[cfe-dev] [analyzer][RFC] The future of StdLibraryFunctionsChecker and other checkers responsibility

[Artem Dergachev via cfe-dev]

No matter how far do we want to push the flexibility of our summaries, 
this checker will always have limitations. I suggest we treat it as yet 
another method of interprocedural analysis, i.e. simply an improvement 
over conservative evaluation of the call.

I don't see a need for a specific one-size-fits-all policy here. We can 
decide every time on per-function basis. Doing evalCall in two different 
checkers is obviously bad (and we have an assertion that protects 
against that, so it won't be "anarchy" but merely a crash, at least in 
non-release builds but i'd expect such crash to be fairly apparent) but 
any other separation of concerns sounds reasonable to me.

Like, for some functions it makes sense to have 
StdLibraryFunctionsChecker do the evalCall and the ranges and let other 
checkers model various aspects of the call that are of interest to them 
in their PreCall/PostCall. Specific checkers can also do a better job at 
reporting bugs than the generic checker. For functions for which 
StdLibraryFunctionsChecker doesn't do evalCall this makes even more sense.

It probably doesn't make much sense to keep StdLibraryFunctionsChecker's 
pre/post-call modeling if we have a specific checker for that function.

I suspect that if you end up setting up dependencies, it's almost always 
better to move the functionality to one checker. But there's no need to 
merge different aspects of modeling that are completely orthogonal. Note 
that state splits are idempotent which means that it's often ok to 
duplicate the post-call state-splitting code in multiple checkers 
without making them aware of each other.

On 6/25/20 7:19 AM, Kristóf Umann via cfe-dev wrote:
>
> +Balázs Kéri
>
> On Thu, 25 Jun 2020 at 10:39, Gábor Márton <martongabesz at gmail.com 
> <mailto:martongabesz at gmail.com>> wrote:
>
>     One of my goals with StdLibraryFunctionsChecker is to add argument
>     constraints to functions in the C, C++ and POSIX standard. It is
>     very easy to gather these argument constraints and apply them
>     massively. By doing this, the analysis can be more precise, so, I
>     see a lot of gain here. However, some of these argument
>     constraints could be handled in more specific checkers too, e.g.
>     in CStringModeling, MallocChecker or in StreamChecker.
>
>
> For those that didn't follow development closely, argument constraints 
> in this context mean the following: For select functions, we define a 
> signature we can use to match a call to this function (for C 
> functions, that is return value, function name and argument types), 
> and a so called "summary", which is a list of constraints on the 
> arguments or the return value. For a function like fread(), the 
> signature is size_t fread(void *, size_t, size_t, FILE *), and the 
> summary is that the stream argument and the buffer must be non-null, 
> and that the buffer must be at least the size of (2nd arg * 3rd arg). 
> We can also constrain the return value, that is is at least (2nd arg * 
> 3rd arg).
>
> If on the function call any of the preconditions isn't met, we can 
> emit a warning, otherwise we can increase the precision of the 
> analysis by constraining the arguments.
>
> This is a form of summary based analysis, and its pretty easy to pull 
> off on standard functions because they have, well, standardized 
> signatures and constraints.
>
> (this isn't pulled from the checker, just a theoretical example)
>
>     Let's take for instance `popen`. In the mentioned patch I am
>     adding a simple summary with argument constraints. However, sooner
>     or later popen could be modeled in StreamChecker once that is
>     mature enough. (1) Should we remove the summary
>     from StdLibraryFunctionsChecker when that happens? (2) Or should
>     we have a declaration that ranges and nullability are checked with
>     summaries while more specific things are checked in their
>     respective checkers? (3) Or the specialized checkers could be
>     dependencies to the generic StdLibraryFunctionsChecker, so any bug
>     related to e.g. popen is prioritized to StreamChecker?
>
>
> Yes, this this the big one. What role should batch modeling checkers 
> play in the analyzer alongside specific modeling checkers? Lets take 
> an example that has already presents a problem, and the most glaring 
> of them, to me at least, is stream modeling functions.
>
> Take for instance the above mentioned fread. The *numerical* pre- and 
> postconditions can be enforced just as I detailed. However, the most 
> important effect of this function is of course the stream operation, 
> what happens depending on the validity of the stream object (which is 
> not a numerical property), and what happens to it after the call has 
> finished, successful or not. If the stream hits EOF during the read, 
> the return value is less then (2nd arg * 3rd arg), and is equal to it 
> if the read was successful. What this implies to me is that 
> StdLibraryFunctionChecker cannot really take on the responsibility of 
> establishing a post condition, **unless** the function is question can 
> be described by numerical properties only (like isalpha()).
>
> This is something that this checker already does well I think. Its 
> clear that we don't want to expand it to cover non-numeric cases. So, 
> onto the topic that brought us here.
>
> (2.) This sounds great. I think precondition checking is something 
> that introduces a lot of code duplication in checkers, and I don't 
> think that fread()ing a nullpointer is a stream mismanagement issue 
> necessarily. A notable challenge to overcome, however, is how can we 
> ask StreamChecker to add NoteTags from other checkers' reports (see: 
> https://reviews.llvm.org/D81407?id=270161#inline-750832).
>
> (1.) I think the only thing we could do with this idea is to implement 
> an incomplete post condition modeling until a checker comes along and 
> does it properly. However, even this would only make sense if we have 
> a single post-condition (https://reviews.llvm.org/D79432#inline-757474).
>
> (3.) If we go with option (2), we must introduce strong dependencies 
> so that numerical preconditions are checked beforehand. However, if we 
> *duplicate* the same checking in multiple checkers and want to use 
> dependencies to make the correct code runs first, we would introduce 
> deadcode, or possibly running the same check twice, and the problem of 
> inconsistently distributing the responsibility of analysis gets even 
> worse.
>
>     My other goal would be to make it possible to add argument
>     constraints (or branches/cases) to any library functions. These
>     libraries may never be modeled in the analyzer. In this case the
>     above problem is non-existent. Perhaps, we should divide the
>     checker to two different checkers once we reach that point,
>     shouldn't we? We are very close to
>     rename StdLibraryFunctionsChecker to LibraryFunctionsChecker
>     anyway by now.
>
>
> If we want to implement summary based analysis, it would only be 
> fitting to bear the name SpeculativeSummaryBasedFunctionModeling and 
> StdSummaryBasedFunctionModeling or something, right? ;) Anyway, this 
> is the least of the worries.
>
> I agree, pure functions (like the above mentioned isalpha()) is the 
> real learning opportunities for generalized summary based analysis for 
> the time being.
>
>     There is more. In `evalCall` if a checker returns true then that
>     function is no longer processed by any subsequent checkers. That
>     is the case with pure functions in StdLibraryFunctionsChecker. In
>     this case, however, it would be anarchy if another checker also
>     returns true from its own evalCall callback. Only one can rule. It
>     would be great if we could enforce that no two checkers evaluate
>     the same function this way. But I don't see any clear solution to
>     achieve that right now, this is really challenging.
>
>
> Yup, the problem is that checker callbacks from CheckerManager's 
> perspective (I might be wrong here!) takes a set of ingoing nodes and 
> returns a set of outgoing ones. So the solution might not be terribly 
> complicated, just ugly (large #ifs).
>
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