linux-kernel - Re: [PATCH] net: fix unix socket bpf implementation: ensure reliable wake-up signaling

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Message-ID: <01100196baa40a87-e4dfc972-74d0-40b9-a78f-83cfe5649dfe-000000@eu-north-1.amazonses.com>
Date: Sat, 10 May 2025 14:40:34 +0000
From: Ozgur Kara <ozgur@...sey.org>
To: Ozgur Kara <ozgur@...sey.org>
Cc: John Fastabend <john.fastabend@...il.com>, 
	Jakub Sitnicki <jakub@...udflare.com>, 
	Kuniyuki Iwashima <kuniyu@...zon.com>, 
	"David S. Miller" <davem@...emloft.net>, 
	Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, 
	Paolo Abeni <pabeni@...hat.com>, Simon Horman <horms@...nel.org>, 
	netdev@...r.kernel.org, bpf@...r.kernel.org, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] net: fix unix socket bpf implementation: ensure reliable
 wake-up signaling

Hello,
I'm sorry but actually please ignore this patch because i realized
that i can put an atomic process with finish_wait() instead of
prepare_to_wait() because its located in af_unix.h and i'm trying to
understand it now.
Can we preserve wake-up with schedule() by registering towards wait
queue by using finish_wait() instead of prepare_to_wait()?
i will figure out the wait in af_unix.h and send a new patch.

Sorry,

Ozgur

Ozgur Kara <ozgur@...sey.org>, 10 May 2025 Cmt, 17:20 tarihinde şunu yazdı:
>
> From: Ozgur Kara <ozgur@...sey.org>
>
> This patch addresses a race condition in the unix socket bpf
> implementation where wake-up signals could be missed. specifically,
> after releasing mutex (`mutex_unlock(&u->iolock)`) and before
> acquiring it again (`mutex_lock(&u->iolock)`) another thread can
> insert data and send a wake-up signal. if this signal occurs before
> `wait_woken()` is called, it may be lost and cause the thread to
> remain unnecessarily blocked.
>
> to fix this patch introduces a safer wait mechanism using
> `prepare_to_wait()` and `finish_wait()` which ensures that the wakeup
> signal is not missed. this prevents unnecessary blocking and reduces
> the risk of potential deadlocks in high-load or multi-processor
> environments.
>
> such race conditions can lead to performance degradation or, in rare
> cases, deadlocks, especially under heavy load or on multi-cpu systems
> where the problem may be difficult to reproduce.
>
> also there was a space in the last line so i added a checkpatch correction :)
>
> Signed-off-by: Ozgur Kara <ozgur@...sey.org>
> --
> diff --git a/net/unix/unix_bpf.c b/net/unix/unix_bpf.c
> index e0d30d6d22ac..04f2b38803d2 100644
> --- a/net/unix/unix_bpf.c
> +++ b/net/unix/unix_bpf.c
> @@ -26,14 +26,29 @@ static int unix_msg_wait_data(struct sock *sk,
> struct sk_psock *psock,
>         if (!timeo)
>                 return ret;
>
> +       /* wait queue is waited */
>         add_wait_queue(sk_sleep(sk), &wait);
>         sk_set_bit(SOCKWQ_ASYNC_WAITDATA, sk);
> +
> +       /* control while locked */
>         if (!unix_sk_has_data(sk, psock)) {
> +               set_current_state(TASK_INTERRUPTIBLE);
>                 mutex_unlock(&u->iolock);
> -               wait_woken(&wait, TASK_INTERRUPTIBLE, timeo);
> +
> +               if (!schedule_timeout(timeo))
> +                       ret = 0; /* timeout set */
> +               else
> +                       ret = signal_pending(current) ? -ERESTARTSYS : 1;
> +
>                 mutex_lock(&u->iolock);
> -               ret = unix_sk_has_data(sk, psock);
> +
> +               if (ret > 0)
> +                       ret = unix_sk_has_data(sk, psock);
> +       } else {
> +               ret = 1; /* return data */
>         }
> +
> +       __set_current_state(TASK_RUNNING);
>         sk_clear_bit(SOCKWQ_ASYNC_WAITDATA, sk);
>         remove_wait_queue(sk_sleep(sk), &wait);
>         return ret;
> @@ -198,5 +213,4 @@ void __init unix_bpf_build_proto(void)
>  {
>         unix_dgram_bpf_rebuild_protos(&unix_dgram_bpf_prot, &unix_dgram_proto);
>         unix_stream_bpf_rebuild_protos(&unix_stream_bpf_prot,
> &unix_stream_proto);
> -
>  }
> --
>
>
>