[-Wunsafe-buffer-usage] Fix a bug and suppress libc warnings for C files

- Fix a bug in UnsafeBufferUsage.cpp related to casting to PointerType
- Suppress -Wunsafe-buffer-usage-in-libc-call for C files

(rdar://117182250)

Author: ziqingluo-90

clang/lib/Analysis/UnsafeBufferUsage.cpp

@@ -250,7 +250,9 @@ AST_MATCHER_P(Stmt, ignoreUnsafeBufferInContainer,
 
 AST_MATCHER_P(Stmt, ignoreUnsafeLibcCall, const UnsafeBufferUsageHandler *,
               Handler) {
-  return Handler->ignoreUnsafeBufferInLibcCall(Node.getBeginLoc());
+  if (Finder->getASTContext().getLangOpts().CPlusPlus)
+    return Handler->ignoreUnsafeBufferInLibcCall(Node.getBeginLoc());
+  return true; /* Only warn about libc calls for C++ */
 }
 
 AST_MATCHER_P(CastExpr, castSubExpr, internal::Matcher<Expr>, innerMatcher) {
@@ -789,7 +791,7 @@ AST_MATCHER_P(CallExpr, hasUnsafePrintfStringArg,
   if (!FristParmTy->isPointerType())
     return false; // possibly some user-defined printf function
 
-  QualType FirstPteTy = (cast<PointerType>(FristParmTy))->getPointeeType();
+  QualType FirstPteTy = FristParmTy->getAs<PointerType>()->getPointeeType();
 
   if (!Ctx.getFILEType()
            .isNull() && //`FILE *` must be in the context if it is fprintf
@@ -865,7 +867,7 @@ AST_MATCHER(CallExpr, hasUnsafeSnprintfBuffer) {
   if (!FirstParmTy->isPointerType())
     return false; // Not an snprint
 
-  QualType FirstPteTy = cast<PointerType>(FirstParmTy)->getPointeeType();
+  QualType FirstPteTy = FirstParmTy->getAs<PointerType>()->getPointeeType();
   const Expr *Buf = Node.getArg(0), *Size = Node.getArg(1);
 
   if (FirstPteTy.isConstQualified() || !Buf->getType()->isPointerType() ||

clang/lib/Sema/AnalysisBasedWarnings.cpp

@@ -2549,6 +2549,7 @@ void clang::sema::AnalysisBasedWarnings::IssueWarnings(
   DiagnosticOptions &DiagOpts = Diags.getDiagnosticOptions();
 
   // UnsafeBufferUsage analysis settings.
+  bool IsCXXLang = S.getLangOpts().CPlusPlus;
   bool UnsafeBufferUsageCanEmitSuggestions = S.getLangOpts().CPlusPlus20;
   bool UnsafeBufferUsageShouldEmitSuggestions =  // Should != Can.
       UnsafeBufferUsageCanEmitSuggestions &&
@@ -2567,8 +2568,9 @@ void clang::sema::AnalysisBasedWarnings::IssueWarnings(
                          Node->getBeginLoc()) ||
         !Diags.isIgnored(diag::warn_unsafe_buffer_usage_in_container,
                          Node->getBeginLoc()) ||
-        !Diags.isIgnored(diag::warn_unsafe_buffer_libc_call,
-                         Node->getBeginLoc())) {
+        (!Diags.isIgnored(diag::warn_unsafe_buffer_libc_call,
+                          Node->getBeginLoc()) &&
+         IsCXXLang /* we only warn about libc calls in C++ files */)) {
       clang::checkUnsafeBufferUsage(Node, R,
                                     UnsafeBufferUsageShouldEmitSuggestions);
     }

clang/test/SemaCXX/warn-unsafe-buffer-usage-no-libc-functions-in-c.c

@@ -0,0 +1,9 @@
+// RUN: %clang_cc1 -Wunsafe-buffer-usage %s -verify %s
+
+void* memcpy(void *dst,const void *src, unsigned long size);
+
+void f(int *p, int *q) {
+
+  memcpy(p, q, 10); // no libc warn in C
+  ++p[5];           // expected-warning{{unsafe buffer access}}
+}