Re: [Qemu-devel] [PATCH v3 09/12] tests/libqos/pci: Clean up string interpolation into QMP input

[Eric Blake]

On 07/31/2017 02:29 AM, Markus Armbruster wrote:
> Eric Blake <address@hidden> writes:
> 
>> On 07/28/2017 11:35 AM, Eric Blake wrote:
>>>>> +    QObject *extra_args = qobject_from_jsonf("{ 'shm': '%s', 'size': 
>>>>> '1M' }",
>>>>> +                                             tmpshm);
>>>>
>>
>>> Passing '%s' through qobject_from_jsonf() is generally wrong (it would
>>> produce ''...'' instead of the intended '...').
>>>
>>> Looks like something to fix on the next round.
>>>
>>
>> What's scary is that the testsuite didn't start failing.  That's not
>> good; we'll want to figure out why the bug didn't impact the test.
> 
> Good idea.

The intended result: the created QDict would include
"shm":"/qtest-11387-3641365368" (or comparable string).  The actual
result: the created QDict includes "shm":"%s" - a literal 2-character
string, because the lexer does not do interpolation inside strings.  And
apparently, naming your ivshmem shared memory a literal "%s" (rather
than a name beginning with "/") is not a semantic error, so the test passes.

In other words, qobject_from_jsonf() does NOT do % interpolation of
anything embedded inside '', and basically blindly ignores the tmpshm
vararg.  It would be neat if we could make qobject_from_jsonf() detect a
mismatch in varargs, even though we don't (can't) require a NULL
sentinel (we're limited to fitting in with -Wformat paradigms already).
So while we can't flag it at compile time, I do think we can flag it at
runtime: if, in qobject_from_jsonf() (but NOT in qobject_from_json()),
we reject any JSON string from the lexer that contains "%" but not "%%",
we will have caught all the places where gcc paired a % sequence with a
vararg parameter even though our lexer did not make the same pairing.
If we WANT a literal % in a string (rare, probably only in the
testsuite), we write it as %% and then qobject_from_jsonf() interpolates
it.  Then, since bare % is NOT valid JSON, we don't have to enhance the
lexer to recognize anything new; our changes are limited to
documentation and the vararg parser.  It still means we only get a
runtime, rather than a compile-time, detection of misuse of % passed to
the format argument, but it should be an easy enough proof of concept
that such a runtime failure would have been enough to make ivshmem-test
fail and flag the error during 'make check' rather than escaping review.

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

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