From: Alexander Bluhm <alexander.bluhm@gmx.net>
Subject: Re: Use sorflush() instead of direct unp_scan() to discard dead unix(4) sockets
To: Vitaliy Makkoveev <mvs@openbsd.org>
Cc: tech@openbsd.org
Date: Fri, 22 Mar 2024 13:51:44 +0100

On Tue, Mar 19, 2024 at 05:51:48PM +0300, Vitaliy Makkoveev wrote:
> This is the part of making `so_rcv' sockbuf of unix(4) socket protected
> by `sb_mtx' mutex(9) only. This work will remove relocking from the most
> of send/receive path and make possible simultaneous send/receive on one
> socket in the most cases.
> 
> I need this because the `so_rcv' sockbuf will be protected by `sb_mtx'
> mutex(9), but unp_scan() does M_WAITOK memory allocation.

Both unp_scan() and sorflush() may sleep, so I don't see the
difference.  Maybe it becomes clear with your next diff.

Is your goal is to put so_rcv.sb_mtx around this?
        m = sb->sb_mb;
        memset(&sb->sb_startzero, 0,
             (caddr_t)&sb->sb_endzero - (caddr_t)&sb->sb_startzero);

> Actually, this is the same unp_scan(..., unp_discard) call, but with
> the significant difference. The mbuf(9) chain is unlinked from the
> receive socket. It is much safer because currently the buffers of
> discarded sockets still contains addresses of file descriptors.

OK bluhm@

> Index: sys/kern/uipc_socket.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/uipc_socket.c,v
> retrieving revision 1.320
> diff -u -p -r1.320 uipc_socket.c
> --- sys/kern/uipc_socket.c	12 Feb 2024 22:48:27 -0000	1.320
> +++ sys/kern/uipc_socket.c	19 Mar 2024 14:33:25 -0000
> @@ -65,7 +65,6 @@ void	sotask(void *);
>  void	soreaper(void *);
>  void	soput(void *);
>  int	somove(struct socket *, int);
> -void	sorflush(struct socket *);
>  
>  void	filt_sordetach(struct knote *kn);
>  int	filt_soread(struct knote *kn, long hint);
> Index: sys/kern/uipc_usrreq.c
> ===================================================================
> RCS file: /cvs/src/sys/kern/uipc_usrreq.c,v
> retrieving revision 1.201
> diff -u -p -r1.201 uipc_usrreq.c
> --- sys/kern/uipc_usrreq.c	17 Mar 2024 19:47:08 -0000	1.201
> +++ sys/kern/uipc_usrreq.c	19 Mar 2024 14:33:26 -0000
> @@ -1438,7 +1438,7 @@ unp_gc(void *arg __unused)
>  				 */
>  				so = unp->unp_socket;
>  				solock(so);
> -				unp_scan(so->so_rcv.sb_mb, unp_discard);
> +				sorflush(so);
>  				sounlock(so);
>  			}
>  		}
> Index: sys/sys/socketvar.h
> ===================================================================
> RCS file: /cvs/src/sys/sys/socketvar.h,v
> retrieving revision 1.124
> diff -u -p -r1.124 socketvar.h
> --- sys/sys/socketvar.h	12 Feb 2024 22:48:27 -0000	1.124
> +++ sys/sys/socketvar.h	19 Mar 2024 14:33:26 -0000
> @@ -394,6 +394,7 @@ int	sosend(struct socket *, struct mbuf 
>  	    struct mbuf *, struct mbuf *, int);
>  int	sosetopt(struct socket *, int, int, struct mbuf *);
>  int	soshutdown(struct socket *, int);
> +void	sorflush(struct socket *);
>  void	sowakeup(struct socket *, struct sockbuf *);
>  void	sorwakeup(struct socket *);
>  void	sowwakeup(struct socket *);