Messages in this thread | | | Date | Thu, 10 Mar 2011 12:25:17 -0800 | From | Tony Jones <> | Subject | Re: PATCH [1/1]: audit: acquire creds selectively to reduce atomic op overhead |
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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
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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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