[cfe-dev] clang-tidy or static analyzer or ...
Billy O'Mahony via cfe-dev
cfe-dev at lists.llvm.org
Tue Sep 10 14:47:06 PDT 2019
Hi Artem,
(or anyone else :D ).
I managed to create a dummy clang-tidy check and run it.
For the tests I want to write I've decided the easiest first step is to
just check for more than one return statement in my functions (this will
only be applying to specific functions all with a common prefix e.g
"api_foo".
So I managed to write a matcher which seems to work ok.
Finder->addMatcher(returnStmt().bind("x"), this);
And get it to print a warning for each rtn statement in MyCheck::check:
const auto *MatchedDecl = Result.Nodes.getNodeAs<ReturnStmt>("x");
diag(MatchedDecl->getReturnLoc(), "found rtn stmt");
So next I want to be able to find the name of the function that contains
the returnStatement but I can't figure out how to do this.
Or maybe I should start with a FunctionDecl matcher that matches fns named
(api_...) and then somehow traverse that function's AST and count the
returnStmt's. ?
I've found a few examples of clang-tidy checks on-line I think this one
https://bbannier.github.io/blog/2015/05/02/Writing-a-basic-clang-static-analysis-check.html
is the best. But recommendations for better resources also appreciated.
Thanks for any help,
Billy.
On Sat, 6 Jul 2019 at 13:41, Billy O'Mahony <billy.omahony at gmail.com> wrote:
> Hi Artem,
>
> super. Good to know I'm on the right path. And thanks for the extra info -
> plenty of clang goodness to be looking into there.
>
> Cheers,
> Billy.
>
> On Sat, 6 Jul 2019 at 04:05, Artem Dergachev <noqnoqneo at gmail.com> wrote:
>
>> > 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
>> behavior.
>>
>> 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> 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 listcfe-dev at lists.llvm.orghttps://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>
>>>
>>>
>>
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