ext4: implementation of a new ioctl called EXT4_IOC_SWAP_BOOT

Add a new ioctl, EXT4_IOC_SWAP_BOOT which swaps i_blocks and
associated attributes (like i_blocks, i_size, i_flags, ...) from the
specified inode with inode EXT4_BOOT_LOADER_INO (#5). This is
typically used to store a boot loader in a secure part of the
filesystem, where it can't be changed by a normal user by accident.
The data blocks of the previous boot loader will be associated with
the given inode.

This usercode program is a simple example of the usage:

int main(int argc, char *argv[])
{
  int fd;
  int err;

  if ( argc != 2 ) {
    printf("usage: ext4-swap-boot-inode FILE-TO-SWAP\n");
    exit(1);
  }

  fd = open(argv[1], O_WRONLY);
  if ( fd < 0 ) {
    perror("open");
    exit(1);
  }

  err = ioctl(fd, EXT4_IOC_SWAP_BOOT);
  if ( err < 0 ) {
    perror("ioctl");
    exit(1);
  }

  close(fd);
  exit(0);
}

[ Modified by Theodore Ts'o to fix a number of bugs in the original code.]

Signed-off-by: Dr. Tilmann Bubeck <t.bubeck@reinform.de>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>

1 files changed, 10 insertions, 0 deletions

diff --git a/Documentation/filesystems/ext4.txt b/Documentation/filesystems/ext4.txt
index 34ea4f1fa6ea..5dd957d8b25b 100644
--- a/Documentation/filesystems/ext4.txt
+++ b/Documentation/filesystems/ext4.txt
@@ -587,6 +587,16 @@ Table of Ext4 specific ioctls
                               bitmaps and inode table, the userspace tool thus
                               just passes the new number of blocks.
 
+EXT4_IOC_SWAP_BOOT            Swap i_blocks and associated attributes
+                              (like i_blocks, i_size, i_flags, ...) from
+                              the specified inode with inode
+                              EXT4_BOOT_LOADER_INO (#5). This is typically
+                              used to store a boot loader in a secure part of
+                              the filesystem, where it can't be changed by a
+                              normal user by accident.
+                              The data blocks of the previous boot loader
+                              will be associated with the given inode.
+
 ..............................................................................
 
 References