Re: [Libguestfs] [PATCH 1/2] spec: Recommend cap on NBD_REPLY_TYPE_BLOCK_STATUS length

On Fri, Apr 8, 2022 at 6:47 PM Eric Blake <eblake(a)redhat.com&gt; wrote: > > On Fri, Apr 08, 2022 at 04:48:59PM +0300, Nir Soffer wrote: ... > > > BTW attached is an nbdkit plugin that creates an NBD server that > > > responds with massive numbers of byte-granularity extents, in case > > > anyone wants to test how nbdkit and/or clients respond: > > > > > > $ chmod +x /var/tmp/lots-of-extents.py > > > $ /var/tmp/lots-of-extents.py -f > > > > > > $ nbdinfo --map nbd://localhost | head > > > 0 1 3 hole,zero > > > 1 1 0 data > > > 2 1 3 hole,zero > > > 3 1 0 data > > > 4 1 3 hole,zero > > > 5 1 0 data > > > 6 1 3 hole,zero > > > 7 1 0 data > > > 8 1 3 hole,zero > > > 9 1 0 data > > > $ nbdinfo --map --totals nbd://localhost > > > 524288 50.0% 0 data > > > 524288 50.0% 3 hole,zero > > > > This is a malicious server. A good client will drop the connection when > > receiving the first 1 byte chunk. > > Depends on the server. Most servers don't serve 1-byte extents, and > the NBD spec even recommends that extents be at least 512 bytes in > size, and requires that extents be a multiple of any minimum block > size if one was advertised by the server. > > But even though most servers don't have 1-byte extents does not mean > that the NBD protocol must forbid them. Forbidding this simplifies clients without limiting real world use cases.

What is a reason to allow this?