Re: [Qemu-devel] [PATCH 5/5] qtest: Document calling conventions

[Eric Blake]

On 07/20/2017 03:37 PM, Eric Blake wrote:
>>> + * @fmt...: QMP message to send to qemu; only recognizes formats
>>> + * understood by json-lexer.c
>>>   *
>>>   * Sends a QMP message to QEMU and consumes the response.
>>>   */
>>
>> These formats are chosen so that gcc can help us check them.  Please add
>> GCC_FMT_ATTR().  Precedence: qobject_from_jsonf().
> 
> Will do.  (This patch was originally part of my larger series trying to
> get rid of qobject_from_jsonf() - I may still succeed at that, but
> separating the easy stuff from the controversial means that the easy
> stuff needs tweaking).

Bleargh.  It's not that simple.

With printf-style, hmp("literal") and hmp("%s", "literal") produce the
same results.

But with json-lexer style, %s MODIFIES its input.
The original qmp("{'execute':\"foo\"}") sends a JSON object
 {'execute':"foo"}
but counterpart qmp("%s", "{'execute':'foo'}") sends a JSON string with
added \ escaping:
 "{'execute':\"foo\"}"

So adding the format immediately causes lots of warnings, such as:

  CC      tests/vhost-user-test.o
tests/vhost-user-test.c: In function ‘test_migrate’:
tests/vhost-user-test.c:668:5: error: format not a string literal and no
format arguments [-Werror=format-security]
     rsp = qmp(cmd);
     ^~~

  CC      tests/boot-order-test.o
tests/boot-order-test.c: In function ‘test_a_boot_order’:
tests/boot-order-test.c:46:26: error: zero-length gnu_printf format
string [-Werror=format-zero-length]
     qmp_discard_response("");   /* HACK: wait for event */
                          ^~

but we CAN'T rewrite those to qmp("%s", command).

Now you can sort-of understand why my larger series wanted to get rid of
qobject_from_jsonf(), since our use of GCC_FMT_ATTR() there is a lie?

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

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