fs: add support loop mount offset

When loop mounting, allow to specify an offset into the file, similarly
to the losetup offset option. Multiplicative suffixes are supported.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>

2 files changed, 13 insertions, 7 deletions

diff --git a/fs/devfs-core.c b/fs/devfs-core.c
index be56edd18d..ea5887c720 100644
--- a/fs/devfs-core.c
+++ b/fs/devfs-core.c
@@ -464,7 +464,7 @@ static const struct file_operations loop_ops = {
 	.lseek = dev_lseek_default,
 };
 
-struct cdev *cdev_create_loop(const char *path, ulong flags)
+struct cdev *cdev_create_loop(const char *path, ulong flags, loff_t offset)
 {
 	struct cdev *new;
 	struct loop_priv *priv;
@@ -486,15 +486,15 @@ struct cdev *cdev_create_loop(const char *path, ulong flags)
 	new->priv = priv;
 
 	ofs = lseek(priv->fd, 0, SEEK_END);
-	if (ofs < 0) {
+	if (ofs < 0 || ofs <= offset) {
 		free(new);
 		free(priv);
 		return NULL;
 	}
-	lseek(priv->fd, 0, SEEK_SET);
+	lseek(priv->fd, offset, SEEK_SET);
 
 	new->size = ofs;
-	new->offset = 0;
+	new->offset = offset;
 	new->dev = NULL;
 	new->flags = 0;
 
diff --git a/fs/fs.c b/fs/fs.c
index 7d0d97906d..d188fa995f 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -1279,11 +1279,13 @@ static const char *detect_fs(const char *filename, const char *fsoptions)
 	enum filetype type;
 	struct driver_d *drv;
 	struct fs_driver_d *fdrv;
-	bool loop;
+	bool loop = false;
+	unsigned long long offset = 0;
 
 	parseopt_b(fsoptions, "loop", &loop);
+	parseopt_llu_suffix(fsoptions, "offset", &offset);
 	if (loop)
-		type = file_name_detect_type(filename);
+		type = file_name_detect_type_offset(filename, offset);
 	else
 		type = cdev_detect_type(filename);
 
@@ -1302,9 +1304,13 @@ static const char *detect_fs(const char *filename, const char *fsoptions)
 
 int fsdev_open_cdev(struct fs_device_d *fsdev)
 {
+	unsigned long long offset = 0;
+
 	parseopt_b(fsdev->options, "loop", &fsdev->loop);
+	parseopt_llu_suffix(fsdev->options, "offset", &offset);
 	if (fsdev->loop)
-		fsdev->cdev = cdev_create_loop(fsdev->backingstore, O_RDWR);
+		fsdev->cdev = cdev_create_loop(fsdev->backingstore, O_RDWR,
+					       offset);
 	else
 		fsdev->cdev = cdev_open(fsdev->backingstore, O_RDWR);
 	if (!fsdev->cdev)