Re: [PATCH bpf-next v2 2/3] bpf, sockmap: Fix FIONREAD for sockmap

[bot+bpf-ci]

> diff --git a/include/linux/skmsg.h b/include/linux/skmsg.h
> index 0323a2b6c..49069b519 100644
> --- a/include/linux/skmsg.h
> +++ b/include/linux/skmsg.h

[ ... ]

> diff --git a/net/core/skmsg.c b/net/core/skmsg.c
> index d73e03f77..4390deaeb 100644
> --- a/net/core/skmsg.c
> +++ b/net/core/skmsg.c

[ ... ]

> @@ -820,8 +821,11 @@ static void __sk_psock_purge_ingress_msg(struct sk_psock 
> *psock)
>               if (!msg->skb)
>                       atomic_sub(msg->sg.size, &psock->sk->sk_rmem_alloc);
>               sk_msg_free(psock->sk, msg);
> +             sk_psock_inc_msg_size(psock, -msg->sg.size);
>               kfree(msg);
>       }
> +
> +     WARN_ON_ONCE(psock->ingress_size);
>  }

Can msg->sg.size be read after sk_msg_free()? Looking at sk_msg_free():

  int sk_msg_free(struct sock *sk, struct sk_msg *msg)
  {
      return __sk_msg_free(sk, msg, msg->sg.start, true);
  }

And __sk_msg_free() at net/core/skmsg.c:207:

  static int __sk_msg_free(struct sock *sk, struct sk_msg *msg, u32 i,
                           bool charge)
  {
      ...
      while (msg->sg.size) {
          msg->sg.size -= sge->length;
          ...
      }
      consume_skb(msg->skb);
      sk_msg_init(msg);    // <-- this zeros the entire msg structure
      return freed;
  }

And sk_msg_init() at include/linux/skmsg.h:193:

  static inline void sk_msg_init(struct sk_msg *msg)
  {
      BUILD_BUG_ON(ARRAY_SIZE(msg->sg.data) - 1 != NR_MSG_FRAG_IDS);
      memset(msg, 0, sizeof(*msg));    // <-- zeros everything
      sg_init_marker(msg->sg.data, NR_MSG_FRAG_IDS);
  }

After sk_msg_free() completes, msg->sg.size is zero, so the line
"sk_psock_inc_msg_size(psock, -msg->sg.size)" subtracts 0 instead of
the actual message size. This means ingress_size will never be
decremented during cleanup, and WARN_ON_ONCE(psock->ingress_size) will
trigger whenever the ingress_msg queue contained messages.

The fix would be to save msg->sg.size before calling sk_msg_free():

  u32 msg_size = msg->sg.size;
  if (!msg->skb)
      atomic_sub(msg_size, &psock->sk->sk_rmem_alloc);
  sk_msg_free(psock->sk, msg);
  sk_psock_inc_msg_size(psock, -msg_size);
  kfree(msg);

[ ... ]


---
AI reviewed your patch. Please fix the bug or email reply why it's not a bug.
See: https://github.com/kernel-patches/vmtest/blob/master/ci/claude/README.md

CI run summary: https://github.com/kernel-patches/bpf/actions/runs/19558612316