[cfe-dev] [analyzer] Proof-of-concept for Matcher-like checker API (graph matchers)
Alexey Sidorin via cfe-dev
cfe-dev at lists.llvm.org
Thu May 17 16:11:02 PDT 2018
Hi Gabor,
Thank you for the reply! You can find my answers inline.
17.05.2018 15:28, Gábor Horváth пишет:
> 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
> <mailto: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?
Allowing checkers to generate new nodes and sinks leads to coverage
issues: the coverage changes if we disable or enable checkers. The
ability to modify state (Environment and Store) also makes us think
about how checkers will interact with each other.
Regarding traits - path matchers are not completely trait-less. They
just use other GDM and can only store DynTypedNodes (extended by
path-sensitive nodes). The bindings that matchers have added while
matching separate graph nodes are propagated across ExplodedGraph.
Actually, path bindings act very similar to checker traits, and they
offer a subset of GDM abilities.
>
> --------- 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.
In my experience, nearly 2/3 path-sensitive checkers we wrote follow
state machine abstraction. But it is possible to implement other matcher
operator that will use another matching strategy (grammar-like, for
example), and declare a path matcher based on it.
>
> 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.
Yes, there is never ideal solution. But we can just use descriptive
matcher names to notify users that they are going to use path-sensitive
analysis. Even in this small PoC, path matchers are placed into a
separate header and into a separate namespace (and are even part of
clangStaticAnalyzer, not clangASTMatchers), so one should really
understand what he does to make path matchers work.
Also, we can often meet some hand-written AST matching inside checker
callbacks. We can just make it more consise.
>
> 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?
I'm not sure I understood you correctly. If you mean that stateful
checkers are checkers that need to modify state, then I think we need to
put state modification out of checkers if possible to get reproducible
coverage independently on checkers enabled or disabled.
>
> 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.
It is not only possible, but is already implemented - partially, of
course. You can take a look at canBeZero() matcher in TestAfterDivZeroV2.
> 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.
In the implemented checkers - yes, the matching is done on the
ExplodedGraph already built. But nothing prevents GraphMatchFinder to be
used as a part of CheckerManager or ExprEngine. It has a method called
advance() that performs a single transition to the new node not
processed before, so it can be fed with single nodes that engine creates
during graph building. The matching on-the-fly is possible, just was not
implemented.
>
> 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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