tempfile.c

/* A secure tmpfile() replacement.
 *
 * Copyright (c) 2015-2021  Joachim Wiberg <troglobit@gmail.com>
 *
 * Permission to use, copy, modify, and/or distribute this software for any
 * purpose with or without fee is hereby granted, provided that the above
 * copyright notice and this permission notice appear in all copies.
 *
 * THE SOFTWARE IS PROVIDED "AS IS" AND THE AUTHOR DISCLAIMS ALL WARRANTIES
 * WITH REGARD TO THIS SOFTWARE INCLUDING ALL IMPLIED WARRANTIES OF
 * MERCHANTABILITY AND FITNESS. IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR
 * ANY SPECIAL, DIRECT, INDIRECT, OR CONSEQUENTIAL DAMAGES OR ANY DAMAGES
 * WHATSOEVER RESULTING FROM LOSS OF USE, DATA OR PROFITS, WHETHER IN AN
 * ACTION OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF
 * OR IN CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE.
 */
#define _GNU_SOURCE
#include <errno.h>
#include <paths.h>
#include <fcntl.h>		/* O_TMPFILE requires -D_GNU_SOURCE */
#include <stdio.h>		/* fdopen() */
#include <stdlib.h>		/* mkstemp() */
#include <unistd.h>   /* mkstemp() */
#include <sys/stat.h>		/* umask() */

static FILE *fallback(void)
{
	char nm[15] = _PATH_TMP "XXXXXXXX";
	int fd;

	fd = mkstemp(nm);
	if (-1 == fd)
		return NULL;

	if (-1 == fcntl(fd, F_SETFD, FD_CLOEXEC)) {
		close(fd);
		return NULL;
	}
	return fdopen(fd, "w+");
}

/**
 * A secure tmpfile() replacement
 *
 * This is the secure replacement for tmpfile() that does not exist in
 * GLIBC.  It uses the Linux specific @c O_TMPFILE and @c O_EXCL to hide
 * the filename.  When the @c FILE is fclose()'ed, the file contents is
 * lost.  The file is hidden in the @c _PATH_TMP ("/tmp") directory in
 * the system.
 *
 * This function requires Linux 3.11, or later, due to @c O_TMPFILE.
 * Not all file systems support hidden inodes, in which case this
 * function defaults to call tmpfile() as a fallback.
 *
 * @returns An open @c FILE pointer, or @c NULL on error.
 */
FILE *tempfile(void)
{
#ifdef O_TMPFILE	  /* Only on Linux, with fairly recent (G)LIBC */
	int fd;
	mode_t oldmask;

	oldmask = umask(0077);
	fd = open(_PATH_TMP, O_TMPFILE | O_RDWR | O_EXCL | O_CLOEXEC, S_IRUSR | S_IWUSR);
	umask(oldmask);
	if (-1 == fd) {
		if (errno == EOPNOTSUPP)
			return fallback();

		return NULL;
	}

	return fdopen(fd, "w+");
#else
	return fallback();
#endif
}

/**
 * Local Variables:
 *  indent-tabs-mode: t
 *  c-file-style: "linux"
 * End:
 */