[-v3] ext4: implementation of a new ioctl called EXT4_IOC_SWAP_BOOT

From: "Dr. Tilmann Bubeck" <t.bubeck@reinform.de>

Hi Tilmann,

I had to make a number of changes to your patches:

#1) If the boot inode is uninitialized, i_flags was set to zero, but
the i_data field was initialized via ext4_ext_tree_init().  As a
result, if you ran e2fsck on a freshly created file system after
running ext4-swap-boot-inode, e2fsck would complain about a corrupted
file system.  (Lesson to be learned: always run e2fsck afterwards
testing the code.  The file system _must_ be consistent afterwards).

#2) Unconditionally calling ext4_ext_tree_init() means the inode will
be initialized to use extents even if the file system does not support
extents (for example, if it is an ext2 or ext3 mapped file system).
This will also cause a file system that will fail an e2fsck for a file
system formatted as ext2 or ext3 but mounted using the ext4 file
system driver.

#3) The original version of the code called truncate_inode_pages()
after releasing i_mutex.  This violates a requirement which is very
clearly documented in the comments of that function.  (Lesson to be
learned: always check the implementation of functions that you're not
familiar with.  This is also why defensive programming such as
including mutex_is_locked() assertions in functions is useful, since
developers don't always read function documentation...)

#4) filemap_flush() needs to be called before you swap the inode data.
That's because the page cache is indexed by inode number and page
number.  So if there is dirty page data, it needs to be flushed to the
inode *before* the messing with the inode data.

#5) I needed to port the patch to deal with the extent status tree.
I'm guessing you probably based your patch off of the 3.7 version of
the kernel, and not the 3.8-rcX development tree.

Here's the modified patch; if someone else on the ext4 development
team could review the patch after my modifications, I'd appreciate it.

						- Ted

From 3de49c9eeb1303b751b92b2f554c57c686e2997d Mon Sep 17 00:00:00 2001
From: "Dr. Tilmann Bubeck" <t.bubeck@reinform.de>
Date: Sat, 6 Apr 2013 22:44:18 -0400
Subject: [PATCH] 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 associated 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 the user on 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>
---
 Documentation/filesystems/ext4.txt |  10 ++
 fs/ext4/ext4.h                     |   8 ++
 fs/ext4/inode.c                    |  11 ++-
 fs/ext4/ioctl.c                    | 197 +++++++++++++++++++++++++++++++++++++
 fs/ext4/move_extent.c              |  48 ++++-----
 5 files changed, 248 insertions(+), 26 deletions(-)

[-v3] ext4: implementation of a new ioctl called EXT4_IOC_SWAP_BOOT

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