[PATCH V4] mke2fs: disallow creating FS on a loop mounted file with no option

[Kazuya Mio <k-mio@sx.jp.nec.com>]

When /etc/mtab is a symlink of /proc/mounts, mke2fs without -FF option
can create a filesystem on the image file that is mounted.
According to mke2fs man page, we should specify -FF option in this case.

This patch protects filesystem from unintended mke2fs caused by human error.

How to reproduce:
  # mke2fs -t ext4 -Fq fs.img
  # mount -o loop fs.img /mnt/mp1
  # mke2fs -t ext4 -Fq fs.img && echo "mke2fs success"
  mke2fs success

Signed-off-by: Kazuya Mio <k-mio@sx.jp.nec.com>
---
 configure.in           |    1 +
 lib/ext2fs/ismounted.c |   41 +++++++++++++++++++++++++++++++++++++++++
 2 files changed, 42 insertions(+)
diff --git a/configure.in b/configure.in
index 049dc11..fef8d9b 100644
--- a/configure.in
+++ b/configure.in
@@ -920,6 +920,7 @@ AC_CHECK_HEADERS(m4_flatten([
 	linux/falloc.h
 	linux/fd.h
 	linux/major.h
+	linux/loop.h
 	net/if_dl.h
 	netinet/in.h
 	sys/disklabel.h
diff --git a/lib/ext2fs/ismounted.c b/lib/ext2fs/ismounted.c
index 2c1bd75..6c6ecff 100644
--- a/lib/ext2fs/ismounted.c
+++ b/lib/ext2fs/ismounted.c
@@ -21,6 +21,13 @@
 #ifdef HAVE_LINUX_FD_H
 #include <linux/fd.h>
 #endif
+#ifdef HAVE_LINUX_LOOP_H
+#include <linux/loop.h>
+#include <sys/ioctl.h>
+#ifdef HAVE_LINUX_MAJOR_H
+#include <linux/major.h>
+#endif /* HAVE_LINUX_MAJOR_H */
+#endif /* HAVE_LINUX_LOOP_H */
 #ifdef HAVE_MNTENT_H
 #include <mntent.h>
 #endif
@@ -35,6 +42,36 @@
 #include "ext2_fs.h"
 #include "ext2fs.h"
 
+/*
+ * Check to see if a regular file is mounted.
+ * If /etc/mtab/ is a symlink of /proc/mounts, you will need the following check
+ * because the name in /proc/mounts is a loopback device not a regular file.
+ */
+static int check_loop_mounted(const char *mnt_fsname, dev_t mnt_rdev,
+				dev_t file_dev, ino_t file_ino)
+{
+#if defined(HAVE_LINUX_LOOP_H) && defined(HAVE_LINUX_MAJOR_H)
+	struct loop_info64 loopinfo;
+	int loop_fd, ret;
+
+	if (major(mnt_rdev) == LOOP_MAJOR) {
+		loop_fd = open(mnt_fsname, O_RDONLY);
+		if (loop_fd < 0)
+			return -1;
+
+		ret = ioctl(loop_fd, LOOP_GET_STATUS64, &loopinfo);
+		close(loop_fd);
+		if (ret < 0)
+			return -1;
+
+		if (file_dev == loopinfo.lo_device &&
+				file_ino == loopinfo.lo_inode)
+			return 1;
+	}
+#endif /* defined(HAVE_LINUX_LOOP_H) && defined(HAVE_LINUX_MAJOR_H) */
+	return 0;
+}
+
 #ifdef HAVE_SETMNTENT
 /*
  * Helper function which checks a file in /etc/mtab format to see if a
@@ -82,6 +119,10 @@ static errcode_t check_mntent_file(const char *mtab_file, const char *file,
 #ifndef __GNU__
 				if (file_rdev && (file_rdev == st_buf.st_rdev))
 					break;
+				if (check_loop_mounted(mnt->mnt_fsname,
+						st_buf.st_rdev, file_dev,
+						file_ino) == 1)
+					break;
 #endif	/* __GNU__ */
 			} else {
 				if (file_dev && ((file_dev == st_buf.st_dev) &&