[Qemu-devel] [PATCH] qcow2: avoid misaligned 64bit bswap

[Qemu-devel] [PATCH] qcow2: avoid misaligned 64bit bswap

[John Snow]

If we create a buffer directly on the stack by using 12 bytes, there's
no guarantee the 64bit value we want to swap will be aligned, which
could cause errors with undefined behavior.

Spotted with clang -fsanitize=undefined and observed in iotests 15, 26,
44, 115 and 121.

Signed-off-by: John Snow <jsnow@redhat.com>
---
 block/qcow2-refcount.c | 11 +++++++----
 1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 4b81c8d..6e0e5bd 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -560,13 +560,16 @@ static int alloc_refcount_block(BlockDriverState *bs,
     }
 
     /* Hook up the new refcount table in the qcow2 header */
-    uint8_t data[12];
-    cpu_to_be64w((uint64_t*)data, table_offset);
-    cpu_to_be32w((uint32_t*)(data + 8), table_clusters);
+    struct QEMU_PACKED {
+        uint64_t d64;
+        uint32_t d32;
+    } data;
+    cpu_to_be64w(&data.d64, table_offset);
+    cpu_to_be32w(&data.d32, table_clusters);
     BLKDBG_EVENT(bs->file, BLKDBG_REFBLOCK_ALLOC_SWITCH_TABLE);
     ret = bdrv_pwrite_sync(bs->file->bs,
                            offsetof(QCowHeader, refcount_table_offset),
-                           data, sizeof(data));
+                           &data, sizeof(data));
     if (ret < 0) {
         goto fail_table;
     }
-- 
2.4.3

Re: [Qemu-devel] [PATCH] qcow2: avoid misaligned 64bit bswap

[Eric Blake]

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On 11/02/2015 04:32 PM, John Snow wrote:
> If we create a buffer directly on the stack by using 12 bytes, there's
> no guarantee the 64bit value we want to swap will be aligned, which
> could cause errors with undefined behavior.
> 
> Spotted with clang -fsanitize=undefined and observed in iotests 15, 26,
> 44, 115 and 121.
> 
> Signed-off-by: John Snow <jsnow@redhat.com>
> ---
>  block/qcow2-refcount.c | 11 +++++++----
>  1 file changed, 7 insertions(+), 4 deletions(-)

Reviewed-by: Eric Blake <eblake@redhat.com>

-- 
Eric Blake   eblake redhat com    +1-919-301-3266
Libvirt virtualization library http://libvirt.org


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Re: [Qemu-devel] [PATCH] qcow2: avoid misaligned 64bit bswap

[Kevin Wolf]

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Am 03.11.2015 um 00:46 hat Eric Blake geschrieben:
> On 11/02/2015 04:32 PM, John Snow wrote:
> > If we create a buffer directly on the stack by using 12 bytes, there's
> > no guarantee the 64bit value we want to swap will be aligned, which
> > could cause errors with undefined behavior.
> > 
> > Spotted with clang -fsanitize=undefined and observed in iotests 15, 26,
> > 44, 115 and 121.
> > 
> > Signed-off-by: John Snow <jsnow@redhat.com>
> > ---
> >  block/qcow2-refcount.c | 11 +++++++----
> >  1 file changed, 7 insertions(+), 4 deletions(-)
> 
> Reviewed-by: Eric Blake <eblake@redhat.com>

Thanks, applied to the block branch.

Kevin

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