mount: automatically detect and loop-mount regular files

This patch allows to automatically create a loop device from a regular
file if a filesystem type is not specified, for example:

   mount /path/disk.img /mnt

If the filesystem type is specified than "-o loop" is required.

Note that there is not a restriction (on kernel side) that prevents
regular file as a mount(2) source argument. A filesystem that is able
to mount regular files could be implemented.

Based on a patch from Adam Jackson <ajax@redhat.com>.

Signed-off-by: Karel Zak <kzak@redhat.com>

2 files changed, 41 insertions, 11 deletions

diff --git a/mount/mount.8 b/mount/mount.8
index 07edb5c2..9675cc62 100644
--- a/mount/mount.8
+++ b/mount/mount.8
@@ -2536,18 +2536,37 @@ Since Linux version 2.1.21 xiafs is no longer part of the kernel source.
 .SH "THE LOOP DEVICE"
 One further possible type is a mount via the loop device. For example,
 the command
-
-.nf
-.B "  mount /tmp/fdimage /mnt -t vfat -o loop=/dev/loop3
-.fi
-
+.RS
+.sp
+.B "mount /tmp/disk.img /mnt -t vfat -o loop=/dev/loop"
+.sp
+.RE
 will set up the loop device
 .I /dev/loop3
 to correspond to the file
-.IR /tmp/fdimage ,
+.IR /tmp/disk.img ,
 and then mount this device on
 .IR /mnt .
 
+If no explicit loop device is mentioned
+(but just an option `\fB\-o loop\fP' is given), then
+.B mount
+will try to find some unused loop device and use that, for example
+.RS
+.sp
+.B "mount /tmp/disk.img /mnt -o loop"
+.sp
+.RE
+The mount command
+.B automatically
+creates a loop device from a regular file if a filesystem type is
+not specified, for example:
+.RS
+.sp
+.B "mount /tmp/disk.img /mnt"
+.sp
+.RE
+
 This type of mount knows about four options, namely
 .BR loop ", " offset ", " sizelimit " and " encryption ,
 that are really options to
@@ -2555,11 +2574,6 @@ that are really options to
 (These options can be used in addition to those specific
 to the filesystem type.)
 
-If no explicit loop device is mentioned
-(but just an option `\fB\-o loop\fP' is given), then
-.B mount
-will try to find some unused loop device and use that.
-
 Since Linux 2.6.25 is supported auto-destruction of loop devices and
 then any loop device allocated by
 .B mount
diff --git a/mount/mount.c b/mount/mount.c
index a73b606c..6226ea72 100644
--- a/mount/mount.c
+++ b/mount/mount.c
@@ -1103,6 +1103,22 @@ loop_check(const char **spec, const char **type, int *flags,
   *loop = ((*flags & MS_LOOP) || *loopdev || opt_offset || opt_sizelimit || opt_encryption);
   *loopfile = *spec;
 
+  /* Automatically create a loop device from a regular file if a filesystem
+   * is not specified.
+   *
+   * Note that there is not a restriction (on kernel side) that prevents regular
+   * file as a mount(2) source argument. A filesystem that is able to mount
+   * regular files could be implemented.
+   *
+   * If the filesystem type is specified than "-o loop" is required to create a
+   * loop device.
+   */
+  if (!*loop && (!*type || strcmp(*type, "auto") == 0)) {
+    struct stat st;
+    if (stat(*loopfile, &st) == 0)
+      *loop = S_ISREG(st.st_mode);
+  }
+
   if (*loop) {
     *flags |= MS_LOOP;
     if (fake) {