[cfe-dev] clang-tidy or static analyzer or ...

Artem Dergachev via cfe-dev cfe-dev at lists.llvm.org
Fri Jul 5 20:05:28 PDT 2019

 > It sounds like clang-tidy will be the sweet spot between usefulness 
and effort to implement.

Yup, i approve. You can experiment with clang-query real quick and it 
should give you an idea. If you find out that some of the useful 
ASTMatchers are missing, feel free to add them - either locally or 
upstreamed, as it's super easy.

 > By CFG you mean the Clang Static Analyzer?

Clang CFG is https://clang.llvm.org/doxygen/classclang_1_1CFG.html .

It's a "source-level" control flow graph that consists of pointers to 
AST statements, ordered in the control flow order (i.e., in order of 
execution). A directed path in the CFG from statement A to statement B 
would tell you that "B can potentially be executed after A". This allows 
you to use your favorite graph algorithms to reason about the program's 

The Static Analyzer works over Clang CFG, but it's much more than that: 
the Static Analyzer also understands the semantics of every statement 
and can model their effects over the "symbolic" state of the program. 
For example, in the following code:

void foo(bool x) {
   if (x) {
   if (x) {

the CFG would be a double-diamond: the control branches off at the first 
statement, then joins back at the end, then branches off again, then 
joins back. But the Static Analyzer would understand that the 
if-conditions are the same, therefore these are simply two unrelated 
execution paths, and the execution path in which the first branch goes 
to true and the second branch goes to false is in fact impossible. And 
if 'x' is overwritten in between, the Static Analyzer would also know that.

Apart from the Static Analyzer, the CFG is used for some clang warnings, 
such as -Wuninitialized (see AnalysisBasedWarnings.cpp). There are some 
ready-made analyses for common data flow problems implemented over Clang 
CFG (such as live variables analysis, dominators analysis, etc.).

Clang CFG is not used during the actual compilation / code generation. 
When Chris Lattner talks about MLIR and mentions that Clang should have 
had a "CIL", he means that the Clang CFG (or something similar) should 
have been used for compilation *as well as* for analysis. This would, in 
particular, allow semantic optimizations that require information that's 
already lost in LLVM IR.

Because the CFG is not used for compilation, it's not necessarily 
correct. There are a few known bugs in it, mostly around complicated C++ 
and also around GNU extensions such as the *binary* operator ?: or 
statement-expressions. But for plain C it should be nearly perfect.

 > Does 'copy  around' include passing to my private fns such as tweak()?

That's entirely up to you. What i was trying to say is that it if you 
allow copying 'dev' into another variable, it will become much harder to 
implement your analysis, so you'd much rather forbid the user to do 
this. You might also allow the user to do this and suffer from 
imprecision in your analysis. At the same time, because your analysis is 
not inter-procedural, passing 'dev' into a function should be fine. The 
function can still return it back "laundered" so the user would be able 
to assign it into a variable behind your back. But these are the usual 
trade-offs of static analysis, don't be too scared by them - after all 
it all boils down to the halting problem :)

On 7/4/19 3:40 AM, Billy O'Mahony wrote:
> Hi Artem,
> thanks for your well thought-out and useful reply. It sounds like 
> clang-tidy will be the sweet spot between usefulness and effort to 
> implement.
> I have a few other responses down below.
> Regards,
> Billy.
> On Wed, 3 Jul 2019 at 23:23, Artem Dergachev <noqnoqneo at gmail.com 
> <mailto:noqnoqneo at gmail.com>> wrote:
>     Hi,
>     It depends on how strict do you want the checking be and on the
>     details of the rule. If you're designing a new API from scratch
>     and stuck with gcc forever, i wouldn't mind using the gcc
>     __attribute__((cleanup())) for your purpose.
> I didn't know about that gcc attrib. I need to read the gcc manual 
> attrib section! I should've added that while we will be developing on 
> gcc the code should be compilable on other toolchains/OSs also, so we 
> are avoiding any gcc extensions (e.g. gcc has extensions for thread 
> local storage but we are not using those for the same reason).
>     The rule you described should be reasonably easy to implement with
>     the Static Analyzer. The good side of it is that you get a lot of
>     semantic modeling for free. For instance, if the developer copies
>     `dev` into a local variable and then uses that local variable
>     outside of api_enter..api_exit, the tool will be able to handle
>     transparently, as it deals with values rather than with variables.
> That is really cool.
>     Also it will probably be the easiest tool for your problem. The
>     downside would be that it's not guaranteed to find all bugs; it'll
>     inevitably give up on complicated code with high cyclomatic
>     complexity :) So if you want strict/paranoid enforcement of rules,
>     the Static Analyzer is not the right tool. But if you want to
>     simply find some bugs for free, it's the right tool.
>     It sounds as if your problem is not inter-procedural. Let me
>     double-check this: would you have another api_enter..api_exit pair
>     in the body of your tweak() function? Or is just one
>     api_enter..api_exit enough? Or is it a bug to call api_enter twice
>     without an api_exit in between?
> Yes in this case tweak() could be another public fn of the api that 
> would also have an enter/exit pair. But we are using recursive mutexes 
> (a thread can acquire the same mutex N times (ie call api_enter)  so 
> long as it also releases N times) so that will be okay.
>     If you have to write api_enter..api_exit in *every* function that
>     deals with devices, then the problem is not inter-procedural,
>     which makes it much easier. In particular, you should be able to
>     come up with a purely syntactic analysis ("every function that
>     accesses a device_t must start with api_enter() and must end in
>     exactly one spot with api_exit()").
> We will only insist on enter/exit in public API functions. Which are 
> the only ones a client application can call. Internal private 
> functions we won't have locking (as they can only be called ultimately 
> from a public function so the device will be locked.) We are going to 
> allow private fns to call back into the api via the public i/face. But 
> we will have the public functions in specific files and have a 
> specific naming prefix.
>     Such analysis should be easily do-able in clang-tidy as long as
>     you're satisfied with this level of aggressiveness. In particular,
>     you'll have to be willing to sacrifice code like this:
>     void foo(device_t *dev) {
>       if (flip_a_coin()) {
>         api_enter(dev);
>         ...
>         api_exit(dev);
>       }
>     }
>     But it may be perfectly fine if you seriously want to enforce a
>     strict structure on all your functions that deal with devices.
> So is it the case that clang-tidy kind of passes info to the 
> checker-extension in a syntactic code-parsing order. Whereas the 
> static analyzer passes information to the checker in a simulated 
> run-time order?
> E.g in your foo() above my proposed checker gets fed 1) theres a 
> function called foo, 2) theres an if with a call to flip_a_coin 3) in 
> the true case there is a call to enter then exit 4) in the else there 
> is nothing 5) there is a return (at which point my checker would need 
> to be pretty smart and hold a lot of state to figure out something was 
> wrong) . And to compare for the static analyzer it's more like 1) 
> there is fn foo 2.1) there is a code path through foo with enter/exit 
> 2.2) there is a code path with just return (at which point my 
> reasonably simple checker would raise an error).
>     I think the truly-truly right tool for your problem would be to
>     come up with a custom analysis over Clang CFG.
>     .
> By CFG you mean the Clang Static Analyzer?
>     It would be harder to implement, but it would allow you to express
>     things like "every execution path within a function that accesses
>     `dev` must have a api_enter before it and an api_exit after it;
>     you are not allowed to copy `dev` around". This would strictly
>     enforce the rule.
> Yes that would be great. But I think just using clang-tidy from what 
> you are saying would get us a long way. And there are heaps of simpler 
> checks we would like to implement also.
>     At the same time it'll allow you to lift the requirement of
>     exactly one return point - you would still be able to ensure that
>     all accesses are covered. If you need to allow to copy `dev`
>     around, it should still be doable, but it will be significantly
>     more difficult to implement
> Does 'copy  around' include passing to my private fns such as 
> tweak()?. We don't need to copy dev anywhere within the public fns but 
> we do need it to pass it to private fns.
>     On 7/2/19 12:13 PM, Billy O'Mahony via cfe-dev wrote:
>>     Hello,
>>     I'd like to write a rule for either clang-tidy or static analyzer
>>     to help catch some potential errors in a project I'm working on.
>>     My questions are:
>>         a) is only one or the other will be able to do what I want to do?
>>         b) if both are feasible which would have the simpler
>>     implementation?
>>     The project involves writing an API that will run in a
>>     multi-threaded application and is responsible for serializing all
>>     access to a device structure. Therefore the first thing in every
>>     function in the API must be to call api_enter (which will among
>>     other things acquire a mutex on the device) and the last thing
>>     before returning must be to call api_exit. Also I want to enforce
>>     single exit point from every API function - or certainly if there
>>     are any return points that bypass the api_exit call.
>>     So here is an example function with errors I want to catch
>>     highlighted.
>>     int api_foo(device_t *dev) {
>>         int ret_val = 0;
>>         bar();  // fn calls & decls before api_enter is ok- just
>>     don't access dev.
>>         dev->bla = 1; // NO! device access before api_enter() called
>>         api_enter(dev);   // error if this call is not present
>>     exactly once
>>         if (dev->bla)
>>             return; // NO! didn't call api_exit before rtn. Also two
>>     return points
>>         if (dev->ma) {
>>             ret_val = 1;
>>             goto cleanup;
>>         }
>>         tweak(dev);
>>     cleanup:
>>         api_exit(dev); // error if this is not present exactly once
>>         dev->bla = 1; //NO! device access after api_exit()
>>         return ret_val;
>>     }
>>     I don't think it matters but the project is C compiled with gcc.
>>     Also if both are feasible any other pointers, tips or good
>>     resources would be appreciated. E.g  is there a totally different
>>     methodology I'm not considering - e.g. would using something like
>>     pycparser be a lot easier - though I'd prefer to keep it in clang
>>     as we plan to use tidy & static analyzer in any case for standard QA.
>>     Thanks for reading,
>>     Billy.
>>     _______________________________________________
>>     cfe-dev mailing list
>>     cfe-dev at lists.llvm.org  <mailto:cfe-dev at lists.llvm.org>
>>     https://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev

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