[clang] [clang] Warn on umask() argument bits outside 0777 (PR #198130)

[Denys Fedoryshchenko via cfe-commits]

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@@ -1490,6 +1490,49 @@ void Sema::checkFortifiedBuiltinMemoryFunction(FunctionDecl *FD,
                           << FunctionName << DestinationStr << SourceStr);
 }
 
+void Sema::checkFortifiedLibcArgument(FunctionDecl *FD, CallExpr *TheCall) {
+  if (TheCall->isValueDependent() || TheCall->isTypeDependent() ||
+      isConstantEvaluatedContext())
+    return;
+  if (!FD->isExternC())
+    return;
+  const IdentifierInfo *II = FD->getIdentifier();
+  if (!II)
+    return;
+
+  // umask(mode_t): warn when the constant-evaluated argument has bits set
+  // outside the file-permission mask (0777). Those bits are ignored.
+  // Require a matching system-header declaration to avoid warning on
+  // user-defined lookalikes.
+  auto AnyDeclInSystemHeader = [&](const FunctionDecl *F) {
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nuclearcat wrote:

Yes, POSIX is the same as C here: umask can be forward-declared without <sys/stat.h>, and then this check goes silent.
Other fortify checks dodge this by recognizing functions via getBuiltinID, but umask isn't a builtin, so that path isn't available :(
Will make documentation changes.

https://github.com/llvm/llvm-project/pull/198130