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[clang] Fix dangling false positives for conditional operators. #120233

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Dec 20, 2024
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[clang] Fix dangling false positives for conditional operators. #120233

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2 changes: 2 additions & 0 deletions clang/docs/ReleaseNotes.rst
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Expand Up @@ -700,6 +700,8 @@ Improvements to Clang's diagnostics
return ptr + index < ptr; // warning
}

- Fix -Wdangling false positives on conditional operators (#120206).

Improvements to Clang's time-trace
----------------------------------

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9 changes: 9 additions & 0 deletions clang/lib/Sema/CheckExprLifetime.cpp
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Expand Up @@ -582,6 +582,15 @@ static void visitFunctionCallArguments(IndirectLocalPath &Path, Expr *Call,
// Temp().ptr; // Here ptr might not dangle.
if (isa<MemberExpr>(Arg->IgnoreImpCasts()))
return;
// Avoid false positives when the object is constructed from a conditional
// operator argument. A common case is:
// // 'ptr' might not be owned by the Owner object.
// std::string_view s = cond() ? Owner().ptr : sv;
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note: I keep the code simple here, and heuristic logic is not perfect and may lead to false negatives, as it filters out more cases than intended. The major case std::string_view sv = cond ? "123" : std::string(); is still covered, I think this is a right tradeoff.

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@Xazax-hun Xazax-hun Dec 17, 2024 •
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I am a bit confused to be honest. Are there any other contexts where Owner().ptr is problematic? I'd expect our analysis to behave the same for a subexpression like that regardless the context. So I am surprised we need to insert special logic for the ternary operator.

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I am a bit confused to be honest. Are there any other contexts where Owner().ptr is problematic? I'd expect our analysis to behave the same for a subexpression like that regardless the context. So I am surprised we need to insert special logic for the ternary operator.

Possibly, but I don’t know of any concrete examples (it’s hard to judge). We have another ad-hoc filter at the end, IsGslPtrValueFromGslTempOwner, which filters out cases where the GSL pointer doesn’t originate from a GSL owner. This works well for simple and common cases, but when combined with lifetimebound, the behavior becomes tricky.

The current fix extends the MemberExpr logic (L583) to handle cases like GSLPointer pointer(Owner().ptr);, but it doesn’t yet address cases like GSLPointer pointer(Cond ? Owner().ptr : GSLPointer());. I think this fix is a reasonable extension to address the issue.

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but it doesn’t yet address cases like GSLPointer pointer(Cond ? Owner().ptr : GSLPointer())

I am wondering if it was possible to use the same or similar MemberExpr filter when we drill down to the branches of the conditional operator. If it is too hard to do, I am OK with the current solution. But to me expanding the MemberExpr logic to handle more expressions sounds more natural than adding logic for a new expression type.

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I am wondering if it was possible to use the same or similar MemberExpr filter when we drill down to the branches of the conditional operator. If it is too hard to do,

This was my first approach. We could add a special MemberExpr filter inside the do-while loop in visitLocalsRetainedByReferenceBinding, but I don’t think it’s a good idea:

  • the filtering logic is GSL-pointer specific, and visitLocalsRetainedByReferenceBinding shouldn’t be aware of it; (layering violation)
  • adding such filtering causes new false negatives in cases like const string_view& sv = Owner().sv;.
  • it would scatter the special filtering logic into another place, making the code harder to reason about.

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Is const string_view& sv = Owner().sv; a false negative? We cannot actually know if sv is actually owned or not because we have no lifetimebound annotation and the gsl owner does not tell us anything about fields, only about conversions/ctors.

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Ah, never mind, I missed the const ref!

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OK, with all of these being said I am OK with the current approach.

if (const auto *Cond =
dyn_cast<AbstractConditionalOperator>(Arg->IgnoreImpCasts());
Cond && isPointerLikeType(Cond->getType()))
return;

auto ReturnType = Callee->getReturnType();

// Once we initialized a value with a non gsl-owner reference, it can no
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29 changes: 29 additions & 0 deletions clang/test/Sema/warn-lifetime-analysis-nocfg.cpp
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Expand Up @@ -777,3 +777,32 @@ void test4() {
}

} // namespace LifetimeboundInterleave

namespace GH120206 {
struct S {
std::string_view s;
};

struct [[gsl::Owner]] Q1 {
const S* get() const [[clang::lifetimebound]];
};
std::string_view test1(int c, std::string_view sv) {
std::string_view k = c > 1 ? Q1().get()->s : sv;
if (c == 1)
return c > 1 ? Q1().get()->s : sv;
Q1 q;
return c > 1 ? q.get()->s : sv;
}

struct Q2 {
const S* get() const [[clang::lifetimebound]];
};
std::string_view test2(int c, std::string_view sv) {
std::string_view k = c > 1 ? Q2().get()->s : sv;
if (c == 1)
return c > 1 ? Q2().get()->s : sv;
Q2 q;
return c > 1 ? q.get()->s : sv;
}

} // namespace GH120206
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