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

Jiri Kosina
2011-04-27T13:12:29Z

On Mon, 21 Mar 2011, Eric Paris 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.
> >
> > 1. The code should be accessing tsk->cred rather than tsk->real_cred.
> > 2. Since tsk is current (or tsk is being created by copy_process) access to
> > tsk->cred without rcu read lock is possible. At the request of the audit
> > maintainer, a new flag has been added to audit_filter_rules in order to make
> > this explicit and guide future code.
> >
> > Signed-off-by: Tony Jones <tonyj@suse.de>
>
> Acked-by: Eric Paris <eparis@redhat.com>

I don't see the patch in linux-next as of today. As it has been acked by
subsystem maintainer, I have picked it up into my tree ("retransmission
mode").

If anyone has any objections, please let me know. Thanks.

>
> > ---
> > kernel/auditsc.c | 27 +++++++++++++++++----------
> > 1 files changed, 17 insertions(+), 10 deletions(-)
> >
> > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
> > index f49a031..281dcf1 100644
> > --- a/kernel/auditsc.c
> > +++ b/kernel/auditsc.c
> > @@ -443,17 +443,25 @@ static int match_tree_refs(struct audit_context *ctx, struct audit_tree *tree)
> >
> > /* Determine if any context name data matches a rule's watch data */
> > /* Compare a task_struct with an audit_rule. Return 1 on match, 0
> > - * otherwise. */
> > + * otherwise.
> > + *
> > + * If task_creation is true, this is an explicit indication that we are
> > + * filtering a task rule at task creation time. This and tsk == current are
> > + * the only situations where tsk->cred may be accessed without an rcu read lock.
> > + */
> > static int audit_filter_rules(struct task_struct *tsk,
> > struct audit_krule *rule,
> > struct audit_context *ctx,
> > struct audit_names *name,
> > - enum audit_state *state)
> > + enum audit_state *state,
> > + bool task_creation)
> > {
> > - const struct cred *cred = get_task_cred(tsk);
> > + const struct cred *cred;
> > int i, j, need_sid = 1;
> > u32 sid;
> >
> > + cred = rcu_dereference_check(tsk->cred, tsk == current || task_creation);
> > +
> > for (i = 0; i < rule->field_count; i++) {
> > struct audit_field *f = &rule->fields[i];
> > int result = 0;
> > @@ -637,10 +645,8 @@ static int audit_filter_rules(struct task_struct *tsk,
> > break;
> > }
> >
> > - if (!result) {
> > - put_cred(cred);
> > + if (!result)
> > return 0;
> > - }
> > }
> >
> > if (ctx) {
> > @@ -656,7 +662,6 @@ static int audit_filter_rules(struct task_struct *tsk,
> > case AUDIT_NEVER: *state = AUDIT_DISABLED; break;
> > case AUDIT_ALWAYS: *state = AUDIT_RECORD_CONTEXT; break;
> > }
> > - put_cred(cred);
> > return 1;
> > }
> >
> > @@ -671,7 +676,8 @@ static enum audit_state audit_filter_task(struct task_struct *tsk, char **key)
> >
> > rcu_read_lock();
> > list_for_each_entry_rcu(e, &audit_filter_list[AUDIT_FILTER_TASK], list) {
> > - if (audit_filter_rules(tsk, &e->rule, NULL, NULL, &state)) {
> > + if (audit_filter_rules(tsk, &e->rule, NULL, NULL,
> > + &state, true)) {
> > if (state == AUDIT_RECORD_CONTEXT)
> > *key = kstrdup(e->rule.filterkey, GFP_ATOMIC);
> > rcu_read_unlock();
> > @@ -705,7 +711,7 @@ static enum audit_state audit_filter_syscall(struct task_struct *tsk,
> > list_for_each_entry_rcu(e, list, list) {
> > if ((e->rule.mask[word] & bit) == bit &&
> > audit_filter_rules(tsk, &e->rule, ctx, NULL,
> > - &state)) {
> > + &state, false)) {
> > rcu_read_unlock();
> > ctx->current_state = state;
> > return state;
> > @@ -743,7 +749,8 @@ void audit_filter_inodes(struct task_struct *tsk, struct audit_context *ctx)
> >
> > list_for_each_entry_rcu(e, list, list) {
> > if ((e->rule.mask[word] & bit) == bit &&
> > - audit_filter_rules(tsk, &e->rule, ctx, n, &state)) {
> > + audit_filter_rules(tsk, &e->rule, ctx, n,
> > + &state, false)) {
> > rcu_read_unlock();
> > ctx->current_state = state;
> > return;

--
Jiri Kosina
SUSE Labs, Novell Inc.