Re: [PATCH] ratelimit printk messages from the audit system
On Wed, 2008-01-23 at 16:05 -0500, Linda Knippers wrote:
Eric Paris wrote:
> Some printk messages from the audit system can become excessive. This
> patch ratelimits those messages. It was found that messages, such as
> the audit backlog lost printk message could flood the logs to the point
> that a machine could take an nmi watchdog hit or otherwise become
> unresponsive.
>
> Signed-off-by: Eric Paris <eparis(a)redhat.com>
>
> ---
> kernel/audit.c | 28 ++++++++++++++++++----------
> 1 files changed, 18 insertions(+), 10 deletions(-)
>
> diff --git a/kernel/audit.c b/kernel/audit.c
> index f93c271..a3d828b 100644
> --- a/kernel/audit.c
> +++ b/kernel/audit.c
> @@ -163,7 +163,8 @@ void audit_panic(const char *message)
> case AUDIT_FAIL_SILENT:
> break;
> case AUDIT_FAIL_PRINTK:
> - printk(KERN_ERR "audit: %s\n", message);
> + if (printk_ratelimit())
> + printk(KERN_ERR "audit: %s\n", message);
> break;
> case AUDIT_FAIL_PANIC:
> panic("audit: %s\n", message);
> @@ -231,11 +232,13 @@ void audit_log_lost(const char *message)
> }
>
> if (print) {
> - printk(KERN_WARNING
> - "audit: audit_lost=%d audit_rate_limit=%d
audit_backlog_limit=%d\n",
> - atomic_read(&audit_lost),
> - audit_rate_limit,
> - audit_backlog_limit);
> + if (printk_ratelimit())
> + printk(KERN_WARNING
> + "audit: audit_lost=%d audit_rate_limit=%d "
This is unrelated to your patch but I think it would be nice if
audit_lost represented the number of audit messages lost since the last
time the message came out or the last time an audit record came out.
Today its a cumulative count since the system was booted. Is it too
much overhead to zero it?
Shouldn't be too much overhead, we are already on a slow/unlikely path.
What's the benefit though? Just don't want to have to do a subtraction?
If we are dropping the 'we lost some messages' message 0'ing the counter
at that time would be a bad idea, certainly not unsolvable, but I don't
see what it buys us.
> + "audit_backlog_limit=%d\n",
> + atomic_read(&audit_lost),
> + audit_rate_limit,
> + audit_backlog_limit);
> audit_panic(message);
> }
> }
> @@ -405,7 +408,11 @@ static int kauditd_thread(void *dummy)
> audit_pid = 0;
> }
> } else {
> - printk(KERN_NOTICE "%s\n", skb->data + NLMSG_SPACE(0));
> + if (printk_ratelimit())
> + printk(KERN_NOTICE "%s\n", skb->data +
> + NLMSG_SPACE(0));
> + else
> + audit_log_lost("printk limit exceeded\n");
If you call audit_log_lost when the printk limit is exceeded, but then
audit_log_lost also checks the printk limit, will this message ever
come out? Does it make sense to print a message saying we couldn't
print a message?
No it won't come out of audit_log_lost() through printk either, but what
it does do is call audit_panic() and we get the lost message accounting.
> kfree_skb(skb);
> }
> } else {
> @@ -1164,7 +1171,7 @@ struct audit_buffer *audit_log_start(struct audit_context
*ctx, gfp_t gfp_mask,
> remove_wait_queue(&audit_backlog_wait, &wait);
> continue;
> }
> - if (audit_rate_check())
> + if (audit_rate_check() && printk_ratelimit())
> printk(KERN_WARNING
> "audit: audit_backlog=%d > "
> "audit_backlog_limit=%d\n",
> @@ -1433,9 +1440,10 @@ void audit_log_end(struct audit_buffer *ab)
> skb_queue_tail(&audit_skb_queue, ab->skb);
> ab->skb = NULL;
> wake_up_interruptible(&kauditd_wait);
> - } else {
> + } else if (printk_ratelimit())
> printk(KERN_NOTICE "%s\n", ab->skb->data + NLMSG_SPACE(0));
> - }
> + else
> + audit_log_lost("printk limit exceeded\n");
Same question here.
I wonder if it would be better to reduce the generation of the messages,
rather than just their output. For example, once we're losing records,
should we just flush the queue, issue one message, and then keep going?
I'd be scared it'd just fill up too quickly again...
Or perhaps issue one message, shut off incoming so we don't
accept new
records until the backlog goes to zero, then start up again?
Well that's sorta what we do now, we throw stuff on the floor until it
gets low, maybe not to 0, i don't remember. I'll take a look.
I'll think about it, but really, as long as we are generating audit
events there isn't a great way to solve this problem other than throwing
stuff on the floor.
At this point I think this patch is good, but I'll look at how we handle
lost messages a little more....