netdev - Re: [PATCH] net, socket, socket_wq: fix missing initialization of flags

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Date:	Mon, 28 Dec 2015 12:19:46 -0500
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Nicolai Stange <nicstange@...il.com>
Cc:	"David S. Miller" <davem@...emloft.net>,
	Eric Dumazet <edumazet@...gle.com>, netdev@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] net, socket, socket_wq: fix missing initialization of
 flags

On Sun, 2015-12-27 at 21:00 +0100, Nicolai Stange wrote:
> Fixes: ceb5d58b2170 ("net: fix sock_wake_async() rcu protection")
> 
> Commit ceb5d58b2170 ("net: fix sock_wake_async() rcu protection") from
> the current 4.4 release cycle introduces a new flags member in
> struct socket_wq and moved SOCKWQ_ASYNC_NOSPACE and SOCKWQ_ASYNC_WAITDATA
> from struct socket's flags member into that new place.
> 
> Unfortunately, the new flags field is never initialized properly, at least
> not for the struct socket_wq instance created in sock_alloc_inode().
> 
> One particular issue I encountered because of this is that my GNU Emacs
> failed to draw anything on my desktop -- i.e. what I got is a transparent
> window, including the title bar. Bisection lead to the commit mentioned
> above and further investigation by means of strace told me that Emacs
> is indeed speaking to my Xorg through an O_ASYNC AF_UNIX socket. This is
> reproducible 100% of times and the fact that properly initializing the
> struct socket_wq ->flags fixes the issue leads me to the conclusion that
> somehow SOCKWQ_ASYNC_NOSPACE got set in the uninitialized ->flags,
> preventing my Emacs from receiving any SIGIO's due to send space becoming
> available again and it got stuck.
> 
> Make sock_alloc_inode() set the newly created struct socket_wq's ->flags
> member to zero.
> 
> Signed-off-by: Nicolai Stange <nicstange@...il.com>
> ---
>  net/socket.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/net/socket.c b/net/socket.c
> index 29822d6..d730ef9 100644
> --- a/net/socket.c
> +++ b/net/socket.c
> @@ -257,6 +257,7 @@ static struct inode *sock_alloc_inode(struct super_block *sb)
>  	}
>  	init_waitqueue_head(&wq->wait);
>  	wq->fasync_list = NULL;
> +	wq->flags = 0;
>  	RCU_INIT_POINTER(ei->socket.wq, wq);
>  
>  	ei->socket.state = SS_UNCONNECTED;

Thanks a lot Nicolai for finding this.
I completely overlooked this initial value.

I checked other places where 'struct socket_wq' were allocated and they
look fine.

Acked-by: Eric Dumazet <edumazet@...gle.com>


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