RE: block ioctl to read/write last sector

Date: Tue Feb 13 2001 - 20:00:02 EST

> > While we can read and write to this sector in the kernel
> > partition code, we have
> > no way for userspace to update this partition block.
> Are you sure?

I'm not sure, but when I asked about this in January, I suggested having an
IOCTL that get/set blksize_size[MAJOR(dev)][MINOR(dev)], which didn't seem
to work for me.

> I need to read/write the last 512-byte
> sector on an odd-sized disk (IDE and/or SCSI) from user space. Employing
> suggestions from you and l-k, I have implemented two IOCTLs that get/set
> blksize_size[MAJOR(dev)][MINOR(dev)] values (via set_blocksize()). In my
> application, I read the hardsector size of a disk device (/dev/sdb) via an
> IOCTL, read the current blksize_size, set it to the hardsector size, and
> then continue, resetting blksize_size back to the original value when
> In between, I use fopen64() to open the device /dev/sdb (an odd-sized
> and lseek64()/fwrite64()/fread64() to write/read the last sector of the
> as reported by the BLKGETSIZE ioctl. The seek succeeds, however, the
> reads/writes fail. Writes fail with "No space left on device". Read
> returns 0, indicating EOF. If I read/write the N-1th sector this way, it
> works just fine. On even-sized disks, this succeeds both with and
> without calling my new IOCTLs, as expected. On odd-sized disks, it
> fails in both cases.

Anton Altaparmakov responded:

> I am sure Andries will correct me if I am wrong but here is
> what I think of the situation at present:
> I suspect that you will find the problem in
> linux/fs/block_dev.c functions
> block_write() and block_read() which AFAIK only get called
> when you use
> usermode to read a /dev/* blockdevice as you probably are
> doing. From the
> kernel these functions AFAIK never get called and hence the problem
> doesn't exist (either way I am surprised that it works as looking at
> generic_make_request() there is a check of simillar type and AFAICS it
> would fail as well for the same reason. I probably don't understand
> something there and/or generic_make_request() is not being
> called either,
> as it obviously works, since you have tried it).
> In block_write() you find this (lines 58ff in the file in 2.4.0):
> [snip]
> if (blk_size[MAJOR(dev)])
> size = ((loff_t) blk_size[MAJOR(dev)][MINOR(dev)] <<
> BLOCK_SIZE_BITS) >> blocksize_bits;
> else
> size = INT_MAX;
> while (count>0) {
> if (block >= size)
> return written ? written : -ENOSPC;
> [snip]
> As you can see when size is calculated blk_size is multiplied
> by 1024 and
> then divided by 512, effectively blk_size is multiplied by 2.
> The effect of this is that size has the lowest bit always
> equal to zero
> and hence it always will be even.
> The subsequent check "if (block >= size)" of course is then
> true and we
> return with -ENOSPC straight away.
> In block_read() you find this (lines 195ff in the file in 2.4.0):
> [snip]
> if (blk_size[MAJOR(dev)])
> size = (loff_t) blk_size[MAJOR(dev)][MINOR(dev)] <<
> else
> size = (loff_t) INT_MAX << BLOCK_SIZE_BITS;
> if (offset > size)
> left = 0;
> [snip]
> if (left <= 0)
> return 0;
> [snip]
> And again size is equal to blk_size multiplied by 1024 and
> hence always
> will be even.

If this analysis is correct (and I think it is), changing the block size
doesn't actually solve the problem.

I've got no problems requiring any IOCTL (either block-size-changing or just
a read/write last sector) to check that a device isn't in use somehow prior
to making these calls. Changing a disk's partition table while partitions
are actively in use on it isn't generally a good idea. But, fdisk and other
partition table-changing apps don't do this kind of check now IIRC.


Matt Domsch
Dell Linux Systems Group
Linux OS Development

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This archive was generated by hypermail 2b29 : Thu Feb 15 2001 - 21:00:23 EST