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From: David Howells <dhowells@redhat.com>
In-Reply-To: <20090105190722.GA11087@us.ibm.com>
References: <20090105190722.GA11087@us.ibm.com> <20090105031827.GA10185@us.ibm.com> <24959.1230694093@redhat.com> <20081230134248.GA30124@lst.de> <21275.1230736542@redhat.com> <alpine.LRH.1.10.0901051301590.14359@tundra.namei.org> <28352.1231159413@redhat.com>
To: "Serge E. Hallyn" <serue@us.ibm.com>
Cc: dhowells@redhat.com, James Morris <jmorris@namei.org>,
       Christoph Hellwig <hch@lst.de>, linux-kernel@vger.kernel.org,
       linux-fsdevel@vger.kernel.org, linux-security-module@vger.kernel.org,
       Stephen Rothwell <sfr@canb.auug.org.au>,
       Andrew Morgan <morgan@kernel.org>
Subject: Re: [PATCH] CRED: Fix regression in cap_capable() as shown up by sys_faccessat() [ver #2]
Date: Mon, 05 Jan 2009 21:12:16 +0000
Message-ID: <6521.1231189936@redhat.com>
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 2037
Lines: 57

Serge E. Hallyn <serue@us.ibm.com> wrote:

> You have the 'acting_as' name for subj/eff, which I like.  Is there
> another name you could use in place of 'real' in the  name
> task_real_capable()?

Ummm...  'Actual' or 'Assigned' perhaps?

> I do find this version much easier to read.  It seems easier to
> track capable+current_cred() vs real_capable+get_task_cred().  Could
> you do a few benchmarks to gauge whether the difference the
> optimization makes?

Yeah...  My main objection is passing around two or three superfluous arguments
in the common case.  Most of the time, the only necessary argument to
sec->capable():

	int (*capable) (struct task_struct *tsk, const struct cred *cred,
			int cap, int audit);

is cap; tsk, cred and audit are all superfluous in the (very) common case.

How about:

	int (*fast_capable) (int cap);

which assumes current, current_cred() and SECURITY_CAP_AUDIT?

Benchmarking is tricky, given that the individual savings will be relatively
small in comparison to the code that calls them.

However, if I can get rid of three arguments passed into each of
security_capable(), selinux_capable() and cap_capable(), that really should
speed things up if you call it enough times, especially as current is held in a
register on some archs.

I'll see what I can do.

> I'm looking at a several-week-old linux-next, but only see one use of
> capable on another task which audits, and that is in commoncap for
> traceme, so it seems reasonable.

Should has_capability() be out of lines and have security_real_capable() merged
into it?  And the same for has_capability_noaudit() and
security_real_capable_noaudit()?

> So yeah, I do like this version better.

Perhaps a separate patch to optimise capable().  As I said, I'll see about
benchmarking it.

David
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