[cfe-dev] Static Analyzer: pointer alias assignment
Gábor Kozár
kozargabor at gmail.com
Wed Jul 24 10:08:15 PDT 2013
Thanks for the info! I missed -analyze-function, because I grep-ed for
"analyzer" (-analyzer-checker, -analyzer-checker-help, etc. - I assumed
it's a general prefix).
> *This example definitely works for me:
*
Something weird is definitely going on here (see my last e-mail). I'll try
to investigate tomorrow.*
> We assume that you are developing against TOT unless stated otherwise. Is
there a reason why you don't?
*
What do you mean by "TOT"?
If you're asking why I'm not using the latest SVN version, it's because my
boss is concerned about stability. How stable is the current SVN build, in
general? Is it reliable, in terms of existing features and functionality?
Obviously I don't expect work-in-progress features to work, but it'd be
nice if they were disabled by default until they are stable - is this
currently done, or what is your policy in this?
2013/7/24 Anna Zaks <ganna at apple.com>
>
> 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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