Add EROFS_IOC_GET_SOURCE_FD ioctl that returns a file descriptor to the
backing image file for file-backed erofs mounts.

Returns -ENOENT for block-device-backed erofs mounts where there is no
backing file.

The UAPI constant is defined in include/uapi/linux/erofs.h.

Signed-off-by: Giuseppe Scrivano <gscrivan@redhat.com>
---
 fs/erofs/inode.c           | 25 +++++++++++++++++++++++++
 include/uapi/linux/erofs.h |  9 +++++++++
 2 files changed, 34 insertions(+)
 create mode 100644 include/uapi/linux/erofs.h

diff --git a/fs/erofs/inode.c b/fs/erofs/inode.c
index 45afe5c50de8..0a38cb5cfc5d 100644
--- a/fs/erofs/inode.c
+++ b/fs/erofs/inode.c
@@ -6,6 +6,8 @@
  */
 #include "xattr.h"
 #include <linux/compat.h>
+#include <linux/file.h>
+#include <uapi/linux/erofs.h>
 #include <trace/events/erofs.h>
 
 static int erofs_fill_symlink(struct inode *inode, void *bptr, unsigned int ofs)
@@ -356,6 +358,27 @@ static int erofs_ioctl_get_volume_label(struct inode *inode, void __user *arg)
 	return ret ? -EFAULT : 0;
 }
 
+static int erofs_ioctl_get_source_fd(struct file *filp)
+{
+	struct erofs_sb_info *sbi = EROFS_I_SB(file_inode(filp));
+	struct file *f;
+	int fd;
+
+	if (!capable(CAP_SYS_ADMIN))
+		return -EPERM;
+
+	if (!erofs_is_fileio_mode(sbi))
+		return -ENOENT;
+
+	fd = get_unused_fd_flags(O_CLOEXEC);
+	if (fd < 0)
+		return fd;
+
+	f = get_file(sbi->dif0.file);
+	fd_install(fd, f);
+	return fd;
+}
+
 long erofs_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 {
 	struct inode *inode = file_inode(filp);
@@ -364,6 +387,8 @@ long erofs_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
 	switch (cmd) {
 	case FS_IOC_GETFSLABEL:
 		return erofs_ioctl_get_volume_label(inode, argp);
+	case EROFS_IOC_GET_SOURCE_FD:
+		return erofs_ioctl_get_source_fd(filp);
 	default:
 		return -ENOTTY;
 	}
diff --git a/include/uapi/linux/erofs.h b/include/uapi/linux/erofs.h
new file mode 100644
index 000000000000..17c835785ea9
--- /dev/null
+++ b/include/uapi/linux/erofs.h
@@ -0,0 +1,9 @@
+/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
+#ifndef _UAPI_LINUX_EROFS_H
+#define _UAPI_LINUX_EROFS_H
+
+#include <linux/ioctl.h>
+
+#define EROFS_IOC_GET_SOURCE_FD	_IO('e', 1)
+
+#endif /* _UAPI_LINUX_EROFS_H */
-- 
2.55.0