On Wed, 27 May 2026 16:29:37 -0300 Ricardo Robaina wrote: > On Mon, May 18, 2026 at 9:35 PM Jakub Kicinski <[email protected]> wrote: > > > > On Wed, 13 May 2026 14:24:43 -0300 Ricardo Robaina wrote: > > > When auditd is bottlenecked (e.g., by slow disk I/O), kauditd blocks on > > > the netlink socket. > > > > Holding socket lock during slow IO sounds very wrong. One could say - > > that's abuse of the socket lock? > > > > > If the wait timeout fully expires (timeo == 0), > > > netlink mistakenly interprets the zeroed timeout as a non-blocking > > > request. It then triggers netlink_overrun that drops the event, > > > completely bypassing the audit subsystem's internal retry queue, and > > > falsely returns ENOBUFS to user-space, resulting in the following error: > > > > > > auditd[]: Error receiving audit netlink packet (No buffer space > > > available) > > > > > > Fix this by detecting when a blocking sender's timeout has expired > > > (timeo == 0 && !nonblock) in netlink_unicast(). In this case, instead > > > of retrying with timeo=0 (which would incorrectly trigger netlink_overrun > > > on the next iteration), safely free the skb and return -EAGAIN, allowing > > > the audit subsystem to gracefully enqueue the pending event into its > > > internal backlog. > > > > The socket _is_ the queue, normally. > > > > Please explore fixing this in audit? > > -- > > pw-bot: cr > > > > Hi Jakub, > > Thanks for reviewing this patch as well. > > First, regarding the lock: kauditd does not hold the socket lock during > slow I/O. The sleep in netlink_attachskb() uses schedule_timeout() on > nlk->wait (a wait queue). No socket lock or mutex is held during the sleep.
So you're saying the queue _is_ actually congested? netlink_attachskb() sleeps because there's no space left in the socket's rcvbuf? So the skbs are moved to audit_retry_queue "temporarily" until user space drains its socket and kernel can succeed sending? Could you confirm this understanding is correct? > Second, regarding an audit-only fix: the symptom manifests as sk->sk_err = > ENOBUFS set inside netlink_overrun() (called from netlink_attachskb when > timeo == 0). Audit has no mechanism to prevent or clear this socket state > from the outside. Potential workarounds all fail: > > (1) Clearing sk_err after the fact is racy and affects other socket ops Why would you clear the sk_err, it's the reader's responsibility to clear the congestion and the reader is AFAIU a user space process. > (2) Avoiding timeouts entirely defeats the anti-deadlock mechanism What's the anti-deadlock mechanism? > (3) A new NETLINK_F_RECV_NO_ENOBUFS socket flag doesn't exist in stable > kernels where this bug is actively impacting users Which commit are you referring to? Isn't that flag itself ancient? > I've submitted v3 [1] with NETLINK_UNICAST_TIMED as an explicit opt-in > constant. It's really not great to fall silent for 10+ days, then respond and immediately posts equally pointless next version of the patch :/
