Setup xloop device with XLOOP_CONFIGURE ioctl call

This feature allows the userspace tool xlosetup to completely setup a
loop device with a single ioctl call, removing the in-between state
where the device can be partially configured, eg. the loop device has
a backing file associated with it, but is reading from the wrong offset.

Besides removing the intermediate state, another big benefit of this
ioctl is that XLOOP_SET_STATUS can be slow. The main reason for this
slowness is that XLOOP_SET_STATUS(64) calls blk_mq_freeze_queue() to
freeze the associated queue. This requires waiting for RCU.

4 files changed, 113 insertions, 52 deletions

diff --git a/src/utils/include/loopdev.h b/src/utils/include/loopdev.h
index 0221e6b..9fa0688 100644
--- a/src/utils/include/loopdev.h
+++ b/src/utils/include/loopdev.h
@@ -75,6 +75,19 @@ struct loop_info64 {
 	uint32_t	lo_file_fmt_type;
 };
 
+#ifndef LOOP_CONFIGURE
+/*
+ * Since Linux v5.8-rc1 (commit 3448914e8cc550ba792d4ccc74471d1ca4293aae)
+ */
+# define LOOP_CONFIGURE		0x4C0A
+struct loop_config {
+  uint32_t fd;
+  uint32_t block_size;
+  struct loop_info64 info;
+  uint64_t __reserved[8];
+};
+#endif
+
 #define LOOPDEV_MAJOR	XLOOP_MAJOR	/* loop major number */
 #define LOOPDEV_DEFAULT_NNODES	CONFIG_BLK_DEV_XLOOP_MIN_COUNT	/* default number of loop devices */
 
@@ -114,7 +127,7 @@ struct loopdev_cxt {
 	unsigned int    control_ok:1;	/* /dev/loop-control success */
 
 	struct path_cxt		*sysfs; /* pointer to /sys/dev/block/<maj:min>/ */
-	struct loop_info64	info;	/* for GET/SET ioctl */
+	struct loop_config 	config;	/* for GET/SET ioctl */
 	struct loopdev_iter	iter;	/* scans /sys or /dev for used/free devices */
 };
 
diff --git a/src/utils/lib/loopdev.c b/src/utils/lib/loopdev.c
index be4e486..194daf9 100644
--- a/src/utils/lib/loopdev.c
+++ b/src/utils/lib/loopdev.c
@@ -100,7 +100,7 @@ int loopcxt_set_device(struct loopdev_cxt *lc, const char *device)
 	lc->has_info = 0;
 	lc->info_failed = 0;
 	*lc->device = '\0';
-	memset(&lc->info, 0, sizeof(lc->info));
+	memset(&lc->config, 0, sizeof(lc->config));
 
 	/* set new */
 	if (device) {
@@ -659,17 +659,17 @@ struct loop_info64 *loopcxt_get_info(struct loopdev_cxt *lc)
 	}
 	errno = 0;
 	if (lc->has_info)
-		return &lc->info;
+		return &lc->config.info;
 
 	fd = loopcxt_get_fd(lc);
 	if (fd < 0)
 		return NULL;
 
-	if (ioctl(fd, LOOP_GET_STATUS64, &lc->info) == 0) {
+	if (ioctl(fd, LOOP_GET_STATUS64, &lc->config.info) == 0) {
 		lc->has_info = 1;
 		lc->info_failed = 0;
 		DBG(CXT, ul_debugobj(lc, "reading loop_info64 OK"));
-		return &lc->info;
+		return &lc->config.info;
 	}
 
 	lc->info_failed = 1;
@@ -1156,7 +1156,7 @@ int loopcxt_set_offset(struct loopdev_cxt *lc, uint64_t offset)
 {
 	if (!lc)
 		return -EINVAL;
-	lc->info.lo_offset = offset;
+	lc->config.info.lo_offset = offset;
 
 	DBG(CXT, ul_debugobj(lc, "set offset=%jd", offset));
 	return 0;
@@ -1169,7 +1169,7 @@ int loopcxt_set_sizelimit(struct loopdev_cxt *lc, uint64_t sizelimit)
 {
 	if (!lc)
 		return -EINVAL;
-	lc->info.lo_sizelimit = sizelimit;
+	lc->config.info.lo_sizelimit = sizelimit;
 
 	DBG(CXT, ul_debugobj(lc, "set sizelimit=%jd", sizelimit));
 	return 0;
@@ -1202,7 +1202,7 @@ int loopcxt_set_blocksize(struct loopdev_cxt *lc, uint64_t blocksize)
 int loopcxt_set_file_fmt_type(struct loopdev_cxt *lc, uint32_t file_fmt_type) {
 	if (!lc)
 		return -EINVAL;
-	lc->info.lo_file_fmt_type = file_fmt_type;
+	lc->config.info.lo_file_fmt_type = file_fmt_type;
 
 	DBG(CXT, ul_debugobj(lc, "set file_fmt_type=%u", (unsigned) file_fmt_type));
 	return 0;
@@ -1220,7 +1220,7 @@ int loopcxt_set_flags(struct loopdev_cxt *lc, uint32_t flags)
 {
 	if (!lc)
 		return -EINVAL;
-	lc->info.lo_flags = flags;
+	lc->config.info.lo_flags = flags;
 
 	DBG(CXT, ul_debugobj(lc, "set flags=%u", (unsigned) flags));
 	return 0;
@@ -1243,9 +1243,9 @@ int loopcxt_set_backing_file(struct loopdev_cxt *lc, const char *filename)
 	if (!lc->filename)
 		return -errno;
 
-	xstrncpy((char *)lc->info.lo_file_name, lc->filename, LO_NAME_SIZE);
+	xstrncpy((char *)lc->config.info.lo_file_name, lc->filename, LO_NAME_SIZE);
 
-	DBG(CXT, ul_debugobj(lc, "set backing file=%s", lc->info.lo_file_name));
+	DBG(CXT, ul_debugobj(lc, "set backing file=%s", lc->config.info.lo_file_name));
 	return 0;
 }
 
@@ -1268,7 +1268,7 @@ static int loopcxt_check_size(struct loopdev_cxt *lc, int file_fd)
 	int dev_fd;
 	struct stat st;
 
-	if (!lc->info.lo_offset && !lc->info.lo_sizelimit)
+	if (!lc->config.info.lo_offset && !lc->config.info.lo_sizelimit)
 		return 0;
 
 	if (fstat(file_fd, &st)) {
@@ -1284,16 +1284,16 @@ static int loopcxt_check_size(struct loopdev_cxt *lc, int file_fd)
 	} else
 		expected_size = st.st_size;
 
-	if (expected_size == 0 || expected_size <= lc->info.lo_offset) {
+	if (expected_size == 0 || expected_size <= lc->config.info.lo_offset) {
 		DBG(CXT, ul_debugobj(lc, "failed to determine expected size"));
 		return 0;	/* ignore this error */
 	}
 
-	if (lc->info.lo_offset > 0)
-		expected_size -= lc->info.lo_offset;
+	if (lc->config.info.lo_offset > 0)
+		expected_size -= lc->config.info.lo_offset;
 
-	if (lc->info.lo_sizelimit > 0 && lc->info.lo_sizelimit < expected_size)
-		expected_size = lc->info.lo_sizelimit;
+	if (lc->config.info.lo_sizelimit > 0 && lc->config.info.lo_sizelimit < expected_size)
+		expected_size = lc->config.info.lo_sizelimit;
 
 	dev_fd = loopcxt_get_fd(lc);
 	if (dev_fd < 0) {
@@ -1361,6 +1361,7 @@ int loopcxt_setup_device(struct loopdev_cxt *lc)
 {
 	int file_fd, dev_fd, mode = O_RDWR, rc = -1, cnt = 0, err, again;
 	int errsv = 0;
+	int fallback = 0;
 
 	if (!lc || !*lc->device || !lc->filename)
 		return -EINVAL;
@@ -1370,7 +1371,7 @@ int loopcxt_setup_device(struct loopdev_cxt *lc)
 	/*
 	 * Open backing file and device
 	 */
-	if (lc->info.lo_flags & LO_FLAGS_READ_ONLY)
+	if (lc->config.info.lo_flags & LO_FLAGS_READ_ONLY)
 		mode = O_RDONLY;
 
 	if ((file_fd = open(lc->filename, mode | O_CLOEXEC)) < 0) {
@@ -1393,10 +1394,10 @@ int loopcxt_setup_device(struct loopdev_cxt *lc)
 
 	if (mode == O_RDONLY) {
 		lc->flags |= LOOPDEV_FL_RDONLY;			/* open() mode */
-		lc->info.lo_flags |= LO_FLAGS_READ_ONLY;	/* kernel loopdev mode */
+		lc->config.info.lo_flags |= LO_FLAGS_READ_ONLY;	/* kernel loopdev mode */
 	} else {
 		lc->flags |= LOOPDEV_FL_RDWR;			/* open() mode */
-		lc->info.lo_flags &= ~LO_FLAGS_READ_ONLY;
+		lc->config.info.lo_flags &= ~LO_FLAGS_READ_ONLY;
 		lc->flags &= ~LOOPDEV_FL_RDONLY;
 	}
 
@@ -1407,8 +1408,8 @@ int loopcxt_setup_device(struct loopdev_cxt *lc)
 			break;
 		if (errno != EACCES && errno != ENOENT)
 			break;
-		/* We have permissions to open /dev/xloop-control, but open
-		 * /dev/xloopN failed with EACCES, it's probably because udevd
+		/* We have permissions to open /dev/loop-control, but open
+		 * /dev/loopN failed with EACCES, it's probably because udevd
 		 * does not applied chown yet. Let's wait a moment. */
 		xusleep(25000);
 	} while (cnt++ < 16);
@@ -1421,44 +1422,70 @@ int loopcxt_setup_device(struct loopdev_cxt *lc)
 	DBG(SETUP, ul_debugobj(lc, "device open: OK"));
 
 	/*
-	 * Set FD
+	 * Atomic way to configure all by one ioctl call
+	 * -- since Linux v5.8-rc1, commit 3448914e8cc550ba792d4ccc74471d1ca4293aae
 	 */
-	if (ioctl(dev_fd, LOOP_SET_FD, file_fd) < 0) {
+	lc->config.fd = file_fd;
+	if (ioctl(dev_fd, LOOP_CONFIGURE, &lc->config) < 0) {
 		rc = -errno;
 		errsv = errno;
-		DBG(SETUP, ul_debugobj(lc, "LOOP_SET_FD failed: %m"));
-		goto err;
+		if (errno != EINVAL) {
+			DBG(SETUP, ul_debugobj(lc, "LOOP_CONFIGURE failed: %m"));
+			goto err;
+		}
+		fallback = 1;
+	} else {
+		if (lc->blocksize > 0
+			&& (rc = loopcxt_ioctl_blocksize(lc, lc->blocksize)) < 0) {
+			errsv = -rc;
+			goto err;
+		}
+		DBG(SETUP, ul_debugobj(lc, "LOOP_CONFIGURE: OK"));
 	}
 
-	DBG(SETUP, ul_debugobj(lc, "LOOP_SET_FD: OK"));
+	/*
+	 * Old deprecated way; first assign backing file FD and then in the
+	 * second step set loop device properties.
+	 */
+	if (fallback) {
+		if (ioctl(dev_fd, LOOP_SET_FD, file_fd) < 0) {
+			rc = -errno;
+			errsv = errno;
+			DBG(SETUP, ul_debugobj(lc, "LOOP_SET_FD failed: %m"));
+			goto err;
+		}
 
-	if (lc->blocksize > 0
-	    && (rc = loopcxt_ioctl_blocksize(lc, lc->blocksize)) < 0) {
-		errsv = -rc;
-		goto err;
-	}
+		DBG(SETUP, ul_debugobj(lc, "LOOP_SET_FD: OK"));
 
-	do {
-		err = ioctl(dev_fd, LOOP_SET_STATUS64, &lc->info);
-		again = err && errno == EAGAIN;
-		if (again)
-			xusleep(250000);
-	} while (again);
-	if (err) {
-		rc = -errno;
-		errsv = errno;
-		DBG(SETUP, ul_debugobj(lc, "LOOP_SET_STATUS64 failed: %m"));
-		goto err;
-	}
+		if (lc->blocksize > 0
+			&& (rc = loopcxt_ioctl_blocksize(lc, lc->blocksize)) < 0) {
+			errsv = -rc;
+			goto err;
+		}
 
-	DBG(SETUP, ul_debugobj(lc, "LOOP_SET_STATUS64: OK"));
+		do {
+			err = ioctl(dev_fd, LOOP_SET_STATUS64, &lc->config.info);
+			again = err && errno == EAGAIN;
+			if (again)
+				xusleep(250000);
+		} while (again);
+
+		if (err) {
+			rc = -errno;
+			errsv = errno;
+			DBG(SETUP, ul_debugobj(lc, "LOOP_SET_STATUS64 failed: %m"));
+			goto err;
+		}
+
+		DBG(SETUP, ul_debugobj(lc, "LOOP_SET_STATUS64: OK"));
+	}
 
 	if ((rc = loopcxt_check_size(lc, file_fd)))
 		goto err;
 
 	close(file_fd);
 
-	memset(&lc->info, 0, sizeof(lc->info));
+	memset(&lc->config, 0, sizeof(lc->config));
 	lc->has_info = 0;
 	lc->info_failed = 0;
 
@@ -1501,7 +1528,7 @@ int loopcxt_ioctl_status(struct loopdev_cxt *lc)
 	DBG(SETUP, ul_debugobj(lc, "device open: OK"));
 
 	do {
-		err = ioctl(dev_fd, LOOP_SET_STATUS64, &lc->info);
+		err = ioctl(dev_fd, LOOP_SET_STATUS64, &lc->config.info);
 		again = err && errno == EAGAIN;
 		if (again)
 			xusleep(250000);
@@ -1654,7 +1681,7 @@ int loopcxt_find_unused(struct loopdev_cxt *lc)
 	}
 
 	if (rc < 0) {
-		DBG(CXT, ul_debugobj(lc, "using loop scan"));
+		DBG(CXT, ul_debugobj(lc, "using xloop scan"));
 		rc = loopcxt_init_iterator(lc, LOOPITER_FL_FREE);
 		if (rc)
 			return rc;
@@ -1705,6 +1732,17 @@ char *loopdev_get_backing_file(const char *device)
 	return res;
 }
 
+int loopdev_has_backing_file(const char *device)
+{
+	char *tmp = loopdev_get_backing_file(device);
+
+	if (tmp) {
+		free(tmp);
+		return 1;
+	}
+	return 0;
+}
+
 /*
  * Returns: TRUE/FALSE
  */
@@ -1911,4 +1949,3 @@ int loopdev_count_by_backing_file(const char *filename, char **loopdev)
 	}
 	return count;
 }
-
diff --git a/src/utils/sys-utils/xlosetup.8 b/src/utils/sys-utils/xlosetup.8
index 4e063e6..c948899 100644
--- a/src/utils/sys-utils/xlosetup.8
+++ b/src/utils/sys-utils/xlosetup.8
@@ -68,6 +68,13 @@ It's possible to create more independent xloop devices for the same backing
 file.
 .B This setup may be dangerous, can cause data loss, corruption and overwrites.
 Use \fB\-\-nooverlap\fR with \fB\-\-find\fR during setup to avoid this problem.
+.sp
+The xloop device setup is not an atomic operation when used with \fB\-\-find\fP, and
+.B xlosetup
+does not protect this operation by any lock.  The number of attempts is
+internally restricted to a maximum of 64.  It is recommended to use for example
+.BR flock (1)
+to avoid a collision in heavily parallel use cases.
 
 .SH OPTIONS
 The \fIsize\fR and \fIoffset\fR
diff --git a/src/utils/sys-utils/xlosetup.c b/src/utils/sys-utils/xlosetup.c
index 60fa0ba..cbc2e69 100644
--- a/src/utils/sys-utils/xlosetup.c
+++ b/src/utils/sys-utils/xlosetup.c
@@ -469,7 +469,7 @@ static int create_loop(struct loopdev_cxt *lc,
 		       uint64_t blocksize, uint32_t file_fmt_type)
 {
 	int hasdev = loopcxt_has_device(lc);
-	int rc = 0;
+	int rc = 0, ntries = 0;
 
 	/* xlosetup --find --noverlap file.img */
 	if (!hasdev && nooverlap) {
@@ -501,7 +501,7 @@ static int create_loop(struct loopdev_cxt *lc,
 				errx(EXIT_FAILURE, _("%s: overlapping encrypted xloop device exists"), file);
 			}
 
-			lc->info.lo_flags &= ~LO_FLAGS_AUTOCLEAR;
+			lc->config.info.lo_flags &= ~LO_FLAGS_AUTOCLEAR;
 			if (loopcxt_ioctl_status(lc)) {
 				loopcxt_deinit(lc);
 				errx(EXIT_FAILURE, _("%s: failed to re-use xloop device"), file);
@@ -571,8 +571,12 @@ static int create_loop(struct loopdev_cxt *lc,
 		rc = loopcxt_setup_device(lc);
 		if (rc == 0)
 			break;			/* success */
-		if (errno == EBUSY && !hasdev)
+
+		if (errno == EBUSY && !hasdev && ntries < 64) {
+			xusleep(200000);
+			ntries++;
 			continue;
+		}
 
 		/* errors */
 		errpre = hasdev && loopcxt_get_fd(lc) < 0 ?