Re: 32-bit bug in iovec iterator changes

James Bottomley
2014-06-22T00:03:23Z

On Sun, 2014-06-22 at 00:49 +0100, Al Viro wrote:
> On Sat, Jun 21, 2014 at 07:09:22PM -0400, Theodore Ts'o wrote:
> > On Sat, Jun 21, 2014 at 06:53:07AM +0100, Al Viro wrote:
> > >
> > > ed include/linux/uio.h <<EOF
> > > /iov_iter_truncate/s/size_t/u64/
> > > w
> > > q
> > > EOF
> > >
> > > Could you check if that fixes the sucker?
> >
> > The following patch (attached at the end) appears to fix the problem,
> > but looking at uio.h, I'm completely confused about *why* it fixes the
> > problem. In particular, iov_iter_iovec() makes no sense to me at all,
> > and I don't understand how the calculation of iov_len makes any sense:
> >
> > .iov_len = min(iter->count,
> > iter->iov->iov_len - iter->iov_offset),
>
> Eh? We have iov[0].iov_base..iov[0].iov_base+iov[0].iov_len - 1 for
> area covered by the first iovec. First iov_offset bytes have already
> been consumed. And at most count bytes matter. So yes, this iov_len
> will give you equivalent first iovec.
>
> Said that, iov_iter_iovec() will die shortly - it's a rudiment of older
> code, with almost no users left. But yes, it is correct.
>
> > It also looks like uio.h is mostly about offsets to memory pointers,
> > and so why this would make a difference when the issue is the block
> > device offset goes above 2**30?
>
> It is, and your patch is a huge overkill.
>
> > There must be deep magic going on here, and so I don't know if your
> > s/size_t/u64/g substitation also extends to the various functions that
> > have size_t in them:
>
> No, it does not. It's specifically about iov_iter_truncate(); moreover,
> it matters to only one caller of that sucker. Namely,
>
> static ssize_t blkdev_read_iter(struct kiocb *iocb, struct iov_iter *to)
> {
> struct file *file = iocb->ki_filp;
> struct inode *bd_inode = file->f_mapping->host;
> loff_t size = i_size_read(bd_inode);
> loff_t pos = iocb->ki_pos;
>
> if (pos >= size)
> return 0;
>
> size -= pos;
> iov_iter_truncate(to, size);
> return generic_file_read_iter(iocb, to);
> }
>
> What happens here is capping to->count, to guarantee that we won't even look
> at anything past the end of block device. Alternative fix would be to
> have
> if (pos >= size)
> return 0;
> if (to->size + pos > size) {
> /* note that size - pos fits into size_t in this case,
> * so it's OK to pass it to iov_iter_truncate().
> */
> iov_iter_truncate(to, size - pos);
> }
> return generic_file_read_iter(iocb, to);
> in there. Other callers are passing it size_t values already, so we don't
> need similar checks there.
>
> Or we can make iov_iter_truncate() take an arbitrary u64 argument, seeing that
> it's inlined anyway. IMO it's more robust that way...
>
> Anyway, does the following alone fix the problem you are seeing?
>
> diff --git a/include/linux/uio.h b/include/linux/uio.h
> index ddfdb53..dbb02d4 100644
> --- a/include/linux/uio.h
> +++ b/include/linux/uio.h
> @@ -94,7 +94,7 @@ static inline size_t iov_iter_count(struct iov_iter *i)
> return i->count;
> }
>
> -static inline void iov_iter_truncate(struct iov_iter *i, size_t count)
> +static inline void iov_iter_truncate(struct iov_iter *i, u64 count)
> {
> if (i->count > count)
> i->count = count;

Al, how can that work? i->count is size_t, which is 32 bit, so we're
going to get truncation errors. I could see this possibly working if
count in struct iov_iter becomes u64 (which is going to have a lot of
knock on consequences, but it seems to me that at least kvec.iov_len and
iov_iter.iov_offset have to become u64 as well.

James