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Re: [Qemu-devel] [PATCH 1/2] glusterfs: fix max_discard

From: Denis V. Lunev
Subject: Re: [Qemu-devel] [PATCH 1/2] glusterfs: fix max_discard
Date: Tue, 3 Feb 2015 10:31:30 +0300
User-agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10.10; rv:31.0) Gecko/20100101 Thunderbird/31.4.0

On 02/02/15 23:46, Denis V. Lunev wrote:
On 02/02/15 23:40, Peter Lieven wrote:
Am 02.02.2015 um 21:09 schrieb Denis V. Lunev:
qemu_gluster_co_discard calculates size to discard as follows
     size_t size = nb_sectors * BDRV_SECTOR_SIZE;
ret = glfs_discard_async(s->fd, offset, size, &gluster_finish_aiocb, acb);

glfs_discard_async is declared as follows:
   int glfs_discard_async (glfs_fd_t *fd, off_t length, size_t lent,
                           glfs_io_cbk fn, void *data) __THROW
This is problematic on i686 as sizeof(size_t) == 4.

Set bl_max_discard to SIZE_MAX >> BDRV_SECTOR_BITS to avoid overflow
on i386.

Signed-off-by: Denis V. Lunev <address@hidden>
CC: Kevin Wolf <address@hidden>
CC: Peter Lieven <address@hidden>
  block/gluster.c | 9 +++++++++
  1 file changed, 9 insertions(+)

diff --git a/block/gluster.c b/block/gluster.c
index 1eb3a8c..8a8c153 100644
--- a/block/gluster.c
+++ b/block/gluster.c
@@ -622,6 +622,11 @@ out:
      return ret;
+static void qemu_gluster_refresh_limits(BlockDriverState *bs, Error **errp)
+    bs->bl.max_discard = MIN(SIZE_MAX >> BDRV_SECTOR_BITS, INT_MAX);
Looking at the gluster code bl.max_transfer_length should have the same limit, but thats a different patch.
ha, the same applies to nbd code too.

I'll do this stuff tomorrow and also I think that some
audit in other drivers could reveal something interesting.

ok. The situation is well rotten here on i686.

The problem comes from the fact that QEMUIOVector
and iovec uses size_t as length. All API calls use
this abstraction. Thus all conversion operations
from nr_sectors to size could bang at any moment.

Putting dirty hands here is problematic from my point
of view. Should we really care about this? 32bit
applications are becoming old good history of IT...

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