[analyzer] Fix stdin declaration in C++ tests (#66074)

The `stdin` declaration should be within `extern "C" {...}`, in C++
mode. In addition, it should be also marked `extern` in both C and
C++ modes.

I tightened the check to ensure we only accept `stdin` if both of these
match. However, from the Juliet test suite's perspective, this commit
should not matter.

#66074

clang/lib/StaticAnalyzer/Checkers/GenericTaintChecker.cpp

@@ -104,8 +104,7 @@ bool isStdin(SVal Val, const ASTContext &ACtx) {
   // variable named stdin with the proper type.
   if (const auto *D = dyn_cast_or_null<VarDecl>(DeclReg->getDecl())) {
     D = D->getCanonicalDecl();
-    // FIXME: This should look for an exact match.
-    if (D->getName().contains("stdin") && D->isExternC()) {
+    if (D->getName() == "stdin" && D->hasExternalStorage() && D->isExternC()) {
       const QualType FILETy = ACtx.getFILEType().getCanonicalType();
       const QualType Ty = D->getType().getCanonicalType();
 

clang/test/Analysis/taint-diagnostic-visitor.c

@@ -13,7 +13,7 @@ size_t strlen( const char* str );
 void *malloc(size_t size );
 void free( void *ptr );
 char *fgets(char *str, int n, FILE *stream);
-FILE *stdin;
+extern FILE *stdin;
 
 void taintDiagnostic(void)
 {

clang/test/Analysis/taint-generic.cpp

@@ -7,6 +7,12 @@ int scanf(const char*, ...);
 int mySource1();
 int mySource3();
 
+typedef struct _FILE FILE;
+extern "C" {
+extern FILE *stdin;
+}
+int fscanf(FILE *stream, const char *format, ...);
+
 bool isOutOfRange2(const int*);
 
 void mySink2(int);
@@ -124,3 +130,9 @@ void testConfigurationMemberFunc() {
   foo.myMemberScanf("%d", &x);
   Buffer[x] = 1; // expected-warning {{Out of bound memory access }}
 }
+
+void testReadingFromStdin(char **p) {
+  int n;
+  fscanf(stdin, "%d", &n);
+  Buffer[n] = 1; // expected-warning {{Out of bound memory access (index is tainted)}}
+}