iov.c

// 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_skip_bytes() - Skip leading bytes of an IO vector
 * @iov:	IO vector
 * @n:		Number of entries in @iov
 * @skip:	Number of leading bytes of @iov to skip
 * @offset:	Offset of first unskipped byte in its @iov entry
 *
 * Return: index I of individual struct iovec which contains the byte at @skip
 *         bytes into the vector (as though all its buffers were contiguous).
 *         If @offset is non-NULL, update it to the offset of that byte within
 *         @iov[I] (guaranteed to be less than @iov[I].iov_len) If the whole
 *         vector has <= @skip bytes, return @n.
 */
size_t iov_skip_bytes(const struct iovec *iov, size_t n,
		      size_t skip, size_t *offset)
{
	size_t off = skip, i;

	for (i = 0; i < n; i++) {
		if (off < iov[i].iov_len)
			break;
		off -= iov[i].iov_len;
	}

	if (offset)
		*offset = off;

	return i;
}

/**
 * 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.
 */
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;
	}

	i = iov_skip_bytes(iov, iov_cnt, offset, &offset);

	/* 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;
	}

	i = iov_skip_bytes(iov, iov_cnt, offset, &offset);

	/* 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.
 */
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_tail_prune() - Remove any unneeded buffers from an IOV tail
 * @tail:	IO vector tail (modified)
 *
 * If an IOV tail's offset is large enough, it may not include any bytes from
 * the first (or first several) buffers in the underlying IO vector.  Modify the
 * tail's representation so it contains the same logical bytes, but only
 * includes buffers that are actually needed.  This will avoid stepping through
 * unnecessary elements of the underlying IO vector on future operations.
 *
 * Return:	true if the tail still contains any bytes, otherwise false
 */
bool iov_tail_prune(struct iov_tail *tail)
{
	size_t i;

	i = iov_skip_bytes(tail->iov, tail->cnt, tail->off, &tail->off);
	tail->iov += i;
	tail->cnt -= i;

	return !!tail->cnt;
}

/**
 * iov_tail_size - Calculate the total size of an IO vector tail
 * @tail:	IO vector tail
 *
 * Returns:    The total size in bytes.
 */
size_t iov_tail_size(struct iov_tail *tail)
{
	iov_tail_prune(tail);
	return iov_size(tail->iov, tail->cnt) - tail->off;
}

/**
 * iov_peek_header_() - Get pointer to a header from an IOV tail
 * @tail:	IOV tail to get header from
 * @len:	Length of header to get, in bytes
 * @align:	Required alignment of header, in bytes
 *
 * @tail may be pruned, but will represent the same bytes as before.
 *
 * Returns: Pointer to the first @len logical bytes of the tail, NULL if that
 *	    overruns the IO vector, is not contiguous or doesn't have the
 *	    requested alignment.
 */
void *iov_peek_header_(struct iov_tail *tail, size_t len, size_t align)
{
	char *p;

	if (!iov_tail_prune(tail))
		return NULL; /* Nothing left */

	if (tail->off + len < tail->off)
		return NULL; /* Overflow */

	if (tail->off + len > tail->iov[0].iov_len)
		return NULL; /* Not contiguous */

	p = (char *)tail->iov[0].iov_base + tail->off;
	if ((uintptr_t)p % align)
		return NULL; /* not aligned */

	return p;
}

/**
 * iov_remove_header_() - Remove a header from an IOV tail
 * @tail:	IOV tail to remove header from (modified)
 * @len:	Length of header to remove, in bytes
 * @align:	Required alignment of header, in bytes
 *
 * On success, @tail is updated so that it longer includes the bytes of the
 * returned header.
 *
 * Returns: Pointer to the first @len logical bytes of the tail, NULL if that
 *	    overruns the IO vector, is not contiguous or doesn't have the
 *	    requested alignment.
 */
void *iov_remove_header_(struct iov_tail *tail, size_t len, size_t align)
{
	char *p = iov_peek_header_(tail, len, align);

	if (!p)
		return NULL;

	tail->off = tail->off + len;
	return p;
}