cursor/os-compatibility.c

/*
 * Copyright © 2012 Collabora, Ltd.
 *
 * Permission is hereby granted, free of charge, to any person obtaining
 * a copy of this software and associated documentation files (the
 * "Software"), to deal in the Software without restriction, including
 * without limitation the rights to use, copy, modify, merge, publish,
 * distribute, sublicense, and/or sell copies of the Software, and to
 * permit persons to whom the Software is furnished to do so, subject to
 * the following conditions:
 *
 * The above copyright notice and this permission notice (including the
 * next paragraph) shall be included in all copies or substantial
 * portions of the Software.
 *
 * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND,
 * EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF
 * MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND
 * NONINFRINGEMENT.  IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS
 * BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN
 * ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN
 * CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 * SOFTWARE.
 */

#define _GNU_SOURCE

#include "config.h"

#include <sys/types.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <string.h>
#include <stdlib.h>

#ifdef HAVE_MEMFD_CREATE
#include <sys/mman.h>
#endif

#include "os-compatibility.h"

#ifndef HAVE_MKOSTEMP
static int
set_cloexec_or_close(int fd)
{
	long flags;

	if (fd == -1)
		return -1;

	flags = fcntl(fd, F_GETFD);
	if (flags == -1)
		goto err;

	if (fcntl(fd, F_SETFD, flags | FD_CLOEXEC) == -1)
		goto err;

	return fd;

err:
	close(fd);
	return -1;
}
#endif

static int
create_tmpfile_cloexec(char *tmpname)
{
	int fd;

#ifdef HAVE_MKOSTEMP
	fd = mkostemp(tmpname, O_CLOEXEC);
	if (fd >= 0)
		unlink(tmpname);
#else
	fd = mkstemp(tmpname);
	if (fd >= 0) {
		fd = set_cloexec_or_close(fd);
		unlink(tmpname);
	}
#endif

	return fd;
}

/*
 * Create a new, unique, anonymous file of the given size, and
 * return the file descriptor for it. The file descriptor is set
 * CLOEXEC. The file is immediately suitable for mmap()'ing
 * the given size at offset zero.
 *
 * The file should not have a permanent backing store like a disk,
 * but may have if XDG_RUNTIME_DIR is not properly implemented in OS.
 *
 * The file name is deleted from the file system.
 *
 * The file is suitable for buffer sharing between processes by
 * transmitting the file descriptor over Unix sockets using the
 * SCM_RIGHTS methods.
 *
 * If the C library implements posix_fallocate(), it is used to
 * guarantee that disk space is available for the file at the
 * given size. If disk space is insufficient, errno is set to ENOSPC.
 * If posix_fallocate() is not supported, program may receive
 * SIGBUS on accessing mmap()'ed file contents instead.
 *
 * If the C library implements memfd_create(), it is used to create the
 * file purely in memory, without any backing file name on the file
 * system, and then sealing off the possibility of shrinking it.  This
 * can then be checked before accessing mmap()'ed file contents, to
 * make sure SIGBUS can't happen.  It also avoids requiring
 * XDG_RUNTIME_DIR.
 */
int
os_create_anonymous_file(off_t size)
{
	static const char template[] = "/wayland-cursor-shared-XXXXXX";
	const char *path;
	char *name;
	int fd;

#ifdef HAVE_MEMFD_CREATE
	fd = memfd_create("wayland-cursor", MFD_CLOEXEC | MFD_ALLOW_SEALING);
	if (fd >= 0) {
		/* We can add this seal before calling posix_fallocate(), as
		 * the file is currently zero-sized anyway.
		 *
		 * There is also no need to check for the return value, we
		 * couldn't do anything with it anyway.
		 */
		fcntl(fd, F_ADD_SEALS, F_SEAL_SHRINK | F_SEAL_SEAL);
	} else
#endif
	{
		path = getenv("XDG_RUNTIME_DIR");
		if (!path) {
			errno = ENOENT;
			return -1;
		}

		name = malloc(strlen(path) + sizeof(template));
		if (!name)
			return -1;

		strcpy(name, path);
		strcat(name, template);

		fd = create_tmpfile_cloexec(name);

		free(name);

		if (fd < 0)
			return -1;
	}

	if (os_resize_anonymous_file(fd, size) < 0) {
		close(fd);
		return -1;
	}

	return fd;
}

int
os_resize_anonymous_file(int fd, off_t size)
{
#ifdef HAVE_POSIX_FALLOCATE
	/* 
	 * Filesystems that do support fallocate will return EINVAL or
	 * EOPNOTSUPP. In this case we need to fall back to ftruncate
	 */
	errno = posix_fallocate(fd, 0, size);
	if (errno == 0)
		return 0;
	else if (errno != EINVAL && errno != EOPNOTSUPP)
		return -1;
#endif
	if (ftruncate(fd, size) < 0)
		return -1;

	return 0;
}