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

[Martin Kletzander]

On Mon, Jul 26, 2021 at 01:45:19PM +0200, Martin Kletzander wrote:
>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.
>

OK, it works for me because while catching up with e-mails post-vacation
I did not yet notice (when replying here) that you already fixed it.
Sorry for the noise.

>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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