[clang] Improve modeling of 'getcwd' in the StdLibraryFunctionsChecker (PR #77040)

[Balázs Kéri via cfe-commits]

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@@ -2516,12 +2516,15 @@ void StdLibraryFunctionsChecker::initFunctionSummaries(
             .ArgConstraint(NotNull(ArgNo(0))));
 
     // char *getcwd(char *buf, size_t size);
-    // FIXME: Improve for errno modeling.
     addToFunctionSummaryMap(
         "getcwd", Signature(ArgTypes{CharPtrTy, SizeTy}, RetType{CharPtrTy}),
         Summary(NoEvalCall)
+            .Case({ReturnValueCondition(BO_EQ, ArgNo(0))},
+                  ErrnoMustNotBeChecked, GenericSuccessMsg)
+            .Case({IsNull(Ret)}, ErrnoNEZeroIrrelevant, GenericFailureMsg)
+            .ArgConstraint(NotNull(ArgNo(0)))
             .ArgConstraint(
-                ArgumentCondition(1, WithinRange, Range(0, SizeMax))));
+                ArgumentCondition(1, WithinRange, Range(1, SizeMax))));
----------------
balazske wrote:

Here a `BufferSize` type of constraint can be used additionally. See `readlink` for example.
Value 0 is allowed for the size. Passing 0 is not undefined behavior, the function shall fail and set `errno`. A new case can be added (like at `readlink`).

https://github.com/llvm/llvm-project/pull/77040