[cfe-dev] [analyzer][GSoC] Problem Statement: Improving BugReporter with static backward program slicing

Artem Dergachev via cfe-dev cfe-dev at lists.llvm.org
Wed Apr 3 14:16:23 PDT 2019

The thing in NoStoreFuncVisitor for IVars looks like it's just saying 
"let's see if there's a data dependency on at least one statement in 
this call". Given that the statement found by such AST matcher was not 
executed on the bug's execution path, there must be at least one control 
flow dependency in this function as well.

I think this pattern-matching may only have false negatives, i.e. it's 
possible to write into a variable/field/ivar without producing an 
assignement-operator to a DeclRefExpr for that variable/field/ivar, but 
it's not possible to write an assignment into a DeclRefExpr for that 
variable/field/ivar without modifying the variable/field/ivar in the 
process. A more sophisticated analysis may find situations when such 
write is always done via an aliasing pointer, but i cannot think of 
anything else. Well, there's also the self-assignment cornercase, i.e. 
`x = x`, but that's esoteric.


Ok, i think i understand: one of the great goals for this project would 
be to correctly highlight control flow dependencies, which we currently 
don't do. I.e.:

*Example 4.*

01  int flag;
03  bool coin();
05  void foo() {
06    flag = coin();
07  }
09  void bar() {
10    int *x = 0;
11    // Set the flag to true.
12    flag = true;
13    foo();
14    if (flag) { // Taking false branch... wait, what?
15      x = new int;
16    }
17    foo();
18    if (flag) { // Now it's taking true branch again?!
19      *x = 1; // Null dereference.
20    }
21  }

Because we wouldn't highlight the data dependency on line 06 of the 
control flow dependency on lines 14 and 18, the positive may be 

This one's tricky to do with a visitor because it's hard to figure out 
that the true-branch of the if-statement on line 14 would have updated 
`x`, given that we didn't execute it.

(1) Again, we can do a syntactic match over the true-branch, like we did 
in NoStoreFuncVisitor, to see if there are any data depenencies within 
it (there are, line 15). Again, this may fail to cover the tricky cases 
(what if this branch instead calls a function that initializes `x`?).

(2) Instead we can also have a look at the execution path on the 
ExplodedGraph that goes through the true-branch and see if the value 
remains null when it exits the if-branch. Kristof, you're saying that 
you plan to do the slicing over the ExplodedGraph - is that what you 
mean? This could work as long as the other branch is actually *explored* 
by the analyzer. If it isn't, it'll be almost impossible to detect, and 
i don't know how often would this happen, but there's a good chance it's 
going to be much more rare than us having problems with such 
highlighting right now.

(3) We can also come up with a completely separate CFG-based analysis 
for this purpose. This is probably the most precise thing to do, because 
the problem is essentially an all-paths problem (which is why loss of 
coverage in the ExplodedGraph would bite us). I cannot estimate how 
difficult such analysis would be to implement (probably pretty scary), 
but i think Kristof wasn't looking in this direction.

Yay, i finally understand how to solve these problems we have!

I'd suggest first starting with a syntax-based approach (1) because it 
sounds to me that the approach that we chose for identifying 
dependencies on paths that aren't part of the bug path is orthogonal to 
actually making use of that information to produce notes. Once we learn 
how to make use of this information, we'll have a taste of this medicine 
and see if it works. Then we'll see if we need to transition to (2) or 
(3) depending on such evaluation.

What i just described sounds like a fairly realistic plan to me. Of 
course, the question still remains about how checker-specific would the 
analysis be. If i learned anything, it's "don't estimate the power of 
the esoteric checker contracts". But the idea that we can often get away 
with just tracking interesting regions and symbols and consuming 
trackExpressionValue() hints sounds like a good thing to try.

On 4/2/19 11:23 PM, Gábor Horváth wrote:
> Yeah, this particular example would be fixed, but we do not know how 
> noisy it would be in general. One way to make it a bit less noisy 
> could be something like what the NoStoreFuncVisitor already does for 
> IVars. We could also add some syntactic checks to see if the 
> method/function actually has the possibility to escape the region.
> I am not familiar with Obj-C, but I suspect it would be possible to 
> trick NoStoreFuncVisitor, or at least the syntactic checks in 
> `potentiallyWritesIntoIvar `.
> I think the main value of slicing here could be to get rid of those 
> fully syntactic heuristics and replace them by something smarter. Does 
> this make sense?
> On Wed, 3 Apr 2019 at 02:25, Artem Dergachev <noqnoqneo at gmail.com 
> <mailto:noqnoqneo at gmail.com>> wrote:
>     One more question about Example 1. Do you think this particular
>     example could be solved by developing some sort of
>     NoStoreFuncVisitor but for MallocChecker? I.e., in addition to
>     emitting notes like "returning without initializing variable `x`",
>     it could emit notes like "returning without escaping pointer `p`".
>     If such visitor is developed, what would be the differences
>     between your proposal and whatever behavior we'll get with such
>     visitor? In other words, is it possible to construct an example
>     with the "use of uninitialized variable" checker that, like
>     Example 1, has vital pieces of information missing, given that
>     this checker already uses NoStoreFuncVisitor?
>     I mean, it's most likely a terrible idea to develop such visitor
>     for MallocChecker, because the only reason this visitor works more
>     or less reliably is the existence of const-qualifiers. For
>     MallocChecker they don't exist, so it's going to be hard to figure
>     out if a function was *supposed* to escape the pointer.
>     On 4/2/19 5:14 PM, Artem Dergachev wrote:
>>     Ahaaaaa, ooooookkkkk, iiiiiiii seeeeeeeeee!
>>     Sounds fascinating, must-have and hard.
>>     My primary question is how much do you think this will put stress
>>     on the checkers. Like, how much more difficult would it be to
>>     write checkers when we require them to include the slicing
>>     criterion with their bug reports? How much of it would we (i.e.,
>>     BugReporter) be able to infer ourselves?
>>     ------------------------
>>     Say, let's take Example 1. You describe the slicing criterion as:
>>         (13, <dynamically allocated int object>)
>>     The value of the dynamically allocated into object remains the
>>     same regardless of whether the object is stored in global_ptr or
>>     not, so the slice doesn't need to include line 5 or 6. Therefore
>>     i think that the slicing criterion you proposed is not what we're
>>     looking for, and the real slicing criterion we're looking for is:
>>         (13, <liveness of <dynamically allocated int object>>)
>>     Like, we're mapping every dynamically allocated object to an
>>     imaginary heap location that represents its current liveness, and
>>     include that imaginary variable, rather than the object itself,
>>     in the slicing criterion. Line 6 would affect liveness of the
>>     object on line 13 (if it's stored into a global, it's alive;
>>     otherwise it's not), therefore it'll be part of the slice. So, am
>>     i understanding correctly that you're proposing a checker API
>>     that'll look like this:
>>       BugReport *R = new BugReport(Node, ...);
>>       R->addLivenessBasedSlicingCriterion(HeapSym);
>>     ?
>>     I guess we can infer the statement of the slicing criterion from
>>     the Node, but i'm not entirely sure; see also below.
>>     I'd actually love it if you elaborate this example further
>>     because it's fairly interesting. Like, we know that the
>>     assignment affects the liveness information, but how would the
>>     slicing algorithm figure this out? Do you have a step-by-step
>>     description of how the algorithm behaves in this case?
>>     ------------------------
>>     Let's take another example i just came up with. Consider
>>     alpha.unix.PthreadLock - a checker that finds various bugs with
>>     mutexes, such as double locks or double unlocks. Consider code:
>>     1  pthread_mutex_t mtx1, mtx2;
>>     2
>>     3  void foo(bool x1, bool x2) {
>>     4    if (x1)
>>     5      pthread_mutex_lock(&mtx1);
>>     6    if (x2)
>>     7      pthread_mutex_lock(&mtx1);
>>     8    // ...
>>     9  }
>>     In this example we'll report a double lock bug due to a
>>     copy-paste error: line 7 should probably lock &mtx2 rather than
>>     &mtx1.
>>     The whole program is relevant to the report. Without line 5,
>>     there would only be a single lock. It transitions the mutex
>>     object from state "unknown" to state "locked".
>>     I don't think the Analyzer would currently do a bad job at
>>     explaining the bug; it's pretty straightforward.
>>     What would be the slicing criterion in this case? As far as i
>>     understand, it'll be
>>         (7, <locked-ness of mtx1>)
>>     In this case "lockedness" is, again, an "imaginary heap location"
>>     that contains metadata for the mutex. More specifically, it is a
>>     GDM map value. How would a checker API look in this case? How
>>     would it even describe to the BugReporter what to look at, given
>>     that currently only the checker understands how to read its GDM
>>     values?
>>     My random guess is:
>>       BugReport *R = new BugReport(Node, ...);
>>     R->addGDMBasedSlicingCriterion<MutexState>(MutexMR);
>>     ------------------------
>>     So it sounds to me that your project is, like many other awesome
>>     projects, brings in, as a dependency, replacing GDM with a
>>     better, smarter data map that the analyzer core *can* introspect
>>     and manipulate without the direct help from the checkers. It
>>     might be that your project actually requires relatively little
>>     amounts of such introspection - i.e., you might be able to get
>>     away with "it's some GDM map and the value for this key has
>>     changed, therefore this statement is a relevant data dependency"
>>     for quite a lot of checkers.
>>     That's the subject i was also bringing up as part of
>>     https://reviews.llvm.org/D59861 - "this was the plan that is part
>>     of the even-more-long-term plan of completely deprecating the
>>     void*-based Generic Data Map...". This was also something that
>>     became a blocker on my old attempt to introduce summary-based
>>     analysis. It required stuffing fairly large chunks of code into
>>     every checker that would teach the checker how to collect summary
>>     information and apply it when the summary is applied, and this
>>     new boilerplate was fairly brain-damaging to implement and hard
>>     to get right or even to explain. This problem also blocks other
>>     projects, such as state merging / loop widening (on one branch
>>     the mutex is locked, on the other branch it is unlocked, hey
>>     checker please teach me how to merge this).
>>     The variety of slicing criteria (that is a consequence of the
>>     variety of checkers that we have) sounds pretty scary to me. Some
>>     of them are really complicated, eg. the liveness criterion.
>>     Making sure that the generic algorithm works correctly with at
>>     least a significant chunk of them is going to be fun. Making sure
>>     it can actually handle arbitrary criterions required by the
>>     checkers without having every checker come with its own
>>     boilerplate for slicing also sounds hard. And it'll be very bad
>>     if we have to tell our checker developers "hey, by the way, you
>>     also need to know what backward slicing is and write down part of
>>     the algorithm in order to ever get your checker enabled by
>>     default" - i'm already feeling pretty bad explaining dead symbols
>>     and pointer escapes (which are pretty much must-have in most
>>     checkers), these are definitely examples of a boilerplate that a
>>     smarter version of GDM could handle automatically. In order to
>>     conquer the world, i think we should stick to our "writing a
>>     checker in 24 hours" utopia: writing a checker should be as easy
>>     as writing down the transfer functions for relevant statements.
>>     In my opinion, we should take it as an important constraint.
>>     On 4/2/19 12:21 PM, Kristóf Umann wrote:
>>>     Somehow the images and the attached files were left out, please
>>>     find them here:
>>>     On Tue, 2 Apr 2019 at 21:16, Kristóf Umann
>>>     <dkszelethus at gmail.com <mailto:dkszelethus at gmail.com>> wrote:
>>>         Hi!
>>>         In this letter, I'd like to describe a particular problem in
>>>         the Clang StaticAnalyzer's BugReporter class that I'd like
>>>         to tackle in a Google Summer of Code project this summer.
>>>         I'll show real-world examples on some of its shortcomings,
>>>         and propose a potential solution using static backward
>>>         program slicing. At last, I'll ask some specific questions.
>>>         I'd love to hear any and all feedback on this!
>>>         This is a /problem statement, /not a /proposal/. I plan to
>>>         send out a formal proposal by Friday (but not later then
>>>         Saturday), that will contain more details on both the
>>>         problem and the solution. I don't introduce myself or my
>>>         previous works within the project, that will also be
>>>         detailed in my upcoming letter. I also plan to incorporate
>>>         the feedback I'll receive to this letter.
>>>         *---=== BugReporter constructs bad reports ===---*
>>>         *
>>>         *
>>>         *What does the BugReporter do?*
>>>         After the Static Analyzer found an error, the BugReporter
>>>         receives an ExplodedNode, which, accompanied by its
>>>         predecessors, contains all the information needed to
>>>         reproduce that error. This /bugpath /is then shortened with
>>>         a variety of heuristics, such as removing unrelated function
>>>         calls, unrelated branches and so on. BugReporter by the end
>>>         of this process will construct a PathDiagnostic object for
>>>         each report, that is, ideally, minimal.
>>>         *
>>>         *
>>>         *Example 1.*
>>>         Consider the following code example:
>>>         1  // leak.cpp
>>>         2  int *global_ptr;
>>>         3
>>>         4  void save_ext(int storeIt, int *ptr) {
>>>         5    if (storeIt)
>>>         6 global_ptr = ptr;
>>>         7  }
>>>         8
>>>         9  void test(int b) {
>>>         10   int *myptr = new int;
>>>         11   save_ext(b, myptr);
>>>         12   delete global_ptr;
>>>         13 }
>>>         It's clear that if test is invoked with b's value set to
>>>         true, there is no error in the program. However, should b be
>>>         false, we'll leak memory.
>>>         $ clang -cc1 -analyze -analyzer-checker=core,cplusplus leak.cpp
>>>         image.png
>>>         The Static Analyzer is able to catch this error, but fails
>>>         to mention the call to save_ext entirely, despite the error
>>>         only occurring because the analyzer assumed that storeIt is
>>>         false. I've also attached the exploded graph leak.svg that
>>>         demonstrates this.
>>>         *Example 2.*
>>>         Consider the following code example:
>>>         1  // divbyzero.cpp
>>>         2  void f() {
>>>         3    int i = 0;
>>>         4    (void) (10 / i);
>>>         5  }
>>>         6
>>>         7  void g() { f(); }
>>>         8  void h() { g(); }
>>>         9  void j() { h(); }
>>>         10 void k() { j(); }
>>>         Its clear that a call to f will result in a division by zero
>>>         error.
>>>         $ clang -cc1 -analyze -analyzer-checker=core divbyzero.cpp
>>>         image.png
>>>         Again, the Static Analyzer is plenty smart enough to catch
>>>         this, but the constructed bug report is littered with a lot
>>>         of useless information -- it would be enough to only show
>>>         the body of f, and, optionally where f itself was called.
>>>         For the sake of completeness, I've attached
>>>         divbyzero.svg that contains the exploded graph for the above
>>>         code snippet.
>>>         The above examples demonstrate that BugReporter sometimes
>>>         reduces the bugpath too much or too little.
>>>         *---=== Solving the above problem with static backward
>>>         program slicing ===---*
>>>         *What is static backward program slicing?*
>>>         A /program slice/ consists of the parts of a program that
>>>         (potentially) affect the values computed at some point of
>>>         interest, called the slicing criterion. /*Program
>>>         slicing*/ is a decomposition technique that elides program
>>>         components not relevant to the slicing criterion (which is a
>>>         pair of (statement, set of variables)), creating a program
>>>         slice[1][2]. *Static *slicing preserves the meaning of the
>>>         variable(s) in the slicing criterion for all possible
>>>         inputs[1]. *Backward *slices answer the question “what
>>>         program components might affect a selected computation?”[1]
>>>         While statement-minimal slices are not necessarily
>>>         unique[3], Weisel developed a popular algorithm that
>>>         constructs one. In essence, his fix-point algorithm
>>>         constructs sets of /relevant variables///for each edge in
>>>         between node /i/ and node /j/ in a CFG graph, from which he
>>>         constructs /relevant statements/. The fix-point of the
>>>         relevant statements set is the slice itself.
>>>         One of the characteristic of his algorithm is that the
>>>         resulting program slice will be executable. However, our
>>>         problem doesn't require the code to be executable, so we
>>>         could use a more "aggressive" approach that creates a
>>>         smaller slice. An improvement to his algorithm is presented
>>>         in [4].
>>>         *
>>>         *
>>>         *How does this relate to BugReporter?*
>>>         We can show that using Weiser's algorithm, issues raised in
>>>         Example 1. and Example 2. can be improved upon.
>>>         For example 1., the statement-minimal program slice with the
>>>         criterion (13, <dynamically allocated int object>) will
>>>         contain the statements 5 and 6, and for example 2., the
>>>         statement-minimal program slice with the criterion (4,i)
>>>         won't contain anything but statements 3 and 4. For the
>>>         latter, we can even improve the algorithm to also contain
>>>         statement 7, where a call to f is made.
>>>         The analyzer, as stated earlier, gives BugReporter an
>>>         ExplodedNode, from which the slicing criterion must be
>>>         constructed. The statement corresponding to this node,
>>>         coupled with the interesting regions the checker itself
>>>         marked could be used to construct this slicing criterion.
>>>         *Challenges*
>>>         While the algorithm Weiser developed along with the
>>>         improvements made by others are interprocedural, I would
>>>         imagine that in implementation, it would be a challenging
>>>         step from an intraprocedural prototype.
>>>         Several articles also describe pointers, references, and
>>>         dynamically allocated regions, as well as gotos and other
>>>         tricky parts of the language, but I still expect to see some
>>>         skeletons falling out of the closet when implementing this
>>>         for C++, not only C.
>>>         *Drawbacks*
>>>         Static slicing, as an algorithm that doesn't know anything
>>>         about input values, suffers from the same issues that all
>>>         static techniques do, meaning that without heuristics, it'll
>>>         have to do very rough guesses, possibly leaving a far too
>>>         big program slice. However, with the symbolic values the
>>>         analyzer holds, this could be improved, turning this into
>>>         /conditioned slicing, /as described in [1]. This however is
>>>         only planned as a followup work after GSoC.
>>>         For this reason, this project would be developed as an
>>>         alternative approach to some of the techniques used in
>>>         BugReporter, as an optional off-by-default analyzer feature.
>>>         *---=== Questions ===---*
>>>         What do you think of this approach?
>>>         Do you think that implementing this algorithm is achievable,
>>>         but tough enough task for GSoC?
>>>         Would you prefer to see a general program slicing library,
>>>         or an analyzer-specific implementation? Traversing the
>>>         ExplodedGraph would be far easier in terms of what I want to
>>>         achieve, but a more general approach that traverses the CFG
>>>         (like llvm::DominatorTree[5]) could be beneficial to more
>>>         developers, but possibly at the cost of not being able to
>>>         improve the prototype with the symbolic value information
>>>         the analyzer holds.
>>>         */References, links/*
>>>         [1] Gallagher, Keith, and David Binkley. "Program slicing."
>>>         /2008 Frontiers of Software Maintenance/.
>>>         [2] Tip, Frank. /A survey of program slicing techniques/.
>>>         Centrum voor Wiskunde en Informatica, 1994.
>>>         [3] Weiser, Mark. "Program slicing." /Proceedings of the 5th
>>>         international conference on Software engineering/. IEEE
>>>         Press, 1981.
>>>         [4] Binkley, David. "Precise executable interprocedural
>>>         slices." /ACM Letters on Programming Languages and Systems
>>>         (LOPLAS)/ 2.1-4 (1993): 31-45.
>>>         [5] http://llvm.org/doxygen/classllvm_1_1DominatorTree.html
>>>         Link to previous GSoC related letter I sent:
>>>         http://lists.llvm.org/pipermail/cfe-dev/2019-February/061464.html
>>>         Cheers,
>>>         Kristóf Umann

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