Static analysis updates in GCC 11

[David Brown]

On 28/01/2021 22:27, David Malcolm wrote:
> On Thu, 2021-01-28 at 22:06 +0100, David Brown wrote:
>> On 28/01/2021 21:23, David Malcolm via Gcc wrote:
>>> I wrote a blog post covering what I've been working on in the
>>> analyzer
>>> in this release:
>>>   https://developers.redhat.com/blog/2021/01/28/static-analysis-updates-in-gcc-11/
>>>
>>
>> As a gcc user, I am always glad to hear of more static analysis and
>> static warning work.  My own work is mostly on small embedded
>> systems,
>> where "malloc" and friends are severely frowned upon in any case and
>> there is no file system, so most of the gcc 10 -fanalyzer warnings
>> are
>> of no direct use to me.  (I still think they are great ideas - even
>> if
>> /I/ don't write much PC code, everyone benefits if there are fewer
>> bugs
>> in programs.)  I will get more use for the new warnings you've added
>> for
>> gcc 11.
>>
>>
>> I wrote a feature request for gcc a while back, involving adding tag
>> attributes to functions in order to ensure that certain classes of
>> functions are only used from specific allowed functions.  The feature
>> request attracted only a little interest at the time.  But I suspect
>> it
>> could work far better along with the kind of analysis you are doing
>> with
>> -fanalyzer than with the normal syntactical analyser in gcc.
>>
>> <https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88391>
> 
> Interesting.  The attribute ideas seem designed to work with the
> callgraph: partitioning the callgraph into families of functions for
> which certain kinds of inter-partition edges are disallowed.  Can a
> function change its tag internally, or is it assumed that a function
> has a single tag throughout its whole body?  I see that you have a case
> in example 3 where a compound statement is marked with an attribute
> (which may be an extension of our syntax).
> 
> One thing I forgot to mention in the blog post is that the analyzer now
> supports plugins; there's an example of a mutex-checking plugin here:
> https://gcc.gnu.org/git/?p=gcc.git;a=commitdiff;h=66dde7bc64b75d4a338266333c9c490b12d49825
> which is similar to your examples 1 and 3.  Your example 2 is also
> reminiscent of the async-signal-unsafe checking that the analyzer has
> (where it detects code paths that are called within a signal handler
> and complains about bad calls within them).  Many of the existing
> checks in the analyzer are modelled as state machines (either global
> state for things like "are we in a signal handler", or per-value state
> for things like "has this pointer been freed"), and your examples could
> be modelled that way too (e.g. "what sections are in RAM" could be a
> global state) - so maybe it could all be done as analyzer plugins, in
> lieu of implementing the RFE you posted.
> 
> Hope this is constructive
> Dave
> 

Thanks for the feedback.

Just to be clear, I am not particularly tied to the syntax I suggested 
in the bugzilla suggestion - it is merely a starting point for ideas. 
"caller_tag" and "callee_tag", for example, are probably too similar and 
easily mixed up by people using the attributes.  "needs_tag" and 
"limit_to_tag" might be a better second suggestion.  Or perhaps someone 
else will think of a completely different arrangement to get a similar 
end result.

Yes, I believe tying an attribute to a compound statement might be a new 
idea, but that could just be because the only "statement attribute" 
currently in gcc (AFAIK, from reading the manual) is "fallthrough" - and 
it is generally applied to a null statement.

I suggested the possibility of attaching attributes to statements within 
a function, so that the developer can make it clear when the tag is 
acquired and released.  But it is not essential - it would be fine to 
simply say that the whole function is tagged, if that makes an 
implementation simpler.

At least some of the potential uses here could be handled by C++ strong 
typing and tag structs (carrying no data, but with restrictions on how 
they can be created and copied around).  But tag attributes, or an 
equivalent mechanism, would let you do this in C as well, and it would 
be less strict and structured - and thus easier to add to existing code.