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Subject[PATCH RFC] loop: make partition scanning reliable
Date
So far, loop-device partition-scanning is skipped with EBUSY if the
bd_mutex cannot be locked. This might happen, if someone runs open() /
close() / ioctl() in parallel to LOOP_SET_FD/LOOP_CHANGE_FD and friends.

__fput() on open files might get delayed arbitrarily, which means,
blkdev_put() might get called at any point in time, locking bd_mutex and
thus preventing any possible partition-rescanning in parallel. This
basically requires user-space to *always* run BLKRRSCAN after
LOOP_SET_FD/LOOP_CHANGE_FD as we cannot know whether someone else is about
to close their FD at the same time.

Fix this by running BLKRRPART without holding lo_ctl_mutex, thus, we will
no longer deadlock if we lock bd_mutex.

Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
---
block/ioctl.c | 9 ++++++---
drivers/block/loop.c | 49 ++++++++++++++++++++++++++++++-------------------
include/linux/fs.h | 1 +
3 files changed, 37 insertions(+), 22 deletions(-)

diff --git a/block/ioctl.c b/block/ioctl.c
index 6c7bf90..01c6550 100644
--- a/block/ioctl.c
+++ b/block/ioctl.c
@@ -150,7 +150,7 @@ static int blkpg_ioctl(struct block_device *bdev, struct blkpg_ioctl_arg __user
}
}

-static int blkdev_reread_part(struct block_device *bdev)
+int blkdev_reread_part(struct block_device *bdev, int skipbusy)
{
struct gendisk *disk = bdev->bd_disk;
int res;
@@ -159,12 +159,15 @@ static int blkdev_reread_part(struct block_device *bdev)
return -EINVAL;
if (!capable(CAP_SYS_ADMIN))
return -EACCES;
- if (!mutex_trylock(&bdev->bd_mutex))
+ if (!skipbusy)
+ mutex_lock(&bdev->bd_mutex);
+ else if (!mutex_trylock(&bdev->bd_mutex))
return -EBUSY;
res = rescan_partitions(disk, bdev);
mutex_unlock(&bdev->bd_mutex);
return res;
}
+EXPORT_SYMBOL(blkdev_reread_part);

static int blk_ioctl_discard(struct block_device *bdev, uint64_t start,
uint64_t len, int secure)
@@ -407,7 +410,7 @@ int blkdev_ioctl(struct block_device *bdev, fmode_t mode, unsigned cmd,
ret = blkpg_ioctl(bdev, (struct blkpg_ioctl_arg __user *) arg);
break;
case BLKRRPART:
- ret = blkdev_reread_part(bdev);
+ ret = blkdev_reread_part(bdev, 1);
break;
case BLKGETSIZE:
size = i_size_read(bdev->bd_inode);
diff --git a/drivers/block/loop.c b/drivers/block/loop.c
index 6cb1beb..4047985 100644
--- a/drivers/block/loop.c
+++ b/drivers/block/loop.c
@@ -637,7 +637,7 @@ out:
* new backing store is the same size and type as the old backing store.
*/
static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,
- unsigned int arg)
+ unsigned int arg, int *rrpart)
{
struct file *file, *old_file;
struct inode *inode;
@@ -676,7 +676,7 @@ static int loop_change_fd(struct loop_device *lo, struct block_device *bdev,

fput(old_file);
if (lo->lo_flags & LO_FLAGS_PARTSCAN)
- ioctl_by_bdev(bdev, BLKRRPART, 0);
+ *rrpart = 1;
return 0;

out_putf:
@@ -821,7 +821,7 @@ static void loop_config_discard(struct loop_device *lo)
}

static int loop_set_fd(struct loop_device *lo, fmode_t mode,
- struct block_device *bdev, unsigned int arg)
+ struct block_device *bdev, unsigned int arg, int *rrpart)
{
struct file *file, *f;
struct inode *inode;
@@ -917,7 +917,7 @@ static int loop_set_fd(struct loop_device *lo, fmode_t mode,
if (part_shift)
lo->lo_flags |= LO_FLAGS_PARTSCAN;
if (lo->lo_flags & LO_FLAGS_PARTSCAN)
- ioctl_by_bdev(bdev, BLKRRPART, 0);
+ *rrpart = 1;

/* Grab the block_device to prevent its destruction after we
* put /dev/loopXX inode. Later in loop_clr_fd() we bdput(bdev).
@@ -1064,7 +1064,8 @@ static int loop_clr_fd(struct loop_device *lo)
}

static int
-loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
+loop_set_status(struct loop_device *lo, const struct loop_info64 *info,
+ int *rrpart)
{
int err;
struct loop_func_table *xfer;
@@ -1123,7 +1124,7 @@ loop_set_status(struct loop_device *lo, const struct loop_info64 *info)
!(lo->lo_flags & LO_FLAGS_PARTSCAN)) {
lo->lo_flags |= LO_FLAGS_PARTSCAN;
lo->lo_disk->flags &= ~GENHD_FL_NO_PART_SCAN;
- ioctl_by_bdev(lo->lo_device, BLKRRPART, 0);
+ *rrpart = 1;
}

lo->lo_encrypt_key_size = info->lo_encrypt_key_size;
@@ -1223,7 +1224,8 @@ loop_info64_to_old(const struct loop_info64 *info64, struct loop_info *info)
}

static int
-loop_set_status_old(struct loop_device *lo, const struct loop_info __user *arg)
+loop_set_status_old(struct loop_device *lo, const struct loop_info __user *arg,
+ int *rrpart)
{
struct loop_info info;
struct loop_info64 info64;
@@ -1231,17 +1233,18 @@ loop_set_status_old(struct loop_device *lo, const struct loop_info __user *arg)
if (copy_from_user(&info, arg, sizeof (struct loop_info)))
return -EFAULT;
loop_info64_from_old(&info, &info64);
- return loop_set_status(lo, &info64);
+ return loop_set_status(lo, &info64, rrpart);
}

static int
-loop_set_status64(struct loop_device *lo, const struct loop_info64 __user *arg)
+loop_set_status64(struct loop_device *lo, const struct loop_info64 __user *arg,
+ int *rrpart)
{
struct loop_info64 info64;

if (copy_from_user(&info64, arg, sizeof (struct loop_info64)))
return -EFAULT;
- return loop_set_status(lo, &info64);
+ return loop_set_status(lo, &info64, rrpart);
}

static int
@@ -1289,15 +1292,15 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
unsigned int cmd, unsigned long arg)
{
struct loop_device *lo = bdev->bd_disk->private_data;
- int err;
+ int err, rrpart = 0;

mutex_lock_nested(&lo->lo_ctl_mutex, 1);
switch (cmd) {
case LOOP_SET_FD:
- err = loop_set_fd(lo, mode, bdev, arg);
+ err = loop_set_fd(lo, mode, bdev, arg, &rrpart);
break;
case LOOP_CHANGE_FD:
- err = loop_change_fd(lo, bdev, arg);
+ err = loop_change_fd(lo, bdev, arg, &rrpart);
break;
case LOOP_CLR_FD:
/* loop_clr_fd would have unlocked lo_ctl_mutex on success */
@@ -1309,7 +1312,8 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
err = -EPERM;
if ((mode & FMODE_WRITE) || capable(CAP_SYS_ADMIN))
err = loop_set_status_old(lo,
- (struct loop_info __user *)arg);
+ (struct loop_info __user *)arg,
+ &rrpart);
break;
case LOOP_GET_STATUS:
err = loop_get_status_old(lo, (struct loop_info __user *) arg);
@@ -1318,7 +1322,8 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
err = -EPERM;
if ((mode & FMODE_WRITE) || capable(CAP_SYS_ADMIN))
err = loop_set_status64(lo,
- (struct loop_info64 __user *) arg);
+ (struct loop_info64 __user *) arg,
+ &rrpart);
break;
case LOOP_GET_STATUS64:
err = loop_get_status64(lo, (struct loop_info64 __user *) arg);
@@ -1334,6 +1339,9 @@ static int lo_ioctl(struct block_device *bdev, fmode_t mode,
mutex_unlock(&lo->lo_ctl_mutex);

out_unlocked:
+ if (rrpart)
+ blkdev_reread_part(bdev, 0);
+
return err;
}

@@ -1429,7 +1437,7 @@ loop_info64_to_compat(const struct loop_info64 *info64,

static int
loop_set_status_compat(struct loop_device *lo,
- const struct compat_loop_info __user *arg)
+ const struct compat_loop_info __user *arg, int *rrpart)
{
struct loop_info64 info64;
int ret;
@@ -1437,7 +1445,7 @@ loop_set_status_compat(struct loop_device *lo,
ret = loop_info64_from_compat(arg, &info64);
if (ret < 0)
return ret;
- return loop_set_status(lo, &info64);
+ return loop_set_status(lo, &info64, rrpart);
}

static int
@@ -1460,14 +1468,17 @@ static int lo_compat_ioctl(struct block_device *bdev, fmode_t mode,
unsigned int cmd, unsigned long arg)
{
struct loop_device *lo = bdev->bd_disk->private_data;
- int err;
+ int err, rrpart = 0;

switch(cmd) {
case LOOP_SET_STATUS:
mutex_lock(&lo->lo_ctl_mutex);
err = loop_set_status_compat(
- lo, (const struct compat_loop_info __user *) arg);
+ lo, (const struct compat_loop_info __user *) arg,
+ &rrpart);
mutex_unlock(&lo->lo_ctl_mutex);
+ if (rrpart)
+ blkdev_reread_part(bdev, 0);
break;
case LOOP_GET_STATUS:
mutex_lock(&lo->lo_ctl_mutex);
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 9ab779e..755a056 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -2153,6 +2153,7 @@ extern const struct file_operations bad_sock_fops;
#ifdef CONFIG_BLOCK
extern int ioctl_by_bdev(struct block_device *, unsigned, unsigned long);
extern int blkdev_ioctl(struct block_device *, fmode_t, unsigned, unsigned long);
+extern int blkdev_reread_part(struct block_device *bdev, int skipbusy);
extern long compat_blkdev_ioctl(struct file *, unsigned, unsigned long);
extern int blkdev_get(struct block_device *bdev, fmode_t mode, void *holder);
extern struct block_device *blkdev_get_by_path(const char *path, fmode_t mode,
--
2.2.2


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