[cfe-dev] Prevent RetainCountChecker from Analyzing Function Bodies when They Have Certain Annotate Attribute
Malhar Thakkar via cfe-dev
cfe-dev at lists.llvm.org
Mon Jul 10 06:26:25 PDT 2017
Dear Dr. Alexandre,
The leak warning is raised by MallocChecker and not the RetainCountChecker
(which performs reference counting).
Adding 'rc_ownership_trusted_implementation' annotation to 'bar' and
returning true (successfully evaluating 'bar') from evalCall() callback in
RetainCountChecker prevents the analyzer from analyzing bar's body. This
causes problems for the MallocChecker as it is unable to find a call to
'free' for the call to 'malloc' in foo().
Using evalCall(), we'll be able to suppress leak false positives raised in
ISL due to obj_free(), obj_cow() and obj_copy(). Now, we want our solution
(to suppress such false positives) to be as generalized as possible. By
generalized, I mean using the same "trusted" annotation across different
codebases.
However, this approach (using evalCall()) can't be generalized for other
codebases in C as it's possible for some codebase to have code similar to
the one mentioned in my previous email. Such a test-case will result in the
MallocChecker raising false positives.
ᐧ
On Mon, Jul 10, 2017 at 6:43 PM, Alexandre Isoard <
alexandre.isoard at gmail.com> wrote:
> Hi Malhar,
>
> I think the idea of trusted implementation is to trust blindly that the
> implementation respects the release/acquire annotation without inlining nor
> looking inside the function. That means you need those functions to be
> properly annotated.
>
> Your bar function do not have any annotations to "trust". Hence the
> problem.
>
> On Mon, Jul 10, 2017, 11:57 Malhar Thakkar via cfe-dev <
> cfe-dev at lists.llvm.org> wrote:
>
>> I've been thinking a lot lately and I feel evalCall-ing is not the right
>> way to proceed (even though it works as desired) as I feel I won't be able
>> to generalize it to work for different kinds of codebases written in C. For
>> example, it won't work for the following case (as you pointed out before).
>>
>> *// evalCall-ing based on annotation*
>>
>> #include <stdlib.h>
>> typedef struct
>> {
>> int ref_count;
>> } rc_struct;
>> void free(void *);
>>
>> __attribute__((annotate("rc_ownership_trusted_implementation")))
>> rc_struct *bar(rc_struct *r) {
>> if (!r)
>> return NULL;
>>
>> if (--r->ref_count > 0)
>> return NULL;
>>
>> free(r);
>> return NULL;
>> }
>>
>> void foo() {
>> rc_struct *r = (rc_struct *)malloc(sizeof(rc_struct));
>> bar(r);
>> } *// Leak warning raised for 'r'*
>>
>>
>> Is it possible to check if the RetainCountChecker entered a function
>> containing a certain annotation by walking through the diagnostic path just
>> before emitting a bug report?
>>
>>
>> On Fri, Jul 7, 2017 at 6:33 PM, Malhar Thakkar <cs13b1031 at iith.ac.in>
>> wrote:
>>
>>>
>>>
>>> On Fri, Jul 7, 2017 at 3:37 PM, Artem Dergachev <noqnoqneo at gmail.com>
>>> wrote:
>>>
>>>> For now RetainCountChecker avoids inlining CoreFoundation's CFRetain()
>>>> by looking at its annotation and performing evalCall().
>>>
>>> Oh, I see.
>>>
>>>> evalCall is the checker callback that allows the checker to completely
>>>> model the function's effects; if the function is evalCall-ed by the
>>>> checker, the analyzer core doesn't try to inline it. I think this is
>>>> exactly what you need to do: the function you're modeling is *the* release,
>>>> and the analyzer doesn't need to know anything about how it is implemented.
>>>>
>>> Yes, I think, this is exactly what I need but I just read that only one
>>> checker can evaluate a call at a given instance and that, as you mentioned
>>> with your example containing MallocChecker, can cause some problems. I just
>>> tried evalCall-ing functions which contain a certain annotation and it
>>> seems to be working on the test-cases which I wrote. But, would that
>>> solution be acceptable to the community(i.e., commit-worthy)? If not, I'll
>>> have to think of something else.
>>>
>>>>
>>>> However, by evalCall-ing based on annotation, you essentially say that
>>>> all functions that wear this annotation are to be modeled similarly. If
>>>> some of them have additional side effects, you may fail to model them. For
>>>> example, if the analyzer has seen the malloc() that corresponds to the
>>>> free() in isl_basic_map_free(), it'd make MallocChecker unhappy - unlike
>>>> the case when you neither evalCall nor inline but evaluate conservatively,
>>>> causing the malloc-ed pointer to "escape" into this function.
>>>
>>> What exactly do you mean by evaluating conservatively?
>>
>>> If this becomes a problem, you may need to look into our body farms -
>>>> the mechanism that synthesizes ASTs of various functions for analysis
>>>> purposes. I imagine farming a function that only does releases, and then
>>>> farming more functions that call this function and do additional side
>>>> effects.
>>>
>>> How would farming a function work exactly? Also, the ISL codebase
>> contains a lot of different functions (one for each ISL data type) which
>> perform releases. So, farming functions for each of them doesn't seem like
>> a good option.
>>
>>>
>>>>
>>>> On 7/7/17 10:23 AM, Malhar Thakkar via cfe-dev wrote:
>>>>
>>>>> Dear all,
>>>>>
>>>>> I wish to prevent the RetainCountChecker from analyzing function
>>>>> bodies if these functions have certain annotate attributes. Consider the
>>>>> following example to get a better idea of why I wish to do that.
>>>>>
>>>>> Below is a small snippet from the Integer Set Library (ISL).
>>>>>
>>>>> typedef struct {
>>>>> int ref;
>>>>> } isl_basic_map;
>>>>>
>>>>> __attribute__((cf_returns_retained))
>>>>> isl_basic_map *isl_basic_map_copy(isl_basic_map *bmap);
>>>>>
>>>>> __attribute__((cf_returns_retained))
>>>>> isl_basic_map *isl_basic_map_cow
>>>>> (__attribute__((cf_consumed)) isl_basic_map *bmap);
>>>>>
>>>>> void free(void *);
>>>>>
>>>>> __attribute__((annotate("rc_ownership_trusted_implementation")))
>>>>> isl_basic_map *isl_basic_map_free
>>>>> (__attribute__((cf_consumed)) isl_basic_map *bmap) {
>>>>> if (!bmap)
>>>>> return NULL;
>>>>>
>>>>> if (--bmap->ref > 0)
>>>>> return NULL;
>>>>>
>>>>> free(bmap);
>>>>> return NULL;
>>>>> }
>>>>>
>>>>> __attribute__((cf_returns_retained))
>>>>> isl_basic_map *foo
>>>>> (__attribute__((cf_consumed)) isl_basic_map *bmap) {
>>>>> // *After this call, 'temp' has a +1 reference count.*
>>>>> isl_basic_map *temp = isl_basic_map_copy(bmap);
>>>>> // *After this call, 'bmap' has a +1 reference count.*
>>>>> bmap = isl_basic_map_cow(bmap);
>>>>> // *After this call, assuming the predicate of the second if branch to
>>>>> be true, 'bmap' has a +1 reference count.*
>>>>> isl_basic_map_free(bmap);
>>>>> return temp; *// Object leaked: 'bmap'*
>>>>> }
>>>>>
>>>>> While running the RetainCountChecker on the above example, it raises a
>>>>> leak warning for 'bmap' in function 'foo'. This warning is a true positive
>>>>> from the checker's perspective in the sense that the reference count of
>>>>> 'bmap' obtained from 'isl_basic_map_cow' is not decremented(from the
>>>>> checker's perspective) in 'isl_basic_map_free' even though it takes the
>>>>> argument 'bmap' as '__attribute__((cf_consumed))'.
>>>>>
>>>>> Actually, '--bmap->ref' does decrement the reference count (from ISL's
>>>>> perspective). Hence, to prevent such false positives (from ISL's
>>>>> perspective) to be raised, I wish to prevent the RetainCountChecker to
>>>>> analyze the bodies of the functions having 'rc_ownership_trusted_implementation'
>>>>> annotate attribute. I want the checker to just look at the declaration of
>>>>> such functions (and not go inside their bodies) to get the necessary
>>>>> information about reference counting.
>>>>>
>>>>> Could someone suggest me a way to achieve my objective?
>>>>>
>>>>>
>>>>> Thank you.
>>>>>
>>>>>
>>>>> Regards,
>>>>> Malhar Thakkar
>>>>> ᐧ
>>>>>
>>>>>
>>>>> _______________________________________________
>>>>> cfe-dev mailing list
>>>>> cfe-dev at lists.llvm.org
>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>>
>>>>
>>>>
>>> ᐧ
>>>
>> ᐧ
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>>
>
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