[cfe-dev] Static Analyzer: pointer alias assignment

[Anna Zaks]

On Jul 24, 2013, at 7:12 AM, Gábor Kozár <kozargabor at gmail.com> wrote:

> Anna,
> 
> > Can this be designed as an extension of the Dereference checker?
> 
> That was my original thought, but looking at the Dereference checker source code, it writes that it's a built-in check in ExprEngine, which discouraged me to try to extend it.

I am not sure which code comment you are referring to but Dereference checker is a very good checker to learn from. I highly recommend to understand how it works before extending it.

> I also have no idea how it works or what it does - I've been unable to have it produce any warnings, except for this trivial case:
> 
> Foo* fp = nullptr;
> fp->baz();
> 
> And now I tried with your code example, and it also produces a warning. By the way, when analyzing a translation unit, which function(s) are treated as entry points?

AnalysisConsumer.cpp contains code that decides on the order of the functions being analyzed, which ones are analyzed as top level, etc.

> It appears to me that all of them are, but are there any restrictions, or ways to control this? I do not see any promising args with clang++ -cc1 --help.
> 
You might find this one helpful:
 clang -cc1 -help | grep "function"
  -analyze-function <value>
                          Run analysis on specific function
                          Specify the function selection heuristic used during inlining
                          Force the static analyzer to analyze functions defined in header files
                          Analyze the definitions of blocks in addition to functions
                          Maximum depth of recursive constexpr function calls
                          Parse templated function definitions at the end of the translation unit 
  -analyzer-display-progress
                          Emit verbose output about the analyzer's progress

Also, AnalyzerOptions.cpp is a good reference for analyzer options. 

> Anyway, this code snippet still does not cause the Dereference checker to raise a warning, even though it is trivial:
> 
> Foo* fp = getFooPtr(); // based on runtime data, either returns nullptr, or an allocated object
> if(!fp)
> {
>       fp->baz();
> }
> 

This example definitely works for me:

zaks$ cat ~/tmp/ex.cpp
struct Foo {
  void baz();
};
Foo* getFooPtr();
void f() {
  Foo* fp = getFooPtr(); // based on runtime data, either returns nullptr, or an allocated object
  if(!fp) {
      fp->baz();
  }
}

$ clang --analyze ~/tmp/ex.cpp
/Users/zaks/tmp/ex.cpp:8:7: warning: Called C++ object pointer is null
      fp->baz();
      ^~~~~~~~~
1 warning generated.

> To the best of my knowledge, the if(!bar) line creates two code paths: one on which the condition is true (i.e. fp == nullptr), and one on which the condition is false (i.e. fp != nullptr). Obviously the first statement on the former code path is the fp->baz() line, analyzing which the Static Analyzer tells me that 'fp' is constrained to be null (using ProgramState::isNull). Therefore, my checker can easily find this issue.
> 
> Browsing through the source code of the DereferenceChecker, I can tell it does not use ProgramState::isNull, but instead ProgramState::assume. However, as far as I can tell, it does exactly nothing - I can see that it's supposed to either raise a warning, or dispatch an ImplicitNullDerefEvent, but neither happens on the code example above (I tested the latter by running both checkers with -analyzer-checker, and in my checker, I subscribe to check::Event<ImplicitNullDerefEvent>).
> 
> So why is ProgramState::assume used instead of ProgramState::isNull?
> 

If you drill down the implementation of isNull, you'll see that it's similar to assume, possibly more efficient and specialized for NULL.

> > Also, the Dereference checker does work fine in presence of aliases... Is there a reason why it's different in your setting?
> 
> I'm using Clang 3.3, so unless significant changes have been made to the Static Analyzer or the Dereference checker, things should work the same. I have not modified the Clang source code, except for adding a bunch of custom checkers.
> 

We assume that you are developing against TOT unless stated otherwise. Is there a reason why you don't?

> Gabor
> 
> 
> 2013/7/23 Anna Zaks <ganna at apple.com>
> 
> On Jul 23, 2013, at 10:33 AM, Gábor Kozár <kozargabor at gmail.com> wrote:
> 
>> Hi Anna,
>> 
>> I'm building a checker that detects inconsistent pointer usages, for example when a pointer is dereferenced, then along the same path is null-checked (without its value changing in between, obviously).
> 
> Can this be designed as an extension of the Dereference checker? 
> 
> Also, the Dereference checker does work fine in presence of aliases... Is there a reason why it's different in your setting?
> 
> int foo(int *p) {
>   int *q = p;
>   if (q)
>     ;
>   return *p;  
> }
> 
> zaks$ clang --analyze ~/tmp/ex.c -Xclang -analyzer-output=text
> /Users/zaks/tmp/ex.c:5:10: warning: Dereference of null pointer (loaded from variable 'p')
>   return *p;  
>          ^~
> /Users/zaks/tmp/ex.c:3:7: note: Assuming 'q' is null
>   if (q)
>       ^
> /Users/zaks/tmp/ex.c:3:3: note: Taking false branch
>   if (q)
>   ^
> /Users/zaks/tmp/ex.c:5:10: note: Dereference of null pointer (loaded from variable 'p')
>   return *p;  
>          ^
> 1 warning generated.
> 
> 
>> Code example:
>> 
>> Foo* f = getFoo();
>> f->bar();
>> 
>> if(f) // warn
>> { ... }
>> 
>> I want to be able to do this with aliases as well, for example:
>> 
>> Foo* f = getFoo();
>> f->bar();
>> 
>> Foo* g = f;
>> if(g) // warn
>> { ... }
>> 
>> What I need is to be able to get the SVals representing 'f' and 'g' when checkBind is called on the Foo* g = f; line. Currently, instead of 'f', Clang gives me the value that was bound to 'f'.
>> 
>> Thanks for your help!
>> 
>> 
>> 2013/7/23 Anna Zaks <ganna at apple.com>
>> 
>> On Jul 23, 2013, at 9:21 AM, Gábor Kozár <kozargabor at gmail.com> wrote:
>> 
>>> During the analysis of a test code, the following two bindings happen (checkBind), with their respective source lines:
>>> 
>>> (Bind: location <= value)
>>> 
>>> Bind: &fp <= &SymRegion{conj_$4{struct Foo *}}
>>> Code: Foo* fp = getFooPtr();
>>> 
>>> Bind: &ap <= &SymRegion{conj_$4{struct Foo *}}
>>> Code: Foo* ap = fp;
>>> 
>>> In the second line, I need to detect that 'ap' is in fact the alias of 'fp'. Unfortunately, I cannot seem to find any way to get Clang SA to tell me that "&SymRegion{conj_$4{struct Foo *}}" is stored in "fp", which seems weird, because the source code is very clear.
>> 
>> As you observe the two binds you see that the same value is stored in both.
>> 
>> The analyzer does not perform alias analyzes as in it does not build sets of aliases. As it models the execution in presence of aliases, we did not find a need for the alias sets. Can you give a bit more background on why you need this info? Maybe your goal can be achieved differently?
>> 
>>> 
>>> Some of the information I extracted, but is not really useful to me:
>>>  - original SVal: &SymRegion{conj_$4{struct Foo *}}
>>>  - getAsRegion(): SymRegion{conj_$4{struct Foo *}}
>>>  - state->getSVal(): &SymRegion{reg_$6<element{SymRegion{conj_$4{struct Foo *}},0 S32b,struct Foo *}>} -- in fact, I have no idea what this is
>>>  - getAsSymbol(): conj_$4{struct Foo *}
>>> 
>>> As a workaround, I can keep track of this information myself, but there must be a built-in way to do this.
>>> Any help would be appreciated. Many thanks!
>>> _______________________________________________
>>> cfe-dev mailing list
>>> cfe-dev at cs.uiuc.edu
>>> http://lists.cs.uiuc.edu/mailman/listinfo/cfe-dev

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