[PATCH] discover: Handle BTRFS root subvolumes

[Sam Mendoza-Jonas]

During install some distributions[0] will create subvolumes when formatting
the root filesystem with BTRFS. In particular this can mean that
bootloader config files will appear (in the case of GRUB) under
/var/petitboot/mnt/dev/$device/@/boot/grub/
rather than the expected
/var/petitboot/mnt/dev/$device/boot/grub/

If this is the case, perform all file operations from the parser
relative to this subvolume rather than the mount point. At the moment
this only supports the trivial case where the subvolume name for root is
blank (ie. '@').

[0] In particular, Ubuntu from at least 14.04

Signed-off-by: Sam Mendoza-Jonas <sam at mendozajonas.com>
---
 discover/device-handler.c | 25 +++++++++++++++++++++++++
 discover/device-handler.h |  1 +
 discover/parser.c         |  2 +-
 3 files changed, 27 insertions(+), 1 deletion(-)

diff --git a/discover/device-handler.c b/discover/device-handler.c
index 5df0700..77f9e3c 100644
--- a/discover/device-handler.c
+++ b/discover/device-handler.c
@@ -1285,6 +1285,28 @@ static inline const char *get_device_path(struct discover_device *dev)
 	return dev->ramdisk ? dev->ramdisk->snapshot : dev->device_path;
 }
 
+static char *check_subvols(struct discover_device *dev)
+{
+	const char *fstype = discover_device_get_param(dev, "ID_FS_TYPE");
+	struct stat sb;
+	char *path;
+	int rc;
+
+	if (strncmp(fstype, "btrfs", strlen("btrfs")))
+		return dev->mount_path;
+
+	/* On btrfs a device's root may be under a subvolume path */
+	path = join_paths(dev, dev->mount_path, "@");
+	rc = stat(path, &sb);
+	if (!rc && S_ISDIR(sb.st_mode)) {
+		pb_debug("Using '%s' for btrfs root path\n", path);
+		return path;
+	}
+
+	talloc_free(path);
+	return dev->mount_path;
+}
+
 static bool check_existing_mount(struct discover_device *dev)
 {
 	struct stat devstat, mntstat;
@@ -1326,6 +1348,7 @@ static bool check_existing_mount(struct discover_device *dev)
 
 		if (mntstat.st_rdev == devstat.st_rdev) {
 			dev->mount_path = talloc_strdup(dev, mnt->mnt_dir);
+			dev->root_path = check_subvols(dev);
 			dev->mounted_rw = !!hasmntopt(mnt, "rw");
 			dev->mounted = true;
 			dev->unmount = false;
@@ -1388,6 +1411,7 @@ static int mount_device(struct discover_device *dev)
 		dev->mounted = true;
 		dev->mounted_rw = false;
 		dev->unmount = true;
+		dev->root_path = check_subvols(dev);
 		return 0;
 	}
 
@@ -1426,6 +1450,7 @@ static int umount_device(struct discover_device *dev)
 
 	talloc_free(dev->mount_path);
 	dev->mount_path = NULL;
+	dev->root_path = NULL;
 
 	return 0;
 }
diff --git a/discover/device-handler.h b/discover/device-handler.h
index e5501ec..12f5ce0 100644
--- a/discover/device-handler.h
+++ b/discover/device-handler.h
@@ -25,6 +25,7 @@ struct discover_device {
 	const char		*label;
 
 	char			*mount_path;
+	char			*root_path;
 	const char		*device_path;
 	struct ramdisk_device	*ramdisk;
 	bool			mounted;
diff --git a/discover/parser.c b/discover/parser.c
index 8e767c6..fbf31b2 100644
--- a/discover/parser.c
+++ b/discover/parser.c
@@ -25,7 +25,7 @@ static char *local_path(struct discover_context *ctx,
 		struct discover_device *dev,
 		const char *filename)
 {
-	return join_paths(ctx, dev->mount_path, filename);
+	return join_paths(ctx, dev->root_path, filename);
 }
 
 int parser_request_file(struct discover_context *ctx,
-- 
2.6.4