[cfe-dev] [analyzer] Proof-of-concept for Matcher-like checker API (graph matchers)
Alexey Sidorin via cfe-dev
cfe-dev at lists.llvm.org
Tue May 22 16:37:43 PDT 2018
22.05.2018 04:35, George Karpenkov пишет:
> Hi Alexey,
>
> As classics say, "Any sufficiently complicated C or Fortran program
> contains an ad-hoc, informally-specified, bug-ridden, slow
> implementation of half of Common Lisp.”
Yes, https://xkcd.com/297/ is what I was always thinking while writing
new AST matchers. Time-honored tradition :)
>
> Jokes aside, I think the concept and what you are doing is great,
> and we could certainly benefit from declarative matchers.
> However, I think the actual implementation and the set of matchers and
> predicates would require quite a bit of bikeshedding.
Sure. If we need this stuff - the design details are subject for
discussion and change.
>
> Are you familiar with similar works in this area?
> E.g. Oracle has a Soufflé project doing a similar task:
> http://souffle-lang.org.
> They have found achieving high performance very challenging, and IIRC
> they resort to generating C++ code from the declaratively described
> matcher.
My goal was not to achieve extreme performance but not to hurt
analyzer's performance too much. Don't know how far I am from this
target, unfortunately.
> Maybe we would have to do the same in tablegen spirit.
>
> I’m sure there are more such works in the literature.
Yes, I know about some researches in this area, but thank you anyway!
>
>> On May 16, 2018, at 4:37 PM, Alexey Sidorin via cfe-dev
>> <cfe-dev at lists.llvm.org <mailto:cfe-dev at lists.llvm.org>> 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.
>
> I agree that this is a worthy goal.
>
>>
>> --------- 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);
>
> Bikeshedding: I think it’s much better for the sequence matcher to run
> from start to end,
> since that’s how we think about the execution of a program.
Not sure that I understand completely what do you mean. The nodes are
written in the same sequence as they should be on the path in ExplodedGraph.
>
>>
>> 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.
>>
>> 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.
>>
>> 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.
>
> Moreover, new matchers could be written without modifying clang. I
> wonder if we should support some kind of a plugin infrastructure which
> support matchers
> defined in a text file, e.g. something along the lines of:
>
> clang -cc1 —analyze -analyzer-checker=core,mymatcher
> -analyzer-load-declarative-checker=mymatcher.txt
I planned to reuse dynamic matchers infrastructure and clang-query for
this. clang-query is a command-line tool but there shouldn't be too much
difference where the matcher string we parse comes from. The problem
here is that, AFAIR, using callbacks with dynamic matchers is
impossible. Still it is a very cool tool for quick matcher prototyping.
>
>>
>> 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.
>> 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("/"))))).
>>
>> 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.
>>
>> 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.
>
> Actually I would disagree with this one: I think it would be better to
> support that, since many errors are considered fatal.
> (but that of course would require running the checkers on a stream of
> events, rather than on the already constructed graph)
As I answered to Artem, this is technically possible (but still
undesirable, I think) because matchers fire their given callbacks on
match, and callbacks are much less limited in their possibilities.
>
>> 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.
>
> I would guess it would affect it hugely, and getting the performance
> right would be the biggest challenge for declarative matchers.
That's sad. I'll try to measure the overhead but I'm not sure I'll be
able to do it soon.
>
>> 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.
>>
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