>,
        =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mic@digikod.net>,
        <linux-kernel@vger.kernel.org>, Aleksa Sarai <cyphar@cyphar.com>,
        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>, Jan Kara <jack@suse.cz>,
        Jann Horn <jannh@google.com>, Jonathan Corbet <corbet@lwn.net>,
        Kees Cook <keescook@chromium.org>,
        Matthew Garrett <mjg59@google.com>,
        Matthew Wilcox <willy@infradead.org>,
        Michael Kerrisk <mtk.manpages@gmail.com>,
        Mimi Zohar <zohar@linux.ibm.com>,
        =?UTF-8?Q?Philippe_Tr=c3=a9buchet?= 
        <philippe.trebuchet@ssi.gouv.fr>,
        Scott Shell <scottsh@microsoft.com>,
        Sean Christopherson <sean.j.christopherson@intel.com>,
        Shuah Khan <shuah@kernel.org>,
        Song Liu <songliubraving@fb.com>,
        Steve Dower <steve.dower@python.org>,
        Steve Grubb <sgrubb@redhat.com>,
        Thibaut Sautereau <thibaut.sautereau@ssi.gouv.fr>,
        Vincent Strubel <vincent.strubel@ssi.gouv.fr>,
        Yves-Alexis Perez <yves-alexis.perez@ssi.gouv.fr>,
        <kernel-hardening@lists.openwall.com>, <linux-api@vger.kernel.org>,
        <linux-security-module@vger.kernel.org>,
        <linux-fsdevel@vger.kernel.org>
References: <20190906152455.22757-1-mic@digikod.net>
 <20190906152455.22757-2-mic@digikod.net>
 <87ef0te7v3.fsf@oldenburg2.str.redhat.com>
 <75442f3b-a3d8-12db-579a-2c5983426b4d@ssi.gouv.fr>
 <f53ec45fd253e96d1c8d0ea6f9cca7f68afa51e3.camel@kernel.org>
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 <alpine.LRH.2.21.1909061202070.18660@namei.org>
 <49e98ece-e85f-3006-159b-2e04ba67019e@ssi.gouv.fr>
 <alpine.LRH.2.21.1909090309260.27895@namei.org>
From:   =?UTF-8?Q?Micka=c3=abl_Sala=c3=bcn?= <mickael.salaun@ssi.gouv.fr>
Message-ID: <073cb831-7c6b-1882-9b7d-eb810a2ef955@ssi.gouv.fr>
Date:   Mon, 9 Sep 2019 12:54:16 +0200
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On 09/09/2019 12:12, James Morris wrote:
> On Mon, 9 Sep 2019, Micka=C3=ABl Sala=C3=BCn wrote:
>
>>
>> On 06/09/2019 21:03, James Morris wrote:
>>> On Fri, 6 Sep 2019, Jeff Layton wrote:
>>>
>>>> The fact that open and openat didn't vet unknown flags is really a bug=
.
>>>>
>>>> Too late to fix it now, of course, and as Aleksa points out, we've
>>>> worked around that in the past. Now though, we have a new openat2
>>>> syscall on the horizon. There's little need to continue these sorts of
>>>> hacks.
>>>>
>>>> New open flags really have no place in the old syscalls, IMO.
>>>
>>> Agree here. It's unfortunate but a reality and Linus will reject any su=
ch
>>> changes which break existing userspace.
>>
>> Do you mean that adding new flags to open(2) is not possible?
>>
>> Does it means that unspecified behaviors are definitely part of the
>> Linux specification and can't be fixed?
>
> This is my understanding.
>
>>
>> As I said, O_MAYEXEC should be ignored if it is not supported by the
>> kernel, which perfectly fit with the current open(2) flags behavior, and
>> should also behave the same with openat2(2).
>
> The problem here is programs which are already using the value of
> O_MAYEXEC, which will break.  Hence, openat2(2).

Well, it still depends on the sysctl, which doesn't enforce anything by
default, hence doesn't break existing behavior, and this unused flags
could be fixed/removed or reported by sysadmins or distro developers.


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