Refcount table entries have a field to store the offset of the
refcount block. The rest of the bits of the entry are currently
reserved.

The offset is always taken from the entry using REFT_OFFSET_MASK to
ensure that we only use the bits that belong to that field.

While that mask is used every time we read from the refcount table, it
is never used when we write to it. Due to the other constraints of the
qcow2 format QEMU can never produce refcount block offsets that don't
fit in that field so any such offset when allocating a refcount block
would indicate a bug in QEMU.
---
 block/qcow2-refcount.c | 3 +++
 1 file changed, 3 insertions(+)

diff --git a/block/qcow2-refcount.c b/block/qcow2-refcount.c
index 46082aeac1..31a2e1f845 100644
--- a/block/qcow2-refcount.c
+++ b/block/qcow2-refcount.c
@@ -367,6 +367,9 @@ static int alloc_refcount_block(BlockDriverState *bs,
         return new_block;
     }
 
+    /* The offset must fit in the offset field of the refcount table entry */
+    assert((new_block & REFT_OFFSET_MASK) == new_block);
+
     /* If we're allocating the block at offset 0 then something is wrong */
     if (new_block == 0) {
         qcow2_signal_corruption(bs, true, -1, -1, "Preventing invalid "
-- 
2.11.0

On 11/13/18 10:45 AM, Alberto Garcia wrote:
> Refcount table entries have a field to store the offset of the
> refcount block. The rest of the bits of the entry are currently
> reserved.
> 
> The offset is always taken from the entry using REFT_OFFSET_MASK to
> ensure that we only use the bits that belong to that field.
> 
> While that mask is used every time we read from the refcount table, it
> is never used when we write to it. Due to the other constraints of the
> qcow2 format QEMU can never produce refcount block offsets that don't
> fit in that field so any such offset when allocating a refcount block
> would indicate a bug in QEMU.
> ---
>   block/qcow2-refcount.c | 3 +++
>   1 file changed, 3 insertions(+)
> 

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

-- 
Eric Blake, Principal Software Engineer
Red Hat, Inc.           +1-919-301-3266
Virtualization:  qemu.org | libvirt.org

On Tue 13 Nov 2018 06:06:54 PM CET, Eric Blake <eblake@redhat.com> wrote:

>> Refcount table entries have a field to store the offset of the
>> refcount block. The rest of the bits of the entry are currently
>> reserved.
>> 
>> The offset is always taken from the entry using REFT_OFFSET_MASK to
>> ensure that we only use the bits that belong to that field.
>> 
>> While that mask is used every time we read from the refcount table, it
>> is never used when we write to it. Due to the other constraints of the
>> qcow2 format QEMU can never produce refcount block offsets that don't
>> fit in that field so any such offset when allocating a refcount block
>> would indicate a bug in QEMU.
>> ---
>>   block/qcow2-refcount.c | 3 +++
>>   1 file changed, 3 insertions(+)
>> 
>
> Reviewed-by: Eric Blake <eblake@redhat.com>

Yes, for 3.1, shall I resend it with the updated subject message?

Berto

Am 14.11.2018 um 08:10 hat Alberto Garcia geschrieben:
> On Tue 13 Nov 2018 06:06:54 PM CET, Eric Blake <eblake@redhat.com> wrote:
> 
> >> Refcount table entries have a field to store the offset of the
> >> refcount block. The rest of the bits of the entry are currently
> >> reserved.
> >> 
> >> The offset is always taken from the entry using REFT_OFFSET_MASK to
> >> ensure that we only use the bits that belong to that field.
> >> 
> >> While that mask is used every time we read from the refcount table, it
> >> is never used when we write to it. Due to the other constraints of the
> >> qcow2 format QEMU can never produce refcount block offsets that don't
> >> fit in that field so any such offset when allocating a refcount block
> >> would indicate a bug in QEMU.

Missing S-o-b.

> >>   block/qcow2-refcount.c | 3 +++
> >>   1 file changed, 3 insertions(+)
> >> 
> >
> > Reviewed-by: Eric Blake <eblake@redhat.com>
> 
> Yes, for 3.1, shall I resend it with the updated subject message?

Honestly, I don't see why an additional assertion should qualify as a
fix? If it changes the behaviour, it's a bug.

You wouldn't have to resend for the updated subject message, but you do
for the missing S-o-b.

Kevin

On Wed 14 Nov 2018 11:49:34 AM CET, Kevin Wolf wrote:
>> > Reviewed-by: Eric Blake <eblake@redhat.com>
>> 
>> Yes, for 3.1, shall I resend it with the updated subject message?
>
> Honestly, I don't see why an additional assertion should qualify as a
> fix? If it changes the behaviour, it's a bug.

We can leave it for after 3.1 if you think it's more appropriate. I'll
resend it with the S-o-b anyway.

Berto