[clang] [clang][analyzer] Add checker 'security.SetgidSetuidOrder'. (PR #91445)

[Donát Nagy via cfe-commits]

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@@ -0,0 +1,170 @@
+// RUN: %clang_analyze_cc1 -analyzer-checker=core,security.SetgidSetuidOrder -verify %s
+
+#include "Inputs/system-header-simulator-setgid-setuid.h"
+
+void correct_order() {
+  if (setgid(getgid()) == -1)
+    return;
+  if (setuid(getuid()) == -1)
+    return;
+  if (setgid(getgid()) == -1)
+    return;
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NagyDonat wrote:

But why do the SEI-CERT best practices mandate that "this should not be recognized as an error"? Could you briefly explain this in a comment? (E.g. "Special case: calling `setgid(getgid())` after an earlier `setgid(getgid()); setuid(getuid())` combination is legitimate, because it... <why is this good>")

https://github.com/llvm/llvm-project/pull/91445