[clang][analyzer] Improve modeling of 'realpath' in StdLibraryFunctionsChecker (#79939)

Author: benshi001

clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp

@@ -2992,12 +2992,16 @@ void StdLibraryFunctionsChecker::initFunctionSummaries(
 
     // char *realpath(const char *restrict file_name,
     //                char *restrict resolved_name);
-    // FIXME: Improve for errno modeling.
+    // FIXME: If the argument 'resolved_name' is not NULL, macro 'PATH_MAX'
+    //        should be defined in "limits.h" to guarrantee a success.
     addToFunctionSummaryMap(
         "realpath",
         Signature(ArgTypes{ConstCharPtrRestrictTy, CharPtrRestrictTy},
                   RetType{CharPtrTy}),
-        Summary(NoEvalCall).ArgConstraint(NotNull(ArgNo(0))));
+        Summary(NoEvalCall)
+            .Case({NotNull(Ret)}, ErrnoMustNotBeChecked, GenericSuccessMsg)
+            .Case({IsNull(Ret)}, ErrnoNEZeroIrrelevant, GenericFailureMsg)
+            .ArgConstraint(NotNull(ArgNo(0))));
 
     QualType CharPtrConstPtr = getPointerTy(getConstTy(CharPtrTy));
 

clang/test/Analysis/errno-stdlibraryfunctions.c

@@ -128,3 +128,13 @@ void errno_pclose(void) {
     if (errno) {} // expected-warning{{An undefined value may be read from 'errno'}}
   }
 }
+
+void errno_realpath(char *Path, char *Buf) {
+  char *Ret = realpath(Path, Buf);
+  if (!Ret) {
+    clang_analyzer_eval(errno != 0);  // expected-warning{{TRUE}}
+    if (errno) {}                     // no-warning
+  } else {
+    if (errno) {} // expected-warning{{An undefined value may be read from 'errno'}}
+  }
+}