[clang] [-Wunsafe-buffer-usage] Fix false warnings when const sized array is safely accessed (array [idx %const]) (PR #140113)
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Thu May 15 10:53:27 PDT 2025
llvmbot wrote:
<!--LLVM PR SUMMARY COMMENT-->
@llvm/pr-subscribers-clang
@llvm/pr-subscribers-clang-analysis
Author: Malavika Samak (malavikasamak)
<details>
<summary>Changes</summary>
The -Wunsafe-buffer-usage analysis currently warns when a const sized array is safely accessed, with an index that is bound by the remainder operator. The false alarm is now suppressed.
Example:
int circular_buffer(int array[10], uint idx) {
return array[idx %10]; // Safe operation
}
rdar://148443453
---
Full diff: https://github.com/llvm/llvm-project/pull/140113.diff
2 Files Affected:
- (modified) clang/lib/Analysis/UnsafeBufferUsage.cpp (+16-1)
- (modified) clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp (+9-1)
``````````diff
diff --git a/clang/lib/Analysis/UnsafeBufferUsage.cpp b/clang/lib/Analysis/UnsafeBufferUsage.cpp
index 5b72382ca9772..2f669637a2731 100644
--- a/clang/lib/Analysis/UnsafeBufferUsage.cpp
+++ b/clang/lib/Analysis/UnsafeBufferUsage.cpp
@@ -600,12 +600,27 @@ static bool isSafeArraySubscript(const ArraySubscriptExpr &Node,
} else if (const auto *BE = dyn_cast<BinaryOperator>(IndexExpr)) {
// For an integer expression `e` and an integer constant `n`, `e & n` and
// `n & e` are bounded by `n`:
- if (BE->getOpcode() != BO_And)
+ if (BE->getOpcode() != BO_And && BE->getOpcode() != BO_Rem)
return false;
const Expr *LHS = BE->getLHS();
const Expr *RHS = BE->getRHS();
+ if(BE->getOpcode() == BO_Rem) {
+ // If n is a negative number, then n % const can be greater than const
+ if(!LHS->getType()->isUnsignedIntegerType()) {
+ return false;
+ }
+
+ if(!RHS->isValueDependent() && RHS->EvaluateAsInt(EVResult, Ctx)) {
+ llvm::APSInt result = EVResult.Val.getInt();
+ if (result.isNonNegative() && result.getLimitedValue() <= limit)
+ return true;
+ }
+
+ return false;
+ }
+
if ((!LHS->isValueDependent() &&
LHS->EvaluateAsInt(EVResult, Ctx)) || // case: `n & e`
(!RHS->isValueDependent() &&
diff --git a/clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp b/clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp
index 3233999ea8ea2..b2358bb9f6be3 100644
--- a/clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp
+++ b/clang/test/SemaCXX/warn-unsafe-buffer-usage-array.cpp
@@ -35,7 +35,15 @@ void constant_idx_safe0(unsigned idx) {
buffer[0] = 0;
}
-int array[10]; // expected-warning {{'array' is an unsafe buffer that does not perform bounds checks}}
+int array[10]; // expected-warning 2{{'array' is an unsafe buffer that does not perform bounds checks}}
+
+void circular_access_safe(unsigned idx) {
+ array[idx % 10];
+}
+
+void circular_access_unsafe(int idx) {
+ array[idx % 10]; // expected-note {{used in buffer access here}}
+}
void masked_idx1(unsigned long long idx, Foo f) {
// Bitwise and operation
``````````
</details>
https://github.com/llvm/llvm-project/pull/140113
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