linux-kernel - Re: Cannot partition 32GB disk on a 32bit machine (correct version of the patch this time)

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Date:	Thu, 19 Jun 2014 09:33:26 +0000
From:	"Cox, Alan" <alan.cox@...el.com>
To:	"linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>
CC:	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"aeb@....nl" <aeb@....nl>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Cannot partition 32GB disk on a 32bit machine (correct version
 of the patch this time)

On Thu, 2014-06-19 at 10:30 +0100, Alan Cox wrote:
> The block code has 32bit cleanness problems with the iterator. This
> prevents things like partitioning a 32GB volume on a 32bit system.
> 
> I hit this with a volume of exactly 32GB in size (easy to duplicate with
> virtual machines). Tracing at step by step through the kernel I found
> the problem lines in blkdev_read_iter which truncates the size value
> into a 32bit value when setting up the iterator.
> 
> The hack below if applied "fixes" this and I think demonstrates that
> this is the problem spot.
> 
> What I'm less clear on is what the correct fix for this should be.

block: fix inability to partition large volumes

From: Alan <alan@...ux.intel.com>

The block code has 32bit cleanness problems with the iterator. This
prevents things like partitioning a 32GB volume on a 32bit system.

This is a simple initial "fix" that clips the problem cases so get
behaviour that is at least sane and trivially backportable.

Signed-off-by: Alan Cox <alan@...ux.intel.com>
---
 fs/block_dev.c |    3 +++
 1 file changed, 3 insertions(+)

diff --git a/fs/block_dev.c b/fs/block_dev.c
index 6d72746..bef2414 100644
--- a/fs/block_dev.c
+++ b/fs/block_dev.c
@@ -1603,6 +1603,9 @@ static ssize_t blkdev_read_iter(struct kiocb
*iocb, struct iov_iter *to)
 
 	size -= pos;
 	iov_iter_truncate(to, size);
+	/* Fix up for 32bit boxes for now */
+	if (to->count < size)
+	        to->count = 0xFFFFFFFF;
 	return generic_file_read_iter(iocb, to);
 }
 

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