iov: add some functions to manage iovec

Introduce functions to copy to/from a buffer from/to an iovec array,
to compute data length in in bytes of an iovec and to copy memory from
an iovec to another.

iov_from_buf(), iov_to_buf(), iov_size(), iov_copy().

Signed-off-by: Laurent Vivier <lvivier@redhat.com>
Message-ID: <20240217150725.661467-2-lvivier@redhat.com>
[dwg: Small changes to suppress cppcheck warnings]
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

Makefile

@@ -45,16 +45,16 @@ FLAGS += -DVERSION=\"$(VERSION)\"
 FLAGS += -DDUAL_STACK_SOCKETS=$(DUAL_STACK_SOCKETS)
 
 PASST_SRCS = arch.c arp.c checksum.c conf.c dhcp.c dhcpv6.c flow.c icmp.c \
-	igmp.c isolation.c lineread.c log.c mld.c ndp.c netlink.c packet.c \
+	igmp.c iov.c isolation.c lineread.c log.c mld.c ndp.c netlink.c \
-	passt.c pasta.c pcap.c pif.c port_fwd.c tap.c tcp.c tcp_splice.c udp.c \
+	packet.c passt.c pasta.c pcap.c pif.c port_fwd.c tap.c tcp.c \
-	util.c
+	tcp_splice.c udp.c util.c
 QRAP_SRCS = qrap.c
 SRCS = $(PASST_SRCS) $(QRAP_SRCS)
 
 MANPAGES = passt.1 pasta.1 qrap.1
 
 PASST_HEADERS = arch.h arp.h checksum.h conf.h dhcp.h dhcpv6.h flow.h \
-	flow_table.h icmp.h inany.h isolation.h lineread.h log.h ndp.h \
+	flow_table.h icmp.h inany.h iov.h isolation.h lineread.h log.h ndp.h \
 	netlink.h packet.h passt.h pasta.h pcap.h pif.h port_fwd.h siphash.h \
 	tap.h tcp.h tcp_conn.h tcp_splice.h udp.h util.h
 HEADERS = $(PASST_HEADERS) seccomp.h

iov.c

@@ -0,0 +1,174 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * iov.h - helpers for using (partial) iovecs.
+ *
+ * Copyright Red Hat
+ * Author: Laurent Vivier <lvivier@redhat.com>
+ *
+ * This file also contains code originally from QEMU include/qemu/iov.h
+ * and licensed under the following terms:
+ *
+ * Copyright (C) 2010 Red Hat, Inc.
+ *
+ * Author(s):
+ *  Amit Shah <amit.shah@redhat.com>
+ *  Michael Tokarev <mjt@tls.msk.ru>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.  See
+ * the COPYING file in the top-level directory.
+ *
+ * Contributions after 2012-01-13 are licensed under the terms of the
+ * GNU GPL, version 2 or (at your option) any later version.
+ */
+#include <sys/socket.h>
+
+#include "util.h"
+#include "iov.h"
+
+/**
+ * iov_from_buf - Copy data from a buffer to an I/O vector (struct iovec)
+ *                efficiently.
+ *
+ * @iov:       Pointer to the array of struct iovec describing the
+ *             scatter/gather I/O vector.
+ * @iov_cnt:   Number of elements in the iov array.
+ * @offset:    Byte offset in the iov array where copying should start.
+ * @buf:       Pointer to the source buffer containing the data to copy.
+ * @bytes:     Total number of bytes to copy from buf to iov.
+ *
+ * Returns:    The number of bytes successfully copied.
+ */
+/* cppcheck-suppress unusedFunction */
+size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
+		    size_t offset, const void *buf, size_t bytes)
+{
+	unsigned int i;
+	size_t copied;
+
+	if (__builtin_constant_p(bytes) && iov_cnt &&
+		offset <= iov[0].iov_len && bytes <= iov[0].iov_len - offset) {
+		memcpy((char *)iov[0].iov_base + offset, buf, bytes);
+		return bytes;
+	}
+
+	/* skipping offset bytes in the iovec */
+	for (i = 0; i < iov_cnt && offset >= iov[i].iov_len; i++)
+		offset -= iov[i].iov_len;
+
+	/* copying data */
+	for (copied = 0; copied < bytes && i < iov_cnt; i++) {
+		size_t len = MIN(iov[i].iov_len - offset, bytes - copied);
+
+		memcpy((char *)iov[i].iov_base + offset, (char *)buf + copied,
+		       len);
+		copied += len;
+		offset = 0;
+	}
+
+	return copied;
+}
+
+/**
+ * iov_to_buf - Copy data from a scatter/gather I/O vector (struct iovec) to
+ *		a buffer efficiently.
+ *
+ * @iov:       Pointer to the array of struct iovec describing the scatter/gather
+ *             I/O vector.
+ * @iov_cnt:   Number of elements in the iov array.
+ * @offset:    Offset within the first element of iov from where copying should start.
+ * @buf:       Pointer to the destination buffer where data will be copied.
+ * @bytes:     Total number of bytes to copy from iov to buf.
+ *
+ * Returns:    The number of bytes successfully copied.
+ */
+/* cppcheck-suppress unusedFunction */
+size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,
+		  size_t offset, void *buf, size_t bytes)
+{
+	unsigned int i;
+	size_t copied;
+
+	if (__builtin_constant_p(bytes) && iov_cnt &&
+		offset <= iov[0].iov_len && bytes <= iov[0].iov_len - offset) {
+		memcpy(buf, (char *)iov[0].iov_base + offset, bytes);
+		return bytes;
+	}
+
+	/* skipping offset bytes in the iovec */
+	for (i = 0; i < iov_cnt && offset >= iov[i].iov_len; i++)
+		offset -= iov[i].iov_len;
+
+	/* copying data */
+	for (copied = 0; copied < bytes && i < iov_cnt; i++) {
+		size_t len = MIN(iov[i].iov_len - offset, bytes - copied);
+		memcpy((char *)buf + copied, (char *)iov[i].iov_base + offset,
+		       len);
+		copied += len;
+		offset = 0;
+	}
+
+	return copied;
+}
+
+/**
+ * iov_size - Calculate the total size of a scatter/gather I/O vector
+ *            (struct iovec).
+ *
+ * @iov:       Pointer to the array of struct iovec describing the
+ *             scatter/gather I/O vector.
+ * @iov_cnt:   Number of elements in the iov array.
+ *
+ * Returns:    The total size in bytes.
+ */
+/* cppcheck-suppress unusedFunction */
+size_t iov_size(const struct iovec *iov, size_t iov_cnt)
+{
+	unsigned int i;
+	size_t len;
+
+	for (i = 0, len = 0; i < iov_cnt; i++)
+		len += iov[i].iov_len;
+
+	return len;
+}
+
+/**
+ * iov_copy - Copy data from one scatter/gather I/O vector (struct iovec) to
+ *            another.
+ *
+ * @dst_iov:      Pointer to the destination array of struct iovec describing
+ *                the scatter/gather I/O vector to copy to.
+ * @dst_iov_cnt:  Number of elements in the destination iov array.
+ * @iov:          Pointer to the source array of struct iovec describing
+ *                the scatter/gather I/O vector to copy from.
+ * @iov_cnt:      Number of elements in the source iov array.
+ * @offset:       Offset within the source iov from where copying should start.
+ * @bytes:        Total number of bytes to copy from iov to dst_iov.
+ *
+ * Returns:       The number of elements successfully copied to the destination
+ *                iov array.
+ */
+/* cppcheck-suppress unusedFunction */
+unsigned iov_copy(struct iovec *dst_iov, size_t dst_iov_cnt,
+		  const struct iovec *iov, size_t iov_cnt,
+		  size_t offset, size_t bytes)
+{
+	unsigned int i, j;
+
+	/* skipping offset bytes in the iovec */
+	for (i = 0; i < iov_cnt && offset >= iov[i].iov_len; i++)
+		offset -= iov[i].iov_len;
+
+	/* copying data */
+	for (j = 0; i < iov_cnt && j < dst_iov_cnt && bytes; i++) {
+		size_t len = MIN(bytes, iov[i].iov_len - offset);
+
+		dst_iov[j].iov_base = (char *)iov[i].iov_base + offset;
+		dst_iov[j].iov_len = len;
+		j++;
+		bytes -= len;
+		offset = 0;
+	}
+
+	return j;
+}

iov.h

@@ -0,0 +1,29 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * iov.c - helpers for using (partial) iovecs.
+ *
+ * Copyrigh Red Hat
+ * Author: Laurent Vivier <lvivier@redhat.com>
+ *
+ * This file also contains code originally from QEMU include/qemu/iov.h:
+ *
+ * Author(s):
+ *  Amit Shah <amit.shah@redhat.com>
+ *  Michael Tokarev <mjt@tls.msk.ru>
+ */
+
+#ifndef IOVEC_H
+#define IOVEC_H
+
+#include <unistd.h>
+#include <string.h>
+
+size_t iov_from_buf(const struct iovec *iov, size_t iov_cnt,
+                    size_t offset, const void *buf, size_t bytes);
+size_t iov_to_buf(const struct iovec *iov, size_t iov_cnt,
+                  size_t offset, void *buf, size_t bytes);
+size_t iov_size(const struct iovec *iov, size_t iov_cnt);
+unsigned iov_copy(struct iovec *dst_iov, size_t dst_iov_cnt,
+		  const struct iovec *iov, size_t iov_cnt,
+		  size_t offset, size_t bytes);
+#endif /* IOVEC_H */