[cfe-dev] [RFC] Adding lifetime analysis to clang

Gábor Horváth via cfe-dev cfe-dev at lists.llvm.org
Fri Mar 1 03:24:03 PST 2019 

On Fri, 1 Mar 2019 at 12:13, Manuel Klimek <klimek at google.com> wrote:

> On Fri, Mar 1, 2019 at 12:10 PM Manuel Klimek <klimek at google.com> wrote:
>
>> -apple George + google George to prevent bounces :)
>>
>> On Fri, Mar 1, 2019 at 12:06 PM Manuel Klimek <klimek at google.com> wrote:
>>
>>> On Fri, Mar 1, 2019 at 11:51 AM Gábor Horváth <xazax.hun at gmail.com>
>>> wrote:
>>>
>>>> Hi Manuel,
>>>>
>>>> On Fri, 30 Nov 2018 at 12:04, Manuel Klimek <klimek at google.com> wrote:
>>>>
>>>>>
>>>>>
>>>>> On Fri, Nov 30, 2018 at 12:00 PM Gábor Horváth <xazax.hun at gmail.com>
>>>>> wrote:
>>>>>
>>>>>> Unfortunately, we do not have real world experience on a large
>>>>>> codebase for the following reasons:
>>>>>> * The implementation we have does not support annotations yet, and
>>>>>> large projects are likely to contain exceptions to the default rules (even
>>>>>> if only a few)
>>>>>> * The analysis has an expected coding style, e.g. it does not reason
>>>>>> about arithmetic on raw pointers
>>>>>>
>>>>>> We did run it on some projects like LLVM and there were a lot of
>>>>>> noise mainly due to the unsupported language features like pointer
>>>>>> arithmetic.
>>>>>> The rest of the false positives were mostly due to the basic
>>>>>> assumption of the analysis that every path is feasible.
>>>>>> We did find some true positives with a similar analysis looking for
>>>>>> use after moves in LLVM. Those true positives were redundant std::forward
>>>>>> calls that are not going to cause any runtime error but still removing them
>>>>>> would make the code cleaner and less likely to misbehave in the future.
>>>>>> See:
>>>>>> https://github.com/llvm-mirror/llvm/blob/master/include/llvm/Support/Error.h#L894
>>>>>> (The same arguments forwarded multiple times in a loop. Since the callee
>>>>>> never actually moves the arguments we will not end up using a moved from
>>>>>> object. But if we never move the argument, why would we use std::forward in
>>>>>> the first place?)
>>>>>>
>>>>>> But even if the cfg-based lifetime analysis end up being a coding
>>>>>> style specific check, the rest of the (cfg-less) clang warnings would
>>>>>> become much more powerful after type categories are upstreamed. They could
>>>>>> catch a lot of errors without having any false positives by default. Thus,
>>>>>> I do see the value going forward, even if we do not have much real world
>>>>>> experience yet.
>>>>>>
>>>>>
>>>>> Could you run the restricted checks on llvm?
>>>>>
>>>>
>>>> Sorry for the delay, but finally I found some time to actually
>>>> implement the CFG-less checks and run them on LLVM. The quick prototype is
>>>> available here:
>>>> https://github.com/mgehre/clang/commits/lifetime-warning-typecategories
>>>> Literally, all of the false positives I encountered after running on
>>>> LLVM and Clang were due to llvm::ValueHandleBase being miscategorized as an
>>>> owner (because of its user-defined destructor).
>>>> So annotating this class would render the LLVM codebase clean of these
>>>> new lifetime errors. Do you think this is a strong enough evidence to start
>>>> upstreaming the type categories or do you think we should do additional
>>>> measurements?
>>>>
>>>
>>> I'd like to get Richard's opinion on this.
>>>
>>
> So just to make sure I understand correctly:
> The result is that in LLVM we have:
> - 1 false positive root cause (causing X false positives)
> - 0 true positives
> ?
>
>

Correct.


>
>>>
>>>> I also plan to look up some lifetime fixes/reverts in the commit
>>>> history to check if some of those problems would have been caught by these
>>>> warnings. I will share the results in a EuroLLVM technical talk.
>>>>
>>>> P.S.:
>>>> Does anybody have a good idea how to get a list of buildbot breaking
>>>> commits where the reason was lifetime related (other than grepping git
>>>> history)?
>>>>
>>>> Regards,
>>>> Gábor
>>>>
>>>>
>>>>>
>>>>>
>>>>>>
>>>>>> Regards,
>>>>>> Gabor
>>>>>>
>>>>>> On Thu, 29 Nov 2018 at 20:58, George Karpenkov <ekarpenkov at apple.com>
>>>>>> wrote:
>>>>>>
>>>>>>> Thanks, the idea looks great!
>>>>>>>
>>>>>>> The heuristic described in a talk on matching returned and accepted
>>>>>>> references looked fragile to me initially,
>>>>>>> but after a bit of time I could not find obvious and common
>>>>>>> counterexamples which would confuse it.
>>>>>>>
>>>>>>> Has the evaluation been done on any large project? E.g. LLVM itself,
>>>>>>> Chrome, Webkit, or any other large C++ codebase?
>>>>>>> What is the false positive rate? What is the annotation burden to
>>>>>>> suppress those?
>>>>>>> How many useful bugs were found?
>>>>>>>
>>>>>>> I’m also very happy that more dataflow analyses are being
>>>>>>> implemented in Clang,
>>>>>>> and I could help with reviews (though obviously I’m not the code
>>>>>>> owner for most of the components).
>>>>>>>
>>>>>>> On Nov 29, 2018, at 8:02 AM, Gábor Horváth via cfe-dev <
>>>>>>> cfe-dev at lists.llvm.org> wrote:
>>>>>>>
>>>>>>> Hi!
>>>>>>>
>>>>>>> This is a proposal to implement Lifetime Analysis [1] defined by
>>>>>>> Herb Sutter in Clang.
>>>>>>> Summary from the paper:
>>>>>>> “This analysis shows how to efficiently diagnose many common cases
>>>>>>> of dangling (use-after-free) in C++ code, using only local analysis to
>>>>>>> report them as deterministic readable errors at compile time. The approach
>>>>>>> is to identify variables that are of generalized “Owner” types (e.g., smart
>>>>>>> pointers, containers, string) and “Pointer” types (e.g., int*, string_view,
>>>>>>> span, iterators, and ranges), and then use a local simple acyclic control
>>>>>>> flow graph (ACFG) analysis to track what each Pointer points to and
>>>>>>> identify when modifying an Owner invalidates a Pointer. The analysis
>>>>>>> leverages C++’s existing strong notions of scopes, object lifetimes, and
>>>>>>> const that carry rich information already available in reasonably modern
>>>>>>> C++ source code. Interestingly, it appears that with minor extension this
>>>>>>> analysis can also detect uses of local moved-from variables
>>>>>>> (use-after-move), which are a form of dangling.”
>>>>>>> More details can be found in the paper [1] or in the CppCon keynote
>>>>>>> [3].
>>>>>>>
>>>>>>> Matthias Gehre and myself had been working on a prototype in Clang
>>>>>>> [2]. The changes are rather large, so we are planning to take an
>>>>>>> incremental approach to upstreaming the features should the community want
>>>>>>> to see this upstream.
>>>>>>>
>>>>>>> *Plans for upstreaming*
>>>>>>>
>>>>>>> 1. Upstream Type Categorization
>>>>>>>
>>>>>>> Clang already performs statement-local lifetime analyses that would
>>>>>>> benefit from type categorization even before adding any other analysis.
>>>>>>>
>>>>>>> This includes annotating types as Owners and Pointers, and
>>>>>>> automatically inferring Owner or Point without annotation to minimize
>>>>>>> annotation burden.
>>>>>>>
>>>>>>> Consider the following code example:
>>>>>>>
>>>>>>> std::reference_wrapper<const int> get_data() {
>>>>>>>    const int i = 3;
>>>>>>>    return {i};
>>>>>>> }
>>>>>>>
>>>>>>> Unfortunately, today compilers do not warn on this case of returning
>>>>>>> a dangling reference. They do warn if we return a raw pointer or reference,
>>>>>>> but the compiler does not know that std::reference_wrapper also is a
>>>>>>> non-owning indirection. In the Lifetime analysis, this is diagnosed because
>>>>>>> std::reference_wrapper is recognized as a Pointer type.
>>>>>>>
>>>>>>> As a first step we would upstream the type categorization part of
>>>>>>> the analysis and make some clang warnings optionally use it. We would also
>>>>>>> upstream a set of annotations to give the users a way to fix potential
>>>>>>> false positives due to miscategorization. (This should be very rare
>>>>>>> according to our experience so far). By default, we could constrain the
>>>>>>> categorization for std types, whose semantics are known.
>>>>>>>
>>>>>>> 2. Extensions of existing CFG-less analyses
>>>>>>>
>>>>>>> 2a. Initialization from temporaries
>>>>>>> The goal is to detect Pointers that dangle on initialization, such as
>>>>>>> std::string_view sv = “test”s;
>>>>>>> By restricting the analysis to single statements, it has a low
>>>>>>> false-positive rate and can be done without building a CFG (i.e. faster).
>>>>>>>
>>>>>>> 2b. Return of locals
>>>>>>> The goal is to detect returning Pointers to local variables, e.g.
>>>>>>> std::reference_wrapper<const int> get_data() {
>>>>>>>    const int i = 3;
>>>>>>>    return {i};
>>>>>>> }
>>>>>>> Similar to 2a also restricted to single statement.
>>>>>>>
>>>>>>> 2c. Member pointer that dangles once construction is complete
>>>>>>> struct X {
>>>>>>>    std::string_view sv;
>>>>>>>    X() : sv("test"s) {} // warning: string_view member bound to
>>>>>>> string temporary whose lifetime ends within the constructor
>>>>>>> };
>>>>>>>
>>>>>>> 2d. New of a Pointer that dangles after the end of the
>>>>>>> full-expression
>>>>>>> new string_view("test"s) // warning: dynamically-allocated
>>>>>>> string_view refers to string whose lifetime ends at the end of the
>>>>>>> full-expression
>>>>>>>
>>>>>>> 3. Intra-function analysis across basic blocks, excluding function
>>>>>>> call expressions
>>>>>>> Propagate point-to sets of Pointers across branches/loops
>>>>>>> intra-function, e.g. analysing
>>>>>>>
>>>>>>> int* p = &i;
>>>>>>> if(condition)
>>>>>>>  p = nullptr;
>>>>>>> *p; // ERROR: p is possibly null
>>>>>>>
>>>>>>> We have some CFG patches and some code traversing the CFG and
>>>>>>> propagating the analysis state. With the type categories already in place,
>>>>>>> this patch should be smaller. We could split these patches further by
>>>>>>> implementing null tracking in a separate patch.
>>>>>>>
>>>>>>> 4. Function calls
>>>>>>>
>>>>>>> auto find(const string& needle, const string& haystack) ->
>>>>>>> string_view [[gsl::lifetime(haystack)]];
>>>>>>>
>>>>>>> string_view sv = find(“needle”, haystack);
>>>>>>> sv[0]; // OK
>>>>>>> string_view sv = find(needle, “temporaryhaystack”);
>>>>>>> sv[0]; // ERROR: sv is dangling
>>>>>>>
>>>>>>> This includes the following subparts.
>>>>>>>
>>>>>>> 4a. Precondition checks
>>>>>>> Check that the psets of the arguments are valid at call site
>>>>>>> according to the lifetime annotations of the callee.
>>>>>>>
>>>>>>> 4b. Postcondition checks
>>>>>>> Check that the psets returned from a function adhere to its
>>>>>>> advertised return/output psets.
>>>>>>> Rigorous checking of not just the function arguments but also the
>>>>>>> returned values is crucial part of the analysis.
>>>>>>>
>>>>>>> 4c. Lifetimes annotations
>>>>>>>
>>>>>>> The analysis gets pretty usable at this point. Most of the time the
>>>>>>> user does not need any annotations, but it is crucial to have them before a
>>>>>>> project can adapt it. For example, the user will occasionally want to
>>>>>>> explicitly state that a member function is “const as far as Lifetime is
>>>>>>> concerned” even though the function itself is not actually declared const
>>>>>>> (e.g., vector::operator[] does not invalidate any Pointers, such as
>>>>>>> iterators or raw pointers).
>>>>>>>
>>>>>>> 5. Implementing use after move analysis and exception support
>>>>>>>
>>>>>>> These parts are not implemented yet in our prototype, but they will
>>>>>>> be useful additions for the analysis.
>>>>>>>
>>>>>>> *Questions*
>>>>>>>
>>>>>>> Does that make sense? What is the criteria for this work to be
>>>>>>> upstreamed? Who is willing to participate in reviewing the patches?
>>>>>>>
>>>>>>> Thanks in advance,
>>>>>>> Gabor, Matthias, and Herb
>>>>>>>
>>>>>>> [1]
>>>>>>> https://github.com/isocpp/CppCoreGuidelines/blob/master/docs/Lifetime.pdf
>>>>>>> [2] https://github.com/mgehre/clang
>>>>>>> [3] https://www.youtube.com/watch?v=80BZxujhY38
>>>>>>> [4] https://godbolt.org/z/90puuu
>>>>>>>
>>>>>>> _______________________________________________
>>>>>>> cfe-dev mailing list
>>>>>>> cfe-dev at lists.llvm.org
>>>>>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-dev
>>>>>>>
>>>>>>>
>>>>>>>
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