Re: i_size misuses

[Mikulas Patocka]

On Sun, 12 Sep 2010, Neil Brown wrote:

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

revalidate_disk() has still the problem that it changes i_size without 
holding i_mutex and other kernel parts (for example generic_file_llseek) 
assume that if we hold the lock, i_size_can't be changed.

The rules for accessing i_size must be specified and followed.

Mikulas
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