[clang] [clang][StaticAnalyzer] Adding getentropy to CStringChecker. (PR #83675)

[Balazs Benics via cfe-commits]

================
@@ -2516,6 +2518,47 @@ void CStringChecker::evalSprintfCommon(CheckerContext &C, const CallEvent &Call,
   C.addTransition(State);
 }
 
+void CStringChecker::evalGetentropy(CheckerContext &C,
+                                    const CallEvent &Call) const {
+  DestinationArgExpr Buffer = {{Call.getArgExpr(0), 0}};
+  SizeArgExpr Size = {{Call.getArgExpr(1), 1}};
+  ProgramStateRef State = C.getState();
+  constexpr int BufferMaxSize = 256;
+
+  SVal SizeVal = C.getSVal(Size.Expression);
+  QualType SizeTy = Size.Expression->getType();
+
+  ProgramStateRef StateZeroSize, StateNonZeroSize;
+  std::tie(StateZeroSize, StateNonZeroSize) =
+      assumeZero(C, State, SizeVal, SizeTy);
+
+  SVal Buff = C.getSVal(Buffer.Expression);
+  State = checkNonNull(C, StateNonZeroSize, Buffer, Buff);
+  if (!State)
+    return;
+
+  State = CheckBufferAccess(C, State, Buffer, Size, AccessKind::write);
+  if (!State)
+    return;
+
+  auto SizeLoc = SizeVal.getAs<nonloc::ConcreteInt>();
+  auto size = SizeLoc->getValue().getExtValue();
+
+  if (size > BufferMaxSize) {
+    ErrorMessage Message;
+    llvm::raw_svector_ostream Os(Message);
+    Os << " destination buffer size is greater than " << BufferMaxSize;
+    emitOutOfBoundsBug(C, StateNonZeroSize, Buffer.Expression, Message);
+    return;
+  }
----------------
steakhal wrote:

```suggestion
```
This hunk seems to be unnecessary as `CheckBufferAccess` supposed to check both ends of the buffer.

https://github.com/llvm/llvm-project/pull/83675