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Re: [Qemu-devel] QMP accepts double dict keys

From: Eric Blake
Subject: Re: [Qemu-devel] QMP accepts double dict keys
Date: Mon, 3 Dec 2018 13:57:13 -0600
User-agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.3.1

On 12/3/18 1:48 PM, Markus Armbruster wrote:
Eric Blake <address@hidden> writes:

On 12/3/18 10:30 AM, Max Reitz wrote:

QMP accepts double keys in dicts without complaining.  The value it is
using is apparently the last one specified:

JSON says it is undefined what happens when a client passes double
keys. We are probably best off if we teach our parser to be strict and
reject doubled keys in QMP as invalid.

Not bug-compatible.  Do we care?

I don't think so. Such a client was already invoking undefined behavior. Relying on first- or last-past-the-post to win is not portable, since JSON parsers are allowed to use hash tables with non-deterministic lookups. I think erroring out is nicer than silently accepting one thing, especially if that might have been different than what the client (incorrectly) expected. I'm not even sure that we would want a deprecation period.

Hmm - can a client abuse QMP with duplicate keys to cause qemu to leak

No.  parse_pair() inserts with qdict_put_obj(), which replaces the old
value without leaking it.

Good to know.

Another test case is iotest 229 which specifies both mode=absolute-paths
and mode=existing (it wants the latter).

We'll have to fix such broken clients, of course. If it is just our iotests (and not libvirt), I'm less worried about the change in behavior.

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

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