linux-kernel - Re: CAP_SYSLOG, 2.6.38 and user space

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Date:	Wed, 9 Feb 2011 14:27:01 -0800 (PST)
From:	david@...g.hm
To:	Gergely Nagy <algernon@...abit.hu>
cc:	"Serge E. Hallyn" <serge@...lyn.com>,
	James Morris <jmorris@...ei.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: CAP_SYSLOG, 2.6.38 and user space

On Wed, 9 Feb 2011, Gergely Nagy wrote:

> On Wed, 2011-02-09 at 13:47 -0800, david@...g.hm wrote:
>> On Wed, 9 Feb 2011, Gergely Nagy wrote:
>>
>>> On Wed, 2011-02-09 at 13:34 -0800, david@...g.hm wrote:
>>>> On Wed, 9 Feb 2011, Gergely Nagy wrote:
>>>>
>>>>> On Wed, 2011-02-09 at 21:23 +0000, Serge E. Hallyn wrote:
>>>>>> So if that's how we're leaning, then the following patch is much more
>>>>>> concise.  I'll send this to Linus and any appropriate -stable tomorrow
>>>>>>  if noone objects.
>>>>>>
>>>>>> From 5166e114d6a7c508addbadd763322089eb0b02f5 Mon Sep 17 00:00:00 2001
>>>>>> From: Serge Hallyn <serge@...lyn.com>
>>>>>> Date: Thu, 3 Feb 2011 09:26:15 -0600
>>>>>> Subject: [PATCH 1/1] cap_syslog: don't refuse cap_sys_admin for now (v2)
>>>>>>
>>>>>> It'd be nice to do that later, but it's not strictly necessary,
>>>>>> and it'll be hard to do without breaking somebody's userspace.
>>>>>>
>>>>>> Signed-off-by: Serge Hallyn <serge@...lyn.com>
>>>>>> ---
>>>>>>  kernel/printk.c |   14 ++++----------
>>>>>>  1 files changed, 4 insertions(+), 10 deletions(-)
>>>>>
>>>>> Personally, I'd prefer the sysctl idea in the long run, because
>>>>> userspace can easily and automatically adapt to the running kernel then.
>>>>> Ie, this patch is fine for 2.6.38, but later on, a sysctl could be
>>>>> introduced, that when set (but defaulting to unset, as to not break
>>>>> userspace), would make CAP_SYS_ADMIN return -EPERM. That way, syslogds
>>>>> could look at the setting, and act accordingly. This would mean that old
>>>>> userspace wouldn't break, and upgraded userspace could work on both old
>>>>> and new kernels, depending on the setting. Distros or admins could then
>>>>> enable the sysctl once they made sure that all neccessary applications
>>>>> have been upgraded.
>>>>
>>>> what is your justification for ever having CAP_SYS_ADMIN return -EPERM?
>>>> what's the value in blocking this.
>>>
>>> Nothing. Come to think of it, the main use of the sysctl would be to
>>> detect CAP_SYSLOG support, so that applications can drop CAP_SYS_ADMIN
>>> and use CAP_SYSLOG only (which, imo, is a good idea - the less
>>> capabilities, the better, and CAP_SYS_ADMIN is quite broad when one only
>>> wants CAP_SYSLOG).
>>>
>>> If there's a better way to allow userspace to easily detect CAP_SYSLOG,
>>> I'm all for that.
>>
>> if userspace wants to detect this, what is wrong with them checking for a
>> kernel >= 2.6.38?
>
> How do I do that, apart from parsing utsname, which I find insultingly
> ugly? It might be just me, but I very much prefer feature tests over
> version sniffing.
>
>> realistically, if the upstream applications (which need to work with many
>> different versions) just support having CAP_SYS_ADMIN, it would be a very
>> minor distro patch to change this to CAP_SYSLOG for a distro release where
>> the distro _knows_ that they don't have to support an older kernel.
>
> That is, indeed, true, and works for distros. But when a software vendor
> provides binaries aswell as source, they do have to support older
> kernels too. And even if that is possible with CAP_SYS_ADMIN, I'd still
> prefer CAP_SYSLOG, if available.
>
> Thus, being able to easily adapt is something I'm very interested in. If
> that's not possible, using CAP_SYS_ADMIN for a long long time still is
> the second best option.

what's wrong with doing a runtime test at startup that tries to read with 
CAP_SYS_ADMIN and if you get -EPERM trying again with CAP_SYSLOG?

creating an ioctl for something like this seems like it's significantly 
overcomplicating the issue.

> I also wish to place as little burden on distros as possible, so
> delegating the decision to them does not appeal to me that much. It's
> certainly an option, but I'm sure we can do better than that.

but why maintain an ioctl forever for something that nobody should care 
about in a few years?

David Lang
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