[cfe-dev] [analyzer] Proof-of-concept for Matcher-like checker API (graph matchers)

[Gábor Horváth via cfe-dev]

Hi Alexey,

In general, I like the idea of having a more declarative way to define new
checks.

On Thu, 17 May 2018 at 01:37, Alexey Sidorin <alexey.v.sidorin at ya.ru> wrote:

> Hello everyone,
>
> I'd like to share some results of my investigation targeted on
> improvement of Clang Static Analyzer checker API. You can find some
> previous conversation on this topic here:
>
> http://clang-developers.42468.n3.nabble.com/analyzer-RFC-Design-idea-separate-modelling-from-checking-td4059122.html.
>
> In my investigation, I tried to solve a particular problem of building a
> checker without generating new nodes.
>

Why do you focus on such checks that does not have traits, does not
generate new nodes.
Do you find this to be the majority of the checks you need to implement?


>
> --------- Introduction and design goals ---------
>
> In brief, I tried to use matchers-like API to make CSA checkers look
> like this:
>
> StatementMatcher NotChdir =
>      callExpr(unless(callee(functionDecl(hasName("::chdir")))));
> Finder.addMatcher(
>      hasSequence(
>          postStmt(hasStatement(
>              callExpr(callee(functionDecl(hasName("::chroot")))))),
>          unless(stmtPoint(hasStatement(callExpr(
>              callee(functionDecl(hasName("::chdir"))),
>              hasArgument(0, hasValue(stringRegion(refersString("/")))))))),
>          explodedNode(anyOf(postStmt(hasStatement(NotChdir)),
>                              callEnter(hasCallExpr(NotChdir))))
>              .bind("bug_node")),
>      &Callback);
> Finder.match(G);
>

I think, a common (design) pitfall of writing checks is to try to match
against
the AST when a symbolic execution callback should be used instead.
I am a bit afraid having an API like this would make this pitfall more
common.
Maybe a separation between the path sensitive, flow sensitive and
AST based checks is good for the checker writers and new users and
I am not sure that the same abstraction fits all of these cases.

In case of path sensitive checks I wonder if a state machine like
abstraction would
fit the use cases better (and also cover checks that are using traits)
where the guarding conditions of the state transitions can include AST
matcher like checks.


>
> and I have managed to make some simple working examples.
>
> The entire diff can be found here:
>
> https://github.com/a-sid/clang/commit/9a0b1d1d9b3cf41b551a663f041f54d5427aa72f
> The code itself: https://github.com/a-sid/clang/tree/a4
>
> There are several reasons why I have tried this approach.
>
> 1. AST Matchers are already extensively used API for AST checking. They
> are available both in Clang-Tidy and CSA. And I wanted to use existing
> functionality as much as possible. So, I decided to extend an existing
> API to make its usage seamless across different kinds of checks:
> path-sensitive, AST-based and CFG-based.
>

I am not sure if this is a good idea. I think the checker author supposed to
be aware if the check is AST based, flow sensitive, or path sensitive.
And this should also be easy to tell by reading the code. I am also not
sure whether there is an abstraction that fits all.

I think the reason why this idea works well for checks that only inspect the
exploded graph, because in this case we are still doing pattern matching on
graphs in a similar manner as doing pattern matching on the AST.

But does this generalize later on to stateful checks?


>
> 2. AST matchers effectively help clients to avoid a lot of checking of
> dyn_cast results. This feature not only makes them more convenient but
> also more safe because, in my experience, forgetting a nullptr/None
> check is a pretty common mistake for checker writers.
>

I think if something cannot be null we should return references, otherwise
the client need to check for the pointer beeing null (unless there are some
dynamic invariant that would make the check redundant). If an API does
not match this philosophy, maybe we should fix the API first.

Regardless of fixing the API, I agree that it would be great to have higher
level APIs for checker authors.


>
> 3. Testing of AST matchers don't require writing C++ code - it can be
> done interactively with clang-query tool. And I believe that we need
> similar functionality for CSA as well.
>

Having a query tool for exploded graphs could be very helpful, I agree.
These graphs tend to be very large and sometimes it is not trivial to
find the relevant parts during debugging.


>
> I didn't want to replace existing checker API. Instead, I tried to make
> new possibilities usable independently or together with existing.
>
> --------- Design and implementation ---------
>
> The implementation consisted of a number of steps.
>
> 1. Allow matching nodes of path-sensitive graph like usual AST nodes. To
> make this possible, three actions were needed:
>   1.1. ASTTypeTraits and DynTypedNode were extended to support
> path-sensitive nodes: ExplodedNode, ProgramState, SVal, SymExpr, etc.
>

I wonder, if in general it is a good idea to pattern match in checks on
SymExpr.
A more general approach here would be to use the constraint solver for
queries
in a similar manner how ProgramState::assume works.


> The implementation with graph node support is moved into a separate
> class (ASTGraphTypeTraits) in ento namespace to resolve cyclic
> dependencies (they are still exist, unfortunately, but are header-only,
> so we can build the PoC).
>   1.2. Some additions to AST matchers were made to add support for new
> kinds of nodes.
>   1.3. To make MatchFinder able to query specific options not supported
> by pure AST, it was augmented with "Contexts". A matcher that needs to
> query the path-sensitive engine asks the Finder for the required Context
> which provides specific helper functions.
>
> As the result of this step, we are able to write matchers like
> expr(hasArgument(0, hasValue(stringRegion(refersString("/"))))).
>

I think this is the part of the proposal that I like the most, it would be
a very concise way to write down guarding conditions.


>
> 2. Create an engine for graph exploration and matching.
>    Unlike normal AST matchers, hasSequence is a path-sensitive matcher.
> It is launched against ExplodedGraph. These matchers are handled by
> GraphMatchFinder object. While searching a graph, it collects matches.
> Each match contains a pointer to the corresponding matcher and State ID
> of this match. The way how matches are translated from one state ID to
> another is determined by matcher operators.
>

Is this matching done after the exploded graph already built? If so,
these checks will be unable to generate sink nodes. I think having sink
nodes sometimes desirable even if the check itself does not have a trait.


>
>    For example, SequenceVariadicOperator, which is the base of
> hasSequence() matcher, has "positive" and "negative" sub-matches. Each
> positive matcher has its corresponding State ID. In order to advance to
> the next State ID, a node being matched should match all "negative"
> matchers before the next "positive" matchers and the next "positive"
> matcher itself. Failure in matching "negative" matcher discards the match.
>
>    The role of GraphMatchFinder is similar to MatchFinder: it is only
> responsible for graph exploration and keeping bound nodes and matchers.
>
> 3. Manage bound nodes.
>    While matching a single graph node, BoundNodes from AST MatchFinder
> are used. AST matchers for path-sensitive nodes support bindings
> out-of-the-box. However, the situation with graph matching is a bit
> different. For graph matching, we have another system of bound nodes.
> Each graph node has a related map of bounds aka GDMTy (yes, the name is
> not a coincidence). GDMTy is a mapping from match ID to BoundNodesMap
> which, in part, is effectively a map from std::string to DynTypedNodes.
> This look pretty much like how GDM is organized in ExplodedGraph, just
> without one level of indirection (it can be added, though).
>
>    MatchFinder contexts should allow support of their own bindings. This
> is how equalsBoundNode() matcher works for path-sensitive nodes: it just
> queries all available contexts for the binding.
>
> Finally, I have managed to make two checkers work in this way:
> ChrootChecker (which is present in the introduction) and
> TestAfterDivZeroChecker. Both them can be found in ChrootCheckerV2.cpp
> and TestAfterDivZeroCheckerV2.cpp correspondingly. They act like normal
> checkers: produce warnings and use visitors. The main difference is that
> they cannot generate sinks and use checkEndAnalysis callback. The code
> of these checkers can be found here:
>
>
> https://github.com/a-sid/clang/blob/a4/lib/StaticAnalyzer/Checkers/ChrootCheckerV2.cpp
>
> https://github.com/a-sid/clang/blob/a4/lib/StaticAnalyzer/Checkers/TestAfterDivZeroCheckerV2.cpp
>
>
> -------- Some features --------
>
> 1.The design of bindings has an interesting consequence: it enables the
> dynamic introspection of GDM which was pretty hard before. (Hello alpha
> renaming?)
> 2. Nothing prevents matchers to be used with existing checker API for
> simplifying conditional checking inside callbacks. The matchers are not
> planned as the replacement for the current API, but look like a nice
> complementary part.
> 3. Implicit conversion of Matcher<ProgramPoint> to Matcher<ExplodedNode>
> and Matcher<SymExpr || MemRegion> to Matcher<SVal> for writing shorter
> code.
>
> -------- Not implemented/not checked yet --------
> I tried to keep the PoC as minimal as possible. As the result, some
> features are not implemented yet, but I believe they can be added.
> 1. Usage of matchers inside checker callbacks
>    This is not exactly unimplemented, but is still untested.
> 2. Dynamic matchers (clang-query interface)
>    In addition to work for supporting dynamic nodes (I don't know how
> many efforts it can take), we need to enable matching against AST nodes,
> not graph. But it doesn't look as a problem, we can just make matchers
> polymorphic.
> 3. Binding to successfully matched paths is not implemented yet - only
> bindings to separate nodes. I wonder if we need path bindings at all.
> 4. Some corner cases are still FIXMEs: single-node sequences, for example.
> 5. GDM is not based on immutable data structures like it is done in CSA
> - it is just an STL map. Its performance can be poor (full copy on every
> new node), but I don't think that changing it to use immutable
> structures is hard.
> 6. Matching on-the-fly
>    GraphMatchFinder should support on-the-fly matching during
> ExplodedGraph building. For this support, we should just call its
> advance() method each time a new node is created. However, this
> possibility wasn't checked yet.
> 7. Matching CFG and CallGraph isn't implemented because it was
> considered to be far out of simple PoC.
> 8. Only sequential matching is available now, and I didn't try to
> implement other operators yet. So, implementing a checker similar to
> PthreadLock can be tricky or even impossible for now.
>
> -------- Known and potential issues --------
>  From matchers' side:
> 1. Some tests don't pass because they rely on the checker ability to
> generate sink nodes. Our matchers cannot do it by design so tests don't
> pass.
> 2. Representing checker bindings as a map can be less effective than
> storing data in structures. I didn't measure the overhead, so I cannot
> give any numbers.
> 3. Matchers are called on every node independently of its type. This is
> not what CheckerManager does. I wonder if this detail can affect
> performance as well.
> 4. Problems with cyclic dependencies. clangStaticAnalyzer has a
> dependency on clangASTMatchers, therefore, clangASTMatchers cannot
> depend on clangStaticAnalyzer. However, if we want ASTMatchers to
> support static analyzer data structures, there should be a backward
> dependency. Now this dependency is header-only.
> 5. Code duplication. This is mostly fine for a sandbox but needs to be
> reduced.
>
>  From analyzer's side:
> 1. Many events are not reflected in the final ExplodedGraph. For
> example, checkers can receive PointerEscape event, but the event itself
> will not be recorded in the ExplodedGraph - only changes made by
> checkers will. That's also true for Stmt nodes: I noticed same issues
> with PostCondition. This makes matching a bit harder. Should we add some
> information into ExplodedGraph?
> 2. (Minor) Some static analyzer data structures don't support
> traditional LLVM casting methods (dyn_cast, isa) because they lack
> classof() method. I have fixed this internally and will put a patch on
> review soon.
>
> -------- Conclusion --------
> Finally, there is a graph matching engine supporting basic functionality
> and two checkers using it. I apologize for not starting the discussion
> earlier, but I just wasn't sure that the approach will work. Is anybody
> interested to have this stuff in upstream (maybe, changed or improved)?
> If yes, I'll be happy to contribute this work patch-by-patch and
> continue its improvement. If no - well, I had enough fun playing with it.
>

Thanks for sharing this results. Regardless of being upstreamed as is or
in a separate form or not at all, this is an interesting experiment for the
community.
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