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, 8 insertions, 3 deletions

diff --git a/fs/ext4/inode.c b/fs/ext4/inode.c
index 769c656..a29bfc2 100644
--- a/fs/ext4/inode.c
+++ b/fs/ext4/inode.c
@@ -4191,8 +4191,9 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 	 * NeilBrown 1999oct15
 	 */
 	if (inode->i_nlink == 0) {
-		if (inode->i_mode == 0 ||
-		    !(EXT4_SB(inode->i_sb)->s_mount_state & EXT4_ORPHAN_FS)) {
+		if ((inode->i_mode == 0 ||
+		     !(EXT4_SB(inode->i_sb)->s_mount_state & EXT4_ORPHAN_FS)) &&
+		    ino != EXT4_BOOT_LOADER_INO) {
 			/* this inode is deleted */
 			ret = -ESTALE;
 			goto bad_inode;
@@ -4200,7 +4201,9 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 		/* The only unlinked inodes we let through here have
 		 * valid i_mode and are being read by the orphan
 		 * recovery code: that's fine, we're about to complete
-		 * the process of deleting those. */
+		 * the process of deleting those.
+		 * OR it is the EXT4_BOOT_LOADER_INO which is
+		 * not initialized on a new filesystem. */
 	}
 	ei->i_flags = le32_to_cpu(raw_inode->i_flags);
 	inode->i_blocks = ext4_inode_blocks(raw_inode, ei);
@@ -4320,6 +4323,8 @@ struct inode *ext4_iget(struct super_block *sb, unsigned long ino)
 		else
 			init_special_inode(inode, inode->i_mode,
 			   new_decode_dev(le32_to_cpu(raw_inode->i_block[1])));
+	} else if (ino == EXT4_BOOT_LOADER_INO) {
+		make_bad_inode(inode);
 	} else {
 		ret = -EIO;
 		EXT4_ERROR_INODE(inode, "bogus i_mode (%o)", inode->i_mode);