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

Tony Jones
2011-03-10T20:25:38Z

On Tue, Mar 08, 2011 at 06:02:53PM +0000, David Howells wrote:
> Tony Jones <tonyj@suse.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)