Fix race while allocating free loop device.

Fixes Issue #314.

Thanks to Martin Jeřábek for the report.

lib/utils_device.c

@@ -419,25 +419,22 @@ out:
 
 static int device_internal_prepare(struct crypt_device *cd, struct device *device)
 {
-	char *loop_device, *file_path = NULL;
+	char *loop_device = NULL, *file_path = NULL;
 	int r, loop_fd, readonly = 0;
 
 	if (device->init_done)
 		return 0;
 
-	log_dbg("Allocating a free loop device.");
+	if (getuid() || geteuid()) {
-	loop_device = crypt_loop_get_device();
+		log_err(cd, _("Cannot use a loopback device, "
-	if (!loop_device) {
+			      "running as non-root user.\n"));
-		if (getuid() || geteuid())
-			log_err(cd, _("Cannot use a loopback device, "
-				      "running as non-root user.\n"));
-		else
-			log_err(cd, _("Cannot find a free loopback device.\n"));
 		return -ENOTSUP;
 	}
 
+	log_dbg("Allocating a free loop device.");
+
 	/* Keep the loop open, dettached on last close. */
-	loop_fd = crypt_loop_attach(loop_device, device->path, 0, 1, &readonly);
+	loop_fd = crypt_loop_attach(&loop_device, device->path, 0, 1, &readonly);
 	if (loop_fd == -1) {
 		log_err(cd, _("Attaching loopback device failed "
 			"(loop device with autoclear flag is required).\n"));

lib/utils_loop.c

@@ -19,6 +19,7 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA.
  */
 
+#include <stdlib.h>
 #include <string.h>
 #include <stdio.h>
 #include <unistd.h>
@@ -73,7 +74,7 @@ static char *crypt_loop_get_device_old(void)
 	return NULL;
 }
 
-char *crypt_loop_get_device(void)
+static char *crypt_loop_get_device(void)
 {
 	char dev[64];
 	int i, loop_fd;
@@ -99,13 +100,15 @@ char *crypt_loop_get_device(void)
 	return strdup(dev);
 }
 
-int crypt_loop_attach(const char *loop, const char *file, int offset,
+int crypt_loop_attach(char **loop, const char *file, int offset,
 		      int autoclear, int *readonly)
 {
 	struct loop_info64 lo64 = {0};
 	char *lo_file_name;
 	int loop_fd = -1, file_fd = -1, r = 1;
 
+	*loop = NULL;
+
 	file_fd = open(file, (*readonly ? O_RDONLY : O_RDWR) | O_EXCL);
 	if (file_fd < 0 && (errno == EROFS || errno == EACCES) && !*readonly) {
 		*readonly = 1;
@@ -114,9 +117,25 @@ int crypt_loop_attach(const char *loop, const char *file, int offset,
 	if (file_fd < 0)
 		goto out;
 
-	loop_fd = open(loop, *readonly ? O_RDONLY : O_RDWR);
+	while (loop_fd < 0)  {
-	if (loop_fd < 0)
+		*loop = crypt_loop_get_device();
-		goto out;
+		if (!*loop)
+			goto out;
+
+		loop_fd = open(*loop, *readonly ? O_RDONLY : O_RDWR);
+		if (loop_fd < 0)
+			goto out;
+
+		if (ioctl(loop_fd, LOOP_SET_FD, file_fd) < 0) {
+			if (errno != EBUSY)
+				goto out;
+			free(*loop);
+			*loop = NULL;
+
+			close(loop_fd);
+			loop_fd = -1;
+		}
+	}
 
 	lo_file_name = (char*)lo64.lo_file_name;
 	lo_file_name[LO_NAME_SIZE-1] = '\0';
@@ -125,9 +144,6 @@ int crypt_loop_attach(const char *loop, const char *file, int offset,
 	if (autoclear)
 		lo64.lo_flags |= LO_FLAGS_AUTOCLEAR;
 
-	if (ioctl(loop_fd, LOOP_SET_FD, file_fd) < 0)
-		goto out;
-
 	if (ioctl(loop_fd, LOOP_SET_STATUS64, &lo64) < 0) {
 		(void)ioctl(loop_fd, LOOP_CLR_FD, 0);
 		goto out;
@@ -149,6 +165,10 @@ out:
 		close(loop_fd);
 	if (file_fd >= 0)
 		close(file_fd);
+	if (r && *loop) {
+		free(*loop);
+		*loop = NULL;
+	}
 	return r ? -1 : loop_fd;
 }
 

lib/utils_loop.h

@@ -24,10 +24,9 @@
 
 /* loopback device helpers */
 
-char *crypt_loop_get_device(void);
 char *crypt_loop_backing_file(const char *loop);
 int crypt_loop_device(const char *loop);
-int crypt_loop_attach(const char *loop, const char *file, int offset,
+int crypt_loop_attach(char **loop, const char *file, int offset,
 		      int autoclear, int *readonly);
 int crypt_loop_detach(const char *loop);
 int crypt_loop_resize(const char *loop);

tests/api-test.c

@@ -423,16 +423,8 @@ static int _setup(void)
 	if (_system(cmd, 1))
 		return 1;
 
-	if (!THE_LOOP_DEV)
+	fd = crypt_loop_attach(&THE_LOOP_DEV, test_loop_file, 0, 0, &ro);
-		THE_LOOP_DEV = crypt_loop_get_device();
+	close(fd);
-	if (!THE_LOOP_DEV) {
-		printf("Cannot find free loop device.\n");
-		return 1;
-	}
-	if (crypt_loop_device(THE_LOOP_DEV)) {
-		fd = crypt_loop_attach(THE_LOOP_DEV, test_loop_file, 0, 0, &ro);
-		close(fd);
-	}
 
 	tmp_file_1 = strdup(THE_LFILE_TEMPLATE);
 	if ((fd=mkstemp(tmp_file_1)) == -1) {
@@ -447,34 +439,15 @@ static int _setup(void)
 
 	_system("dmsetup create " DEVICE_EMPTY_name " --table \"0 10000 zero\"", 1);
 	_system("dmsetup create " DEVICE_ERROR_name " --table \"0 10000 error\"", 1);
-	if (!DEVICE_1)
+
-		DEVICE_1 = crypt_loop_get_device();
+	_system(" [ ! -e " IMAGE1 " ] && bzip2 -dk " IMAGE1 ".bz2", 1);
-	if (!DEVICE_1) {
+	fd = crypt_loop_attach(&DEVICE_1, IMAGE1, 0, 0, &ro);
-		printf("Cannot find free loop device.\n");
+	close(fd);
-		return 1;
+
-	}
+	_system("dd if=/dev/zero of=" IMAGE_EMPTY " bs=1M count=4 2>/dev/null", 1);
-	if (crypt_loop_device(DEVICE_1)) {
+	fd = crypt_loop_attach(&DEVICE_2, IMAGE_EMPTY, 0, 0, &ro);
-		_system(" [ ! -e " IMAGE1 " ] && bzip2 -dk " IMAGE1 ".bz2", 1);
+	close(fd);
-		fd = crypt_loop_attach(DEVICE_1, IMAGE1, 0, 0, &ro);
+
-		close(fd);
-	}
-	if (!DEVICE_2)
-		DEVICE_2 = crypt_loop_get_device();
-	if (!DEVICE_2) {
-		printf("Cannot find free loop device.\n");
-		return 1;
-	}
-	if (crypt_loop_device(DEVICE_2)) {
-		_system("dd if=/dev/zero of=" IMAGE_EMPTY " bs=1M count=4 2>/dev/null", 1);
-		fd = crypt_loop_attach(DEVICE_2, IMAGE_EMPTY, 0, 0, &ro);
-		close(fd);
-	}
-	if (!DEVICE_3)
-		DEVICE_3 = crypt_loop_get_device();
-	if (!DEVICE_3) {
-		printf("Cannot find free loop device.\n");
-		return 1;
-	}
 	/* Keymaterial offset is less than 8 sectors */
 	_system(" [ ! -e " EVL_HEADER_1 " ] && bzip2 -dk " EVL_HEADER_1 ".bz2", 1);
 	/* keymaterial offset aims into payload area */
@@ -1486,7 +1459,7 @@ static void LuksHeaderBackup(void)
 	crypt_free(cd);
 
 	// exercise luksOpen using backup header on block device
-	fd = crypt_loop_attach(DEVICE_3, BACKUP_FILE, 0, 0, &ro);
+	fd = crypt_loop_attach(&DEVICE_3, BACKUP_FILE, 0, 0, &ro);
 	close(fd);
 	OK_(fd < 0);
 	OK_(crypt_init(&cd, DEVICE_3));