Patch for 2.2 loop driver & loop root filesystem

From: Joshua Hudson
Date: Sat Jan 17 2004 - 23:05:42 EST

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I wrote this patch for the linux 2.2.22 kernel, although I will be replacing it in about ten days
with 2.2.25 or so. There is a little problem with loopback installations
that I discovered, in that it is impossible to unmount the host partition or even mount it
readonly. This patch makes it possible to mount it readonly. I have also included a little utility
that demonstrates the new functionality. Enjoy.

Oh, I'm not on the list, so cc any followups to me.

!-! Start of utility

/* loopro.c - JH
*
*/

/* Can't include gclibc defs. Oh bother. */
#include <linux/types.h>
#include <linux/loop.h>
#define O_RDWR 2

#ifndef LO_CRYPT_KEEP
#define LO_CRYPT_KEEP 127 /* So that we don't depend on the newest k header. */
#endif

char usage[] = "Sets a loop device to readonly\nUsage: loopro device\n";
char cantdev[] = "loopro: Can't open device\n";
char notloop[] = "loopro: ioctl failed. Not a loop device?\n";
char notsup[] = "loopro: your kernel doesn't support this.\n";

int main(int argc, char **argv)
{
int fd;
struct loop_info inf;
if (argc != 2)
{
write(2, usage, sizeof(usage) - 1);
return 2;
}
fd = open(argv[1], O_RDWR);
if (fd < 0)
{
write(2, cantdev, sizeof(cantdev) - 1);
return 1;
}
if (0 > ioctl(fd, LOOP_GET_STATUS, (char *)&inf))
{
write(2, notloop, sizeof(notloop) - 1);
return 1;
}
inf.lo_encrypt_type = LO_CRYPT_KEEP;
inf.lo_flags |= LO_FLAGS_READ_ONLY;
if (0 > ioctl(fd, LOOP_SET_STATUS, (char *)&inf))
{
write(2, notsup, sizeof(notsup) - 1);
return 1;
}
return 0;
}

!-! Start of patch
diff -u -r linux-2.2.22/Documentation/Configure.help linux-2.2.22c/Documentation/Configure.help
--- linux-2.2.22/Documentation/Configure.help Mon Sep 16 09:26:11 2002
+++ linux-2.2.22c/Documentation/Configure.help Sat Jan 17 11:00:40 2004
@@ -262,6 +262,16 @@

Most users will answer N here.

+Enable late setting of readonly
+CONFIG_LOOP_SET_RO
+ Saying Y here enables the loopback driver to switch the device and
+ backing file to read-only mode (but it can't switch back). This is
+ of use mainly for systems with a loopback filesystem as the root
+ filesystem.
+
+ If you will not be using a loopback root filesystem, or you don't
+ know what any of this means, say N here.
+
Network Block Device support
CONFIG_BLK_DEV_NBD
Saying Y here will allow your computer to be a client for network
diff -u -r linux-2.2.22/drivers/block/Config.in linux-2.2.22c/drivers/block/Config.in
--- linux-2.2.22/drivers/block/Config.in Fri Nov 2 08:39:06 2001
+++ linux-2.2.22c/drivers/block/Config.in Sat Jan 17 09:08:29 2004
@@ -98,6 +98,9 @@
comment 'Additional Block Devices'

tristate 'Loopback device support' CONFIG_BLK_DEV_LOOP
+if [ "$CONFIG_BLK_DEV_LOOP" = "y" -o $CONFIG_BLK_DEV_LOOP = "m" ]; then
+ bool ' Enable late setting of readonly' CONFIG_LOOP_SET_RO
+fi
if [ "$CONFIG_NET" = "y" ]; then
tristate 'Network block device support' CONFIG_BLK_DEV_NBD
fi
diff -u -r linux-2.2.22/drivers/block/loop.c linux-2.2.22c/drivers/block/loop.c
--- linux-2.2.22/drivers/block/loop.c Mon Sep 16 09:26:11 2002
+++ linux-2.2.22c/drivers/block/loop.c Sat Jan 17 09:21:35 2004
@@ -49,6 +49,7 @@
#include <asm/uaccess.h>

#include <linux/loop.h>
+#include <linux/smp_lock.h>

#define MAJOR_NR LOOP_MAJOR

@@ -499,7 +500,12 @@
return 0;
}

+#ifdef CONFIG_LOOP_SET_RO
+static int loop_set_status(struct loop_device *lo, struct loop_info *arg,
+ kdev_t dev)
+#else
static int loop_set_status(struct loop_device *lo, struct loop_info *arg)
+#endif
{
struct loop_info info;
int err;
@@ -514,28 +520,66 @@
return -EFAULT;
if ((unsigned int) info.lo_encrypt_key_size > LO_KEY_SIZE)
return -EINVAL;
- type = info.lo_encrypt_type;
- if (type >= MAX_LO_CRYPT || xfer_funcs[type] == NULL)
- return -EINVAL;
- err = loop_release_xfer(lo);
- if (!err)
- err = loop_init_xfer(lo, type, &info);
- if (err)
- return err;
+ type = info.lo_encrypt_type;
+#ifdef CONFIG_LOOP_SET_RO
+ if (info.lo_flags & LO_FLAGS_READ_ONLY &&
+ !(lo->lo_flags & LO_FLAGS_READ_ONLY))
+ {
+ /* The user is trying to turn on readonly. */
+ /* There appears to be no way to verify if activating
+ * readonly is safe. The kernel is sloppy and does not
+ * remove the write flag when remounting readonly. So,
+ * be warned that setting readonly when the device is
+ * mounted readwrite will corrupt your data. */
+
+ lock_kernel();
+ fsync_dev(0); /* Prevent the worst of the damage. */
+ set_device_ro(dev, 1);
+ lo->lo_flags |= LO_FLAGS_READ_ONLY;
+ unlock_kernel();
+ /* Now reopen the backing file ro */
+ if (lo->lo_backing_file)
+ {
+ lo->lo_backing_file->f_mode ^= FMODE_WRITE;
+ put_write_access(lo->lo_backing_file->f_dentry->d_inode
+ );
+ }
+ }

+ if (type != LO_CRYPT_KEEP)
+ { /* Changing encryption */
+#endif
+ if (type >= MAX_LO_CRYPT || xfer_funcs[type] == NULL)
+ return -EINVAL;
+
+ err = loop_release_xfer(lo);
+ if (!err)
+ err = loop_init_xfer(lo, type, &info);
+ if (err)
+ return err;
+#ifdef CONFIG_LOOP_SET_RO
+ }
+#endif
lo->lo_offset = info.lo_offset;
strncpy(lo->lo_name, info.lo_name, LO_NAME_SIZE);

- lo->transfer = xfer_funcs[type]->transfer;
- lo->ioctl = xfer_funcs[type]->ioctl;
- lo->lo_encrypt_key_size = info.lo_encrypt_key_size;
- lo->lo_init[0] = info.lo_init[0];
- lo->lo_init[1] = info.lo_init[1];
- if (info.lo_encrypt_key_size) {
- memcpy(lo->lo_encrypt_key, info.lo_encrypt_key,
- info.lo_encrypt_key_size);
- lo->lo_key_owner = current->uid;
- }
+#ifdef CONFIG_LOOP_SET_RO
+ if (type != LO_CRYPT_KEEP)
+ {
+#endif
+ lo->transfer = xfer_funcs[type]->transfer;
+ lo->ioctl = xfer_funcs[type]->ioctl;
+ lo->lo_encrypt_key_size = info.lo_encrypt_key_size;
+ lo->lo_init[0] = info.lo_init[0];
+ lo->lo_init[1] = info.lo_init[1];
+ if (info.lo_encrypt_key_size) {
+ memcpy(lo->lo_encrypt_key, info.lo_encrypt_key,
+ info.lo_encrypt_key_size);
+ lo->lo_key_owner = current->uid;
+ }
+#ifdef CONFIG_LOOP_SET_RO
+ }
+#endif
figure_loop_size(lo);
return 0;
}
@@ -587,7 +631,12 @@
case LOOP_CLR_FD:
return loop_clr_fd(lo, inode->i_rdev);
case LOOP_SET_STATUS:
+#ifdef CONFIG_LOOP_SET_RO
+ return loop_set_status(lo, (struct loop_info *) arg,
+ inode->i_rdev);
+#else
return loop_set_status(lo, (struct loop_info *) arg);
+#endif
case LOOP_GET_STATUS:
return loop_get_status(lo, (struct loop_info *) arg);
case BLKGETSIZE: /* Return device size */
diff -u -r linux-2.2.22/include/linux/loop.h linux-2.2.22c/include/linux/loop.h
--- linux-2.2.22/include/linux/loop.h Sat Sep 21 05:13:19 2002
+++ linux-2.2.22c/include/linux/loop.h Sat Jan 17 07:42:33 2004
@@ -97,6 +97,7 @@
#define LO_CRYPT_DUMMY 9
#define LO_CRYPT_SKIPJACK 10
#define MAX_LO_CRYPT 20
+#define LO_CRYPT_KEEP 127 /* Changing something besides encryption (!)*/

#ifdef __KERNEL__
/* Support for loadable transfer modules */

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Next message: Yu, Luming: "RE: ACPI: problem on ASUS PR-DLS533"
Previous message: Randy.Dunlap: "[announce] 2.6.1-bk4-kj1 patchset"
Messages sorted by: [ date ] [ thread ] [ subject ] [ author ]