[clang] [clang][analyzer] Add notes to PointerSubChecker (PR #95899)

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llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-clang-static-analyzer-1

Author: Balázs Kéri (balazske)

<details>
<summary>Changes</summary>

Notes are added to indicate the array declarations of the arrays in a found invalid pointer subtraction.

---
Full diff: https://github.com/llvm/llvm-project/pull/95899.diff


2 Files Affected:

- (modified) clang/lib/StaticAnalyzer/Checkers/PointerSubChecker.cpp (+30-15) 
- (added) clang/test/Analysis/pointer-sub-notes.c (+34) 


``````````diff
diff --git a/clang/lib/StaticAnalyzer/Checkers/PointerSubChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/PointerSubChecker.cpp
index b73534136fdf0..a983e66df0818 100644
--- a/clang/lib/StaticAnalyzer/Checkers/PointerSubChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/PointerSubChecker.cpp
@@ -43,8 +43,6 @@ class PointerSubChecker
   bool checkArrayBounds(CheckerContext &C, const Expr *E,
                         const ElementRegion *ElemReg,
                         const MemRegion *Reg) const;
-  void reportBug(CheckerContext &C, const Expr *E,
-                 const llvm::StringLiteral &Msg) const;
 
 public:
   void checkPreStmt(const BinaryOperator *B, CheckerContext &C) const;
@@ -57,6 +55,14 @@ bool PointerSubChecker::checkArrayBounds(CheckerContext &C, const Expr *E,
   if (!ElemReg)
     return true;
 
+  auto ReportBug = [&](const llvm::StringLiteral &Msg) {
+    if (ExplodedNode *N = C.generateNonFatalErrorNode()) {
+      auto R = std::make_unique<PathSensitiveBugReport>(BT, Msg, N);
+      R->addRange(E->getSourceRange());
+      C.emitReport(std::move(R));
+    }
+  };
+
   ProgramStateRef State = C.getState();
   const MemRegion *SuperReg = ElemReg->getSuperRegion();
   SValBuilder &SVB = C.getSValBuilder();
@@ -64,7 +70,7 @@ bool PointerSubChecker::checkArrayBounds(CheckerContext &C, const Expr *E,
   if (SuperReg == Reg) {
     if (const llvm::APSInt *I = SVB.getKnownValue(State, ElemReg->getIndex());
         I && (!I->isOne() && !I->isZero()))
-      reportBug(C, E, Msg_BadVarIndex);
+      ReportBug(Msg_BadVarIndex);
     return false;
   }
 
@@ -77,7 +83,7 @@ bool PointerSubChecker::checkArrayBounds(CheckerContext &C, const Expr *E,
     ProgramStateRef S1, S2;
     std::tie(S1, S2) = C.getState()->assume(*IndexTooLarge);
     if (S1 && !S2) {
-      reportBug(C, E, Msg_LargeArrayIndex);
+      ReportBug(Msg_LargeArrayIndex);
       return false;
     }
   }
@@ -89,22 +95,13 @@ bool PointerSubChecker::checkArrayBounds(CheckerContext &C, const Expr *E,
     ProgramStateRef S1, S2;
     std::tie(S1, S2) = State->assume(*IndexTooSmall);
     if (S1 && !S2) {
-      reportBug(C, E, Msg_NegativeArrayIndex);
+      ReportBug(Msg_NegativeArrayIndex);
       return false;
     }
   }
   return true;
 }
 
-void PointerSubChecker::reportBug(CheckerContext &C, const Expr *E,
-                                  const llvm::StringLiteral &Msg) const {
-  if (ExplodedNode *N = C.generateNonFatalErrorNode()) {
-    auto R = std::make_unique<PathSensitiveBugReport>(BT, Msg, N);
-    R->addRange(E->getSourceRange());
-    C.emitReport(std::move(R));
-  }
-}
-
 void PointerSubChecker::checkPreStmt(const BinaryOperator *B,
                                      CheckerContext &C) const {
   // When doing pointer subtraction, if the two pointers do not point to the
@@ -136,6 +133,9 @@ void PointerSubChecker::checkPreStmt(const BinaryOperator *B,
   if (!checkArrayBounds(C, B->getRHS(), ElemRR, LR))
     return;
 
+  const ValueDecl *DiffDeclL = nullptr;
+  const ValueDecl *DiffDeclR = nullptr;
+
   if (ElemLR && ElemRR) {
     const MemRegion *SuperLR = ElemLR->getSuperRegion();
     const MemRegion *SuperRR = ElemRR->getSuperRegion();
@@ -144,9 +144,24 @@ void PointerSubChecker::checkPreStmt(const BinaryOperator *B,
     // Allow arithmetic on different symbolic regions.
     if (isa<SymbolicRegion>(SuperLR) || isa<SymbolicRegion>(SuperRR))
       return;
+    if (const auto *SuperDLR = dyn_cast<DeclRegion>(SuperLR))
+      DiffDeclL = SuperDLR->getDecl();
+    if (const auto *SuperDRR = dyn_cast<DeclRegion>(SuperRR))
+      DiffDeclR = SuperDRR->getDecl();
   }
 
-  reportBug(C, B, Msg_MemRegionDifferent);
+  if (ExplodedNode *N = C.generateNonFatalErrorNode()) {
+    auto R =
+        std::make_unique<PathSensitiveBugReport>(BT, Msg_MemRegionDifferent, N);
+    R->addRange(B->getSourceRange());
+    if (DiffDeclL)
+      R->addNote("Array at the left-hand side of subtraction",
+                 {DiffDeclL, C.getSourceManager()});
+    if (DiffDeclR)
+      R->addNote("Array at the right-hand side of subtraction",
+                 {DiffDeclR, C.getSourceManager()});
+    C.emitReport(std::move(R));
+  }
 }
 
 void ento::registerPointerSubChecker(CheckerManager &mgr) {
diff --git a/clang/test/Analysis/pointer-sub-notes.c b/clang/test/Analysis/pointer-sub-notes.c
new file mode 100644
index 0000000000000..dfdace3a58deb
--- /dev/null
+++ b/clang/test/Analysis/pointer-sub-notes.c
@@ -0,0 +1,34 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=alpha.core.PointerSub -analyzer-output=text -verify %s
+
+void negative_1() {
+  int a[3];
+  int x = -1;
+  // FIXME: should indicate that 'x' is -1
+  int d = &a[x] - &a[0]; // expected-warning{{Using a negative array index at pointer subtraction is undefined behavior}} \
+                         // expected-note{{Using a negative array index at pointer subtraction is undefined behavior}}
+}
+
+void negative_2() {
+  int a[3];
+  int *p1 = a, *p2 = a;
+  --p2;
+  // FIXME: should indicate that 'p2' is negative
+  int d = p1 - p2; // expected-warning{{Using a negative array index at pointer subtraction is undefined behavior}} \
+                   // expected-note{{Using a negative array index at pointer subtraction is undefined behavior}}
+}
+
+void different_1() {
+  int a[3]; // expected-note{{Array at the left-hand side of subtraction}}
+  int b[3]; // expected-note{{Array at the right-hand side of subtraction}}
+  int d = &a[2] - &b[0]; // expected-warning{{Subtraction of two pointers that do not point into the same array is undefined behavior}} \
+                         // expected-note{{Subtraction of two pointers that do not point into the same array is undefined behavior}}
+}
+
+void different_2() {
+  int a[3]; // expected-note{{Array at the right-hand side of subtraction}}
+  int b[3]; // expected-note{{Array at the left-hand side of subtraction}}
+  int *p1 = a + 1;
+  int *p2 = b;
+  int d = p2 - p1; // expected-warning{{Subtraction of two pointers that do not point into the same array is undefined behavior}} \
+                   // expected-note{{Subtraction of two pointers that do not point into the same array is undefined behavior}}
+}

``````````

</details>


https://github.com/llvm/llvm-project/pull/95899