linux-kernel - Re: PATCH [1/1]: audit: acquire creds selectively to reduce atomic op overhead

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Date:	Thu, 10 Mar 2011 12:25:17 -0800
From:	Tony Jones <tonyj@...e.de>
To:	David Howells <dhowells@...hat.com>
Cc:	linux-kernel@...r.kernel.org, linux-audit@...hat.com,
	Eric Paris <eparis@...hat.com>,
	Al Viro <viro@...iv.linux.org.uk>
Subject: Re: PATCH [1/1]: audit: acquire creds selectively to reduce atomic
 op overhead

On Tue, Mar 08, 2011 at 06:02:53PM +0000, David Howells wrote:
> Tony Jones <tonyj@...e.de> wrote:
> 
> > Commit c69e8d9c01db added calls to get_task_cred and put_cred in
> > audit_filter_rules.  Profiling with a large number of audit rules active on
> > the exit chain shows that we are spending upto 48% in this routine for
> > syscall intensive tests, most of which is in the atomic ops.
> > 
> > The following patch acquires the cred if a rule requires it.  In our
> > particular case above, most rules had no cred requirement and this dropped
> > the time spent in audit_filter_rules down to ~12%.  An alternative would be
> > for the caller to acquire the cred just once for the whole chain and pass
> > into audit_filter_rules.  I can create an alternate patch doing this if
> > required.
> 
> There's no actual need to get a ref on the named task's creds.
> 
> If tsk == current, no locking is needed at all.
> 
> If tsk != current, the RCU read lock is sufficient.  See task_cred_xxx() in
> include/linux/cred.h.
> 
> Hmmm...  I wonder...  The audit filter uses tsk->real_cred, but is that
> correct?  Should it be using tsk->cred?  And is tsk always going to be
> current?

Hi David.

I'm not seeing the 'tsk->real_cred' usage, can you clarify?

I went through the call tree.  Assuming my analysis is correct the only case 
where it's not current is the calls from copy_process.  I believe there is no 
need for rcu in this case either.

Tony

------

audit_filter_rules
	<- audit_filter_task
		<- audit_alloc
			<- copy_process

	<- audit_filter_syscall
		<- audit_get_context 
			<- audit_free 
				<- copy_process (error path)
				<- do_exit (tsk == current)
			<- audit_syscall_exit (tsk == current)

		<- audit_syscall_entry (tsk == current)

	<- audit_filter_inodes
		<- audit_update_watch (tsk == current)
		<- audit_get_context (see above)
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