[Libguestfs] nbdkit: cow.c:591: cow_extents: Assertion `count > 0' failed.

[Martin Kletzander]

On Sat, Jul 24, 2021 at 11:40:49AM +0100, Richard W.M. Jones wrote:
>I wonder if this is an overflow:
>
>$ ./nbdkit sparse-random 3G --filter=cow --run 'nbdinfo --map $uri'
># OK
>
>$ ./nbdkit sparse-random 4G --filter=cow --run 'nbdinfo --map $uri'
>nbdkit: cow.c:591: cow_extents: Assertion `count > 0' failed.
>

This is OK on my machine as well.

>Any size >= 4G seems to cause the crash.
>

What struck me with this is:

cow.c:cow_extents:

   /* To make this easier, align the requested extents to whole blocks.
    * Note that count is a 64 bit variable containing at most a 32 bit
    * value so rounding up is safe here.
    */

I started debugging the cow filter and the values for 4G image I got
are:

count=4294966784; offset=0 // Pre-rounding
end=4294967296; offset=0; count=4294967296; blknum=0 // Post-rounding

It looks like it is not asking for the whole size, but size - 1 block.
That gets rounded to 2^32 and sparse-extents then reports:

sparse-random: extents count=4294963200 offset=0 req_one=0
sparse-random: extents count=4096 offset=4294963200 req_one=0

Which is 2^32 - 4096.  I hope I am not confusing anyone with this, but
it seems weird to me.

Martin

>Rich.
>
>-- 
>Richard Jones, Virtualization Group, Red Hat http://people.redhat.com/~rjones
>Read my programming and virtualization blog: http://rwmj.wordpress.com
>virt-p2v converts physical machines to virtual machines.  Boot with a
>live CD or over the network (PXE) and turn machines into KVM guests.
>http://libguestfs.org/virt-v2v
>
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