[PATCH 3/3] fs: implement new resolve_fsdevice_path helper

[Ahmad Fatoum <a.fatoum@pengutronix.de>]

For use by the EFI loader, we will want to split off the file path
within a device from the device path.

As we have no function that generates a path relative to a mountpoint's
root, add a function that does this.

Signed-off-by: Ahmad Fatoum <a.fatoum@pengutronix.de>
---
 fs/fs.c      | 25 +++++++++++++++++++++++++
 include/fs.h |  1 +
 2 files changed, 26 insertions(+)

diff --git a/fs/fs.c b/fs/fs.c
index 28378989f91b..77f22d69ea49 100644
--- a/fs/fs.c
+++ b/fs/fs.c
@@ -2454,6 +2454,31 @@ struct fs_device *get_fsdevice_by_path(int dirfd, const char *pathname)
 	return fsdev;
 }
 
+struct fs_device *resolve_fsdevice_path(int dirfd, const char *pathname, char **filepath)
+{
+	struct fs_device *fsdev;
+	char *res = NULL;
+	struct path path;
+	int ret;
+
+	fsdev = get_fsdevice_by_path(dirfd, pathname);
+	if (!fsdev)
+		return NULL;
+
+	ret = filename_lookup(dirfd, getname(pathname), LOOKUP_FOLLOW, &path);
+	if (ret)
+		goto out;
+
+	res = dpath(path.dentry, fsdev->vfsmount.mnt_root);
+	if (res)
+		*filepath = res;
+
+	path_put(&path);
+out:
+	errno_set(ret);
+	return res ? fsdev : NULL;
+}
+
 static int vfs_rmdir(struct inode *dir, struct dentry *dentry)
 {
 	int error;
diff --git a/include/fs.h b/include/fs.h
index b9bd14da81a7..9c3c8d95123a 100644
--- a/include/fs.h
+++ b/include/fs.h
@@ -141,6 +141,7 @@ void cdev_print(const struct cdev *cdev);
 char *canonicalize_path(int dirfd, const char *pathname);
 
 struct fs_device *get_fsdevice_by_path(int dirfd, const char *path);
+struct fs_device *resolve_fsdevice_path(int dirfd, const char *pathname, char **filepath);
 
 const char *get_mounted_path(const char *path);
 
-- 
2.47.3