[clang][analyzer] Change modeling of 'fileno' in checkers. (#81842)

Function 'fileno' fails only if invalid pointer is passed, this is a
case that is often ignored in source code. The failure case leads to
many "false positive" reports when `fileno` returns -1 and this is not
checked in the program. Because this, the function is now assumed
to not fail (this is assumption that the passed file pointer is correct).
The change affects `StdCLibraryFunctionsChecker` and
`StreamChecker`.

1 files changed, 6 insertions, 3 deletions

diff --git a/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp b/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp
index 6b8ac26..6cc8867 100644
--- a/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp
+++ b/clang/lib/StaticAnalyzer/Checkers/StdLibraryFunctionsChecker.cpp
@@ -2388,12 +2388,15 @@ void StdLibraryFunctionsChecker::initFunctionSummaries(
             .ArgConstraint(NotNull(ArgNo(0))));
 
     // int fileno(FILE *stream);
+    // According to POSIX 'fileno' may fail and set 'errno'.
+    // But in Linux it may fail only if the specified file pointer is invalid.
+    // At many places 'fileno' is used without check for failure and a failure
+    // case here would produce a large amount of likely false positive warnings.
+    // To avoid this, we assume here that it does not fail.
     addToFunctionSummaryMap(
         "fileno", Signature(ArgTypes{FilePtrTy}, RetType{IntTy}),
         Summary(NoEvalCall)
-            .Case(ReturnsValidFileDescriptor, ErrnoMustNotBeChecked,
-                  GenericSuccessMsg)
-            .Case(ReturnsMinusOne, ErrnoNEZeroIrrelevant, GenericFailureMsg)
+            .Case(ReturnsValidFileDescriptor, ErrnoUnchanged, GenericSuccessMsg)
             .ArgConstraint(NotNull(ArgNo(0))));
 
     // void rewind(FILE *stream);