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(a)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)