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
- 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 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.
- 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 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
- 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.
- 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).
- 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
