Re: i_size misuses

[Neil Brown <neilb@suse.de>]

On Wed, 8 Sep 2010 09:32:13 -0400 (EDT)
Mikulas Patocka <mpatocka@redhat.com> wrote:

> Hi
> 
> when reviewing some i_size problem, I searched the kernel for i_size usage 
> (the variable should really be written with i_size_write and read with 
> i_size_read).
> 
> Properly locked direct use of "i_size" inside memory management or 
> filesystems may not be a problem, but there are many problems in general 
> code outside mm.
> 
> The misuses are:
> SOUND/SOUND_FIRMWARE.C:l = filp->f_path.dentry->d_inode->i_size;
> KERNEL/RELAY.C:buf->dentry->d_inode->i_size = buf->early_bytes;
> KERNEL/RELAY.C:buf->dentry->d_inode->i_size += buf->chan->subbuf_size 
> -buf->padding[old_subbuf];
> DRIVERS/USB/CORE/INODE.C:dev->usbfs_dentry->d_inode->i_size = i_size;
> DRIVERS/MTD/DEVICES/BLOCK2MTD.C:dev->mtd.size = 
> dev->blkdev->bd_inode->i_size & PAGE_MASK;
> DRIVERS/MD/MD.C: many reads of i_size 
> DRIVERS/BLOCK/NBD.C: many writes to i_size without i_size_write
> DRIVERS/BLOCK/DRBD/DRBD_INT.H: return bdev ? bdev->bd_inode->i_size >> 9 : 0;
> DRIVERS/BLOCK/DRBD/DRBD_WRAPPERS.H: mdev->this_bdev->bd_inode->i_size = 
> (loff_t)size << 9;
> BLOCK/BLK-CORE.C:printk(KERN_INFO "%s: rw=%ld, want=%Lu, limit=%Lu\n",
>                 bdevname(bio->bi_bdev, b),
>                 bio->bi_rw,
>                 (unsigned long long)bio->bi_sector + bio_sectors(bio),
>                 (long long)(bio->bi_bdev->bd_inode->i_size >> 9));
> maxsector = bio->bi_bdev->bd_inode->i_size >> 9;
> BLOCK/COMPAT_IOCTL.C: size = bdev->bd_inode->i_size;
> return compat_put_u64(arg, bdev->bd_inode->i_size);
> BLOCK/IOCTL.C: if (start + len > (bdev->bd_inode->i_size >> 9))
> size = bdev->bd_inode->i_size;
> return put_u64(arg, bdev->bd_inode->i_size);
> 
> The problem with this code is that if you read i_size without i_size_read 
> and the size wraps around 32 bits, for example from 0xffffffff to 
> 0x100000000 , there is a possibility on 32-bit machines to read an invalid 
> value (either 0 or 0x1ffffffff). Similarly, if you write i_size without 
> i_size_write, the readers can see intermediate invalid values.
> 
> 
> The original problem that caused this investigation is the question, how a 
> block device driver can change the size of its device. Normal method (used 
> in a few drivers, including dm), consists of
> mutex_lock(&inode->i_mutex);
> i_size_write(inode, new_size);
> mutex_unlock(&inode->i_mutex);

Don't you just do

  set_capacity(gendisk, sectors);
  revalidate(gendisk);

??

NeilBrown

> 
> This is deadlock-prone, because i_mutex is also held on fsync path. 
> Therefore, this deadlock happens: fsync takes i_mutex and issues I/Os, 
> block device driver wants to change its size, so it waits on i_mutex, 
> the I/Os wait until the device driver did its internal maintenance and 
> changed the inode size. The driver doesn't change the size until fsync 
> finished.
> 
> Jens, as a block maintainer, please think about it and propose some 
> specification how to clean this up. Also a clean verifiable rule regarding 
> i_size should be specified and the code should be fixed to conform to the 
> rule: maybe we could rename i_size to __i_size and ban its using.
> 
> Mikulas
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