On Monday, March 23, 2026 11:07:00 AM Eastern Daylight Time Ricardo Robaina wrote: > Currently, determining the optimal `audit_backlog_limit` relies on > instantaneous polling of the queue size. This misses transient > micro-bursts, making it difficult for system administrators to know > if their queue is adequately sized or if they are at risk of > dropping events. > > This patch introduces `backlog_max_depth`, a high-water mark metric > that tracks the maximum number of buffers in the audit queue since > the system was booted or the metric was last reset. To minimize > performance overhead in the fast-path, the metric is updated using > a lockless cmpxchg loop in `__audit_log_end()`. > > Userspace can read-and-clear this metric by sending an `AUDIT_SET` > message with the `AUDIT_STATUS_BACKLOG_MAX_DEPTH` mask. To support > periodic telemetry polling (e.g., statsd, Prometheus), the reset > operation atomically returns the snapshot of the high-water mark > right before zeroing it, ensuring no peaks are lost between polls.
>From a user space point of view, this looks good. User space support was co- developed alongside of this patch to ensure it works as advertised. Acked-by: Steve Grubb <[email protected]> -Steve > Link: https://github.com/linux-audit/audit-kernel/issues/63 > Suggested-by: Steve Grubb <[email protected]> > Signed-off-by: Ricardo Robaina <[email protected]> > --- > include/linux/audit.h | 3 ++- > include/uapi/linux/audit.h | 2 ++ > kernel/audit.c | 32 ++++++++++++++++++++++++++++++++ > 3 files changed, 36 insertions(+), 1 deletion(-) > > diff --git a/include/linux/audit.h b/include/linux/audit.h > index d79218bf075a..53132b303c20 100644 > --- a/include/linux/audit.h > +++ b/include/linux/audit.h > @@ -22,7 +22,8 @@ > AUDIT_STATUS_BACKLOG_LIMIT | \ > AUDIT_STATUS_BACKLOG_WAIT_TIME | \ > AUDIT_STATUS_LOST | \ > - AUDIT_STATUS_BACKLOG_WAIT_TIME_ACTUAL) > + AUDIT_STATUS_BACKLOG_WAIT_TIME_ACTUAL | \ > + AUDIT_STATUS_BACKLOG_MAX_DEPTH) > > #define AUDIT_INO_UNSET ((unsigned long)-1) > #define AUDIT_DEV_UNSET ((dev_t)-1) > diff --git a/include/uapi/linux/audit.h b/include/uapi/linux/audit.h > index e8f5ce677df7..862ca93c0c31 100644 > --- a/include/uapi/linux/audit.h > +++ b/include/uapi/linux/audit.h > @@ -355,6 +355,7 @@ enum { > #define AUDIT_STATUS_BACKLOG_WAIT_TIME 0x0020 > #define AUDIT_STATUS_LOST 0x0040 > #define AUDIT_STATUS_BACKLOG_WAIT_TIME_ACTUAL 0x0080 > +#define AUDIT_STATUS_BACKLOG_MAX_DEPTH 0x0100 > > #define AUDIT_FEATURE_BITMAP_BACKLOG_LIMIT 0x00000001 > #define AUDIT_FEATURE_BITMAP_BACKLOG_WAIT_TIME 0x00000002 > @@ -486,6 +487,7 @@ struct audit_status { > __u32 backlog_wait_time_actual;/* time spent waiting while > * message limit exceeded > */ > + __u32 backlog_max_depth; /* message queue max depth */ > }; > > struct audit_features { > diff --git a/kernel/audit.c b/kernel/audit.c > index e1d489bc2dff..256053cb6132 100644 > --- a/kernel/audit.c > +++ b/kernel/audit.c > @@ -163,6 +163,9 @@ static struct sk_buff_head audit_retry_queue; > /* queue msgs waiting for new auditd connection */ > static struct sk_buff_head audit_hold_queue; > > +/* audit queue high water mark since last startup or reset */ > +static atomic_t audit_backlog_max_depth __read_mostly = ATOMIC_INIT(0); > + > /* queue servicing thread */ > static struct task_struct *kauditd_task; > static DECLARE_WAIT_QUEUE_HEAD(kauditd_wait); > @@ -1286,6 +1289,7 @@ static int audit_receive_msg(struct sk_buff *skb, > struct nlmsghdr *nlh, s.backlog = > skb_queue_len(&audit_queue); > s.feature_bitmap = AUDIT_FEATURE_BITMAP_ALL; > s.backlog_wait_time = audit_backlog_wait_time; > + s.backlog_max_depth = > atomic_read(&audit_backlog_max_depth); > s.backlog_wait_time_actual = > atomic_read(&audit_backlog_wait_time_actual); audit_send_reply(skb, seq, > AUDIT_GET, 0, 0, &s, sizeof(s)); > break; > @@ -1399,6 +1403,12 @@ static int audit_receive_msg(struct sk_buff *skb, > struct nlmsghdr *nlh, audit_log_config_change("backlog_wait_time_actual", > 0, actual, 1); return actual; > } > + if (s.mask == AUDIT_STATUS_BACKLOG_MAX_DEPTH) { > + u32 old_depth = atomic_xchg(&audit_backlog_max_depth, > 0); > + > + audit_log_config_change("backlog_max_depth", 0, > old_depth, 1); > + return old_depth; > + } > break; > } > case AUDIT_GET_FEATURE: > @@ -2761,6 +2771,25 @@ int audit_signal_info(int sig, struct task_struct > *t) return audit_signal_info_syscall(t); > } > > +/* > + * audit_update_backlog_max_depth - update the audit queue high water mark > + * > + * Safely updates the audit_backlog_max_depth metric using a lockless > + * cmpxchg loop. This ensures the high-water mark is accurately tracked > + * even when multiple CPUs are logging audit records concurrently. > + */ > +static inline void audit_update_backlog_max_depth(void) > +{ > + u32 q_len = skb_queue_len(&audit_queue); > + u32 q_max = atomic_read(&audit_backlog_max_depth); > + > + while (unlikely(q_len > q_max)) { > + if (likely(atomic_try_cmpxchg(&audit_backlog_max_depth, > + &q_max, q_len))) > + break; > + } > +} > + > /** > * __audit_log_end - enqueue one audit record > * @skb: the buffer to send > @@ -2777,6 +2806,9 @@ static void __audit_log_end(struct sk_buff *skb) > > /* queue the netlink packet */ > skb_queue_tail(&audit_queue, skb); > + > + /* update backlog high water mark */ > + audit_update_backlog_max_depth(); > } else { > audit_log_lost("rate limit exceeded"); > kfree_skb(skb);
