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Re: [Qemu-devel] [Nbd] question on ioctl NBD_SET_FLAGS

From: Alex Bligh
Subject: Re: [Qemu-devel] [Nbd] question on ioctl NBD_SET_FLAGS
Date: Wed, 4 May 2016 11:29:35 +0100

On 20 Apr 2016, at 17:18, Wouter Verhelst <address@hidden> wrote:

> Hi Eric,
> On Wed, Apr 20, 2016 at 09:42:02AM -0600, Eric Blake wrote:
> [...]
>> But in 3.9, the overlap bug was still present, and the set of global
>> flags had grown to include NBD_FLAG_NO_ZEROS (commit 038e066), which
>> overlaps with NBD_FLAG_READ_ONLY.  Ouch.  This means that a client
>> talking to a server that advertises NO_ZEROES means that the client will
>> mistakenly tell the kernel to treat EVERY export as read-only, even if
>> the client doesn't respond with NBD_FLAG_C_NO_ZEROES.
>> 3.10 fixed things; negotiate() now uses uint16_t *flags (instead of u32
>> *), and no longer tries to merge global flags with transmission flags;
>> only the transmission flags are ever passed to the kernel via
>> NBD_SET_FLAGS.  Maybe it's good that there was only 2 weeks between 3.9
>> and 3.10, so hopefully few distros are trying to ship that broken version.
> Well, yeah, since 3.10 was an "oops" release when 3.9 exposed that bug
> (which indeed had existed for a while) and which was reported quite
> quickly on the list. Released versions of nbd which have the bug exist
> though, and trying to have a 3.8 (or below) client talk to a 3.9 (or
> above) server has the same issue.
> I decided that there was no way in which I could fix it, and that "the
> export is readonly" is bad but not a "critical data loss" kind of bug,
> so releasing 3.10 was pretty much the only sane thing I could do (other
> than delaying NO_ZEROES, which might have worked too).

For what it's worth, Ubuntu 14.04 (still under long term
support) ships with nbd-client 3.7 and I've just had a bug
report filed against gonbdserver where all exports go readonly.

I've added an option to disable NBD_FLAG_NO_ZEROS on nbd-server
but that's pretty horrible. And I think this will affect all
pre-3.10 clients talking to 3.9 and later servers.

I'm going to find a minimal patch to nbd-client and offer that
to Ubuntu / Debian.

This message is here in part so I have something to point them at
on the mailing list :-)

Alex Bligh

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