sctp: Add peeloff-flags socket option

Based on a request raised on the sctp devel list, there is a need to
augment the sctp_peeloff operation while specifying the O_CLOEXEC and
O_NONBLOCK flags (simmilar to the socket syscall). Since modifying the
SCTP_SOCKOPT_PEELOFF socket option would break user space ABI for existing
programs, this patch creates a new socket option
SCTP_SOCKOPT_PEELOFF_FLAGS, which accepts a third flags parameter to
allow atomic assignment of the socket descriptor flags.

Tested successfully by myself and the requestor

Signed-off-by: Neil Horman <[e-mail]>
CC: Vlad Yasevich <[e-mail]>
CC: "David S. Miller" <[e-mail]>
CC: Andreas Steinmetz <[e-mail]>
CC: Marcelo Ricardo Leitner <[e-mail]>
Acked-by: Marcelo Ricardo Leitner <[e-mail]>
Signed-off-by: David S. Miller <[e-mail]>

include/uapi/linux/sctp.h

···
 #define SCTP_RESET_STREAMS	119
 #define SCTP_RESET_ASSOC	120
 #define SCTP_ADD_STREAMS	121
+#define SCTP_SOCKOPT_PEELOFF_FLAGS 122
 
 /* PR-SCTP policies */
 #define SCTP_PR_SCTP_NONE	0x0000
···
 	sctp_assoc_t associd;
 	int sd;
 } sctp_peeloff_arg_t;
+
+typedef struct {
+	sctp_peeloff_arg_t p_arg;
+	unsigned flags;
+} sctp_peeloff_flags_arg_t;
 
 /*
  *  Peer Address Thresholds socket option

net/sctp/socket.c

···
 }
 EXPORT_SYMBOL(sctp_do_peeloff);
 
+static int sctp_getsockopt_peeloff_common(struct sock *sk, sctp_peeloff_arg_t *peeloff,
+					  struct file **newfile, unsigned flags)
+{
+	struct socket *newsock;
+	int retval;
+
+	retval = sctp_do_peeloff(sk, peeloff->associd, &newsock);
+	if (retval < 0)
+		goto out;
+
+	/* Map the socket to an unused fd that can be returned to the user.  */
+	retval = get_unused_fd_flags(flags & SOCK_CLOEXEC);
+	if (retval < 0) {
+		sock_release(newsock);
+		goto out;
+	}
+
+	*newfile = sock_alloc_file(newsock, 0, NULL);
+	if (IS_ERR(*newfile)) {
+		put_unused_fd(retval);
+		sock_release(newsock);
+		retval = PTR_ERR(*newfile);
+		*newfile = NULL;
+		return retval;
+	}
+
+	pr_debug("%s: sk:%p, newsk:%p, sd:%d\n", __func__, sk, newsock->sk,
+		 retval);
+
+	peeloff->sd = retval;
+
+	if (flags & SOCK_NONBLOCK)
+		(*newfile)->f_flags |= O_NONBLOCK;
+out:
+	return retval;
+}
+
 static int sctp_getsockopt_peeloff(struct sock *sk, int len, char __user *optval, int __user *optlen)
 {
 	sctp_peeloff_arg_t peeloff;
-	struct socket *newsock;
-	struct file *newfile;
+	struct file *newfile = NULL;
 	int retval = 0;
 
 	if (len < sizeof(sctp_peeloff_arg_t))
···
 	if (copy_from_user(&peeloff, optval, len))
 		return -EFAULT;
 
-	retval = sctp_do_peeloff(sk, peeloff.associd, &newsock);
+	retval = sctp_getsockopt_peeloff_common(sk, &peeloff, &newfile, 0);
 	if (retval < 0)
 		goto out;
-
-	/* Map the socket to an unused fd that can be returned to the user.  */
-	retval = get_unused_fd_flags(0);
-	if (retval < 0) {
-		sock_release(newsock);
-		goto out;
-	}
-
-	newfile = sock_alloc_file(newsock, 0, NULL);
-	if (IS_ERR(newfile)) {
-		put_unused_fd(retval);
-		sock_release(newsock);
-		return PTR_ERR(newfile);
-	}
-
-	pr_debug("%s: sk:%p, newsk:%p, sd:%d\n", __func__, sk, newsock->sk,
-		 retval);
 
 	/* Return the fd mapped to the new socket.  */
 	if (put_user(len, optlen)) {
···
 		put_unused_fd(retval);
 		return -EFAULT;
 	}
-	peeloff.sd = retval;
+
+	if (copy_to_user(optval, &peeloff, len)) {
+		fput(newfile);
+		put_unused_fd(retval);
+		return -EFAULT;
+	}
+	fd_install(retval, newfile);
+out:
+	return retval;
+}
+
+static int sctp_getsockopt_peeloff_flags(struct sock *sk, int len,
+					 char __user *optval, int __user *optlen)
+{
+	sctp_peeloff_flags_arg_t peeloff;
+	struct file *newfile = NULL;
+	int retval = 0;
+
+	if (len < sizeof(sctp_peeloff_flags_arg_t))
+		return -EINVAL;
+	len = sizeof(sctp_peeloff_flags_arg_t);
+	if (copy_from_user(&peeloff, optval, len))
+		return -EFAULT;
+
+	retval = sctp_getsockopt_peeloff_common(sk, &peeloff.p_arg,
+						&newfile, peeloff.flags);
+	if (retval < 0)
+		goto out;
+
+	/* Return the fd mapped to the new socket.  */
+	if (put_user(len, optlen)) {
+		fput(newfile);
+		put_unused_fd(retval);
+		return -EFAULT;
+	}
+
 	if (copy_to_user(optval, &peeloff, len)) {
 		fput(newfile);
 		put_unused_fd(retval);
···
 		break;
 	case SCTP_SOCKOPT_PEELOFF:
 		retval = sctp_getsockopt_peeloff(sk, len, optval, optlen);
+		break;
+	case SCTP_SOCKOPT_PEELOFF_FLAGS:
+		retval = sctp_getsockopt_peeloff_flags(sk, len, optval, optlen);
 		break;
 	case SCTP_PEER_ADDR_PARAMS:
 		retval = sctp_getsockopt_peer_addr_params(sk, len, optval,