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    SubjectRe: [PATCH] audit: speedup for syscalls when auditing is disabled
    Date
    In message <1282586177.2681.43.camel@localhost.localdomain> you wrote:
    > On Fri, 2010-08-20 at 12:13 +1000, Michael Neuling wrote:
    > > We found that when auditing is disabled using "auditctl -D", that
    > > there's still a significant overhead when doing syscalls. This overhead
    > > is not present when a single never rule is inserted using "auditctl -a
    > > task,never".
    > >
    > > Using Anton's null syscall microbenchmark from
    > > http://ozlabs.org/~anton/junkcode/null_syscall.c we currently have on a
    > > powerpc machine:
    > >
    > > # auditctl -D
    > > No rules
    > > # ./null_syscall
    > > null_syscall: 739.03 cycles 100.00%
    > > # auditctl -a task,never
    > > # ./null_syscall
    > > null_syscall: 204.63 cycles 100.00%
    > >
    > > This doesn't seem right, as we'd hope that auditing would have the same
    > > minimal impact when disabled via -D as when we have a single never rule.
    > >
    > > The patch below creates a fast path when initialising a task. If the
    > > rules list for tasks is empty (the disabled -D option), we mark auditing
    > > as disabled for this task.
    > >
    > > When this is applied, our null syscall benchmark improves in the
    > > disabled case to match the single never rule case.
    > >
    > > # auditctl -D
    > > No rules
    > > # ./null_syscall
    > > null_syscall: 204.62 cycles 100.00%
    > > # auditctl -a task,never
    > > # ./null_syscall
    > > null_syscall: 204.63 cycles 100.00%
    > >
    > > Reported-by: Anton Blanchard <anton@samba.org>
    > > Signed-off-by: Michael Neuling <mikey@neuling.org>
    > > ---
    > > I'm not familiar with the auditing code/infrastructure so I may have
    > > misunderstood something here
    > >
    > > diff --git a/kernel/auditsc.c b/kernel/auditsc.c
    > > index 1b31c13..1cd6ec7 100644
    > > --- a/kernel/auditsc.c
    > > +++ b/kernel/auditsc.c
    > > @@ -666,6 +666,11 @@ static enum audit_state audit_filter_task(struct task_
    struct *tsk, char **key)
    > > enum audit_state state;
    > >
    > > rcu_read_lock();
    > > + /* Fast path. If the list is empty, disable auditing */
    > > + if (list_empty(&audit_filter_list[AUDIT_FILTER_TASK])) {
    > > + rcu_read_unlock();
    > > + return AUDIT_DISABLED;
    > > + }
    > > list_for_each_entry_rcu(e, &audit_filter_list[AUDIT_FILTER_TASK], list)
    {
    > > if (audit_filter_rules(tsk, &e->rule, NULL, NULL, &state)) {
    > > if (state == AUDIT_RECORD_CONTEXT)
    >
    > I don't think this works at all. I don't see how syscall audit'ing can
    > work. What if I have nothing in the AUDIT_FILTER_TASK list but I want
    > to audit all 'open(2)' syscalls? This patch is going to leave the task
    > in the DISABLED state and we won't ever be able to match on the syscall
    > rules.

    Sorry my bad. I'm not too familiar with the audit infrastructure.

    On reflection, we might have a bug in audit_alloc though. Currently we
    have this:

    int audit_alloc(struct task_struct *tsk)
    {
    <snip>
    state = audit_filter_task(tsk, &key);
    if (likely(state == AUDIT_DISABLED))
    return 0;

    <snip>
    set_tsk_thread_flag(tsk, TIF_SYSCALL_AUDIT);
    return 0;
    }

    This gets called on fork. If we have "task,never" rule, we hit this
    state == AUDIT_DISABLED path, return immediately and the tasks
    TIF_SYSCALL_AUDIT flags doesn't get set. On powerpc, we check
    TIF_SYSCALL_AUDIT in asm on syscall entry to fast path not calling the
    syscall audit code.

    This seems wrong to me as a "never" _task_ audit rule shouldn't effect
    _syscall_ auditing? Is there some interaction between task and syscall
    auditing that I'm missing?

    > I wonder if you could get much back, in terms of performance, by moving
    > the
    > context->dummy = !audit_n_rules;
    > line to the top and just returning if context->dummy == 1;

    We get 668.09 cycles with this optimisation, so it comes down a bit, but
    no where near if the auditing is disabled altogether.

    Like I said above, powerpc has a fast path in asm on system call entry
    to check the thread_info flags for if syscall auditing is disabled. If
    it's disabled, we don't call the audit code, hence why it's very fast in
    this case.

    > I'll play a bit, but I thought that was supposed to be a safe thing to
    > do....

    Thanks!

    Mikey



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