netdev - RE: [PATCH net 2/4] tcp: fix race in tcp_write

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Message-ID: <889fbe3feae042ada8d75a8a2184dbaa@AcuMS.aculab.com>
Date: Tue, 28 May 2024 09:19:51 +0000
From: David Laight <David.Laight@...LAB.COM>
To: 'Eric Dumazet' <edumazet@...gle.com>, "David S . Miller"
	<davem@...emloft.net>, Jakub Kicinski <kuba@...nel.org>, Paolo Abeni
	<pabeni@...hat.com>
CC: Neal Cardwell <ncardwell@...gle.com>, "netdev@...r.kernel.org"
	<netdev@...r.kernel.org>, "eric.dumazet@...il.com" <eric.dumazet@...il.com>
Subject: RE: [PATCH net 2/4] tcp: fix race in tcp_write_err()

From: Eric Dumazet
> Sent: 24 May 2024 20:36
> 
> I noticed flakes in a packetdrill test, expecting an epoll_wait()
> to return EPOLLERR | EPOLLHUP on a failed connect() attempt,
> after multiple SYN retransmits. It sometimes return EPOLLERR only.
> 
> The issue is that tcp_write_err():
>  1) writes an error in sk->sk_err,
>  2) calls sk_error_report(),
>  3) then calls tcp_done().
> 
> tcp_done() is writing SHUTDOWN_MASK into sk->sk_shutdown,
> among other things.
> 
> Problem is that the awaken user thread (from 2) sk_error_report())
> might call tcp_poll() before tcp_done() has written sk->sk_shutdown.
> 
> tcp_poll() only sees a non zero sk->sk_err and returns EPOLLERR.
> 
> This patch fixes the issue by making sure to call sk_error_report()
> after tcp_done().

Isn't there still the potential for a program to call poll() at
'just the wrong time' and still see an unexpected status?

...
>  	WRITE_ONCE(sk->sk_err, READ_ONCE(sk->sk_err_soft) ? : ETIMEDOUT);
> -	sk_error_report(sk);
> 
> -	tcp_write_queue_purge(sk);
> -	tcp_done(sk);
> +	tcp_done_with_error(sk);

Is there scope for moving the write to sk->sk_err inside the function?
Looks like it'll need a larger change to tcp_reset().

	David

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