On 07/31/2017 02:29 AM, Markus Armbruster wrote:
> Eric Blake <ebl...@redhat.com> 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
signature.asc
Description: OpenPGP digital signature
